REFINERY PROCESS SIMPLIFIED

Before we begin learning about different units in the refinery, it is very important to understand the process flow. By Process flow I mean, from the point the crude oil comes in, to the final stage where it is taken out as different products. Each product will have a required specification and to obtain that, many additional processes may be carried out. The most important product obtained is gasoline or petrol that is used as fuel. The other products are Jet Fuel, Kerosene, and Diesel Oil.

(Image Courtesy - http://www.reactor-resources.com)

This is a standard diagram that shows the entire process in the refinery. But it might differ in some places according to the requirements. I will be explaining the purpose of each unit further. This description is not comprehensive, but a basic one through which you can only understand the purpose of the unit. To understand each unit in detail, please check out my upcoming posts.

Atmospheric Distillation Unit (ADU)

Crude oil is a mixture of hydrocarbons with different molecular weights.  And distillation as we all know is a fractionator that separates different components based on their relative volatility.

Crude oil is fed to the distillation column and different products are obtained - gas, light naphtha, heavy naphtha, kerosene, jet fuel, diesel oil, and atmospheric gas oil.

The main product of the refinery is gasoline or petroleum. All the fractions obtained other than diesel oil, kerosene and jet fuel are treated to obtain gasoline. The streams may undergo cracking, oxidation, reduction, alkylation, or isomerisation to make it to the final composition (of gasoline).

Vacuum Distillation Unit (VDU)

The inlet to the VDU is the bottoms of the ADU. That means it is the heaviest part of the crude oil. ADU has a temperature limit it can operate on. We all know that crude is separated into different components based on the boiling point or the temperature. Beyond a point, the temperature cannot be increased as the feed will thermally crack and hence we resort to vacuum columns that operate under vacuum conditions. Remember vacuum means a pressure lower than the atmospheric pressure.

Hydrotreater

Petroleum streams contain organosulfur compounds that have to be removed as they can pose a challenge to catalysts in further steps or for environmental reasons.

Hydrotreaters are used to remove impurities like sulfur, nitrogen, and other olefins by selectively treating the inlet with hydrogen. A catalyst is required here and it varies according to which component should be removed – sulfur or nitrogen.

Isomerization Plant

As the name suggests an isomerization unit converts the inlet stream into its isomers when reacted with hydrogen in the presence of a catalyst. One important point to be remembered is that the inlet stream should not contain olefins as it may disturb the platinum catalyst used.

Merox Treaters

Mercaptan Oxidation or Merox process removes the mercaptans from the inlet stream. This is usually done to meet environmental regulations, especially to decrease the level of sulfur. Whenever the content of sulfur is less, we call it sweet (the sour odor is removed as the mercaptans or hydrogen sulfide is removed), and whenever it is more, we call it sour. So merox treater is essentially a sweetening process. An alkaline environment is required for the merox treater and is created by adding caustic or ammonia.

Amine Treater

CO₂ and H₂S are removed from the gaseous feed mixture by treating it with an amine solution in an Amine Treater. Amine with great affinity to CO₂ and H₂S absorbs it in the liquid and is then stripped off in two stages. This process is also known as sweetening or acid gas removal. H₂S is a lethal gas and can be corrosive. CO₂ may freeze up and hamper the working of the plant.

Claus Sulfur Plant

The H₂S gas is converted into elemental sulfur in a Claus Sulfur/ Sulfur Recovery Unit. The process was patented by Carl Friedrich Claus and hence the name. H₂S is burned with air and gets converted into S0₂ and then to elemental Sulfur in the presence of a catalyst usually alumina.                 

Catalytic Reformer

The catalytic reformer is used to treat the low-octane naphtha feed from CDU. It converts the inlet stream into a high-octane product called reformate. The reformate is a major component of gasoline or petrol. The process is the rearrangement and breakage of hydrocarbons with the production of hydrogen as a byproduct. The rearrangement results in more complex structures and thus high-octane fuel. Platinum is the most commonly used catalyst.

Hydrocracker

Hydrocracker cracks the high molecular weight hydrocarbons into low molecular weight hydrocarbons like gasoline, kerosene, diesel oil, etc in the presence of hydrogen and a catalyst.  Sulfur and nitrogen present in the inlet are also hydrogenated and removed as Hydrogen sulfide or Ammonia.

Fluid Catalytic Cracker

The process here is similar to the Hydrocracker. The heavy feed is cracked into lighter ones in the presence of a catalyst. Hydrocracker converts VGO to diesel and kerosene but FCC converts VGO into better quality gasoline along with other products. However, the FCC unit is a much bigger one and usually consists of a reactor, a regenerator (for the catalyst), and a fractionator (to separate the different cracked products). Hydrocracking is an exothermic process whereas catalytic cracking is endothermic.

