CB&I's Virtual Refinery

Crude Storage

The crude storage area is designed to hold the many barrels of crude oil that are supplied to a refinery from pipeline, ship or barge.  These tanks hold the crude oil until it is sent to the process units for upgrading into refined products that meet customer and government specifications.  The crude storage area is necessary for any refinery to maintain a constant supply of crude oil to the process units and maximize product production.

There are many different types of tanks available to store crude oil and refined products.  Some products require storage at low temperatures, so they are stored in refrigerated tanks.  Other highly volatile products, which could easily evaporate, are stored in spherical pressure vessels.  To minimize environmental risks and maximize safety, the design of the tank must take into account the properties of the stored liquid.

Crude oil is generally stored in flat bottom tanks in a refinery. Most flat bottom tanks are constructed out of steel plates that are formed into shape and welded together. One type of flat bottom tank has a fixed roof which is a stationary roof welded into place on top of the tank.

Besides fixed roofs, some flat bottom tanks have a floating roof, which means the roof of the tank moves up and down as the amount of liquid in the tank changes.  Since there is no vapor space