Alkylation

Alkylation as the name suggests is the process by which an alkyl group is added. The light hydrocarbons like propylene, butane, isobutane, etc are combined to form heavy hydrocarbons in the presence of a catalyst.

Delayed Coker

The coker unit in refineries cracks the heavy bottom residues into coker gas oil and pet coke. This is essentially a thermal cracking process carried out at high temperatures and pressure in the coker drum. The light ends produced (vapor phase) are removed and further refined for end use. The coke deposited in the coker drum is removed using cutters. A delayed coker unit is a batch process with two drums – one offline and one online. 

 

PROCESS FLOW

The crude oil is fractionated in the Atmospheric Distillation Unit (ADU) or the Crude Distillation Unit (CDU). The different fractions produced are further treated to produce the required product.

Gases

The top product of the ADU is gasses that are sent into a gas processing unit along with other gases. Here C3-C6 hydrocarbons are recovered (LPG constituents). The gas stream is then sent to the Merox treater to remove the mercaptans. The final product obtained is LPG and Butane. Mercaptans are organo-sulfur compounds with carbon, hydrogen, and sulfur.

The Fuel gas produced from the gas processing unit is sent to the Amine Treating unit where CO₂ and H₂S are removed. The remaining product is taken as refinery fuel and the H₂S is sent to the Claus sulfur plant where sulfur is recovered.

Light Naphtha

Naphtha denotes a group of volatile flammable liquid hydrocarbons. Light Naphtha contains low molecular weight components, unlike heavy naphtha which has high molecular weight components.

The light naphtha from the CDU is sent to the Hydrotreater where impurities like sulfur, nitrogen, and others are removed. The treated naphtha is then sent to the isomerization plant. Here the octane number of light naphtha is upgraded and the benzene content is simultaneously reduced through saturation. The final product isomerate, a major component of gasoline, is then sent to the gasoline blending pool. 

Heavy Naphtha

Heavy naphtha from the ADU is hydrotreated to remove impurities before sending it to the Catalytic Reformer. The reformer converts the inlet into a high-octane product, which is the major component of gasoline. The reformate is then sent to the Gasoline Blending Pool just like light naphtha.

Jet Fuel/Kerosene

This fraction taken from the CDU is sent to the Merox Treater to reduce the sulfur level, mainly to comply with environmental standards. The treated Jet fuel/Kerosene can be used as the final product.

Diesel Oil

The diesel oil from CDU is hydrotreated to remove impurities like sulfur and nitrogen before being taken out as the final product.

Atmospheric Bottoms

The bottoms from the ADU are sent to the Vacuum Distillation Unit or VDU. The VDU operates under zero pressure and fractionates the inlet into light vacuum gas oil, heavy vacuum gas oil, and Vacuum residuum.

The Light vacuum gas oil along with the atmospheric gas oil is sent to the Hydrotreater to remove impurities like sulfur and nitrogen. This treated inlet is then routed to the Fluid Catalytic Cracker (FCC). The FCC cracks the heavy inlet into lighter components in the presence of a catalyst. Different products produced are Pentenes, Naphtha, and FCC gas oil.

The naphtha fraction from FCC is hydrotreated and the gasoline thus obtained is sent to the Gasoline Blending Pool.

The heavy vacuum gas oil from the VDU is sent to the Hydrocracker. Hydrocracker, just like the FCC cracks the inlet into lighter components. The hydrocracked gasoline is then sent to the Gasoline Blending Pool and the diesel oil produced is taken as the final product. Butenes are obtained as a bottom product from the Hydrocracker.

The butenes from the hydrocracker and the pentenes from the FCC are sent to the alkylation unit. Here the inlet is combined to form heavy hydrocarbon. The Alkylate thus obtained is also sent to the Gasoline blending Pool.

A part of the Vacuum residuum is sent to the delayed coker unit. The residuum is the heaviest part of crude oil. The products from the coker are Coker Naphtha, coker gas oil, and petroleum coke. Naphtha is hydrotreated and reformed before being sent to the gasoline blending pool. Coker gas oil is used as fuel oil. The bottom product, petroleum coke is directly used as the final product.

The remaining vacuum residuum from the VDU is sent to the Asphalt Blowing unit. This is an oxidation process that blows air into the asphalt to remove the contaminants and increase its viscosity. The final product asphalt or bitumen is taken for end use.

The refineries usually will have some subunits that support the overall plant. The hydrogen production unit and sour water strippers are the most common ones.

Biomass Feed

With climate change taking the front seat, gasoline is tried to be derived from biomass feed. Here, an HDO or hydrodeoxygenation is used to convert the feed into renewable fuel. This fuel is blended with gasoline and used as the final product. HDO is a combination of different processes such as hydrogenation, hydrogenolysis, decarbonylation, and dehydration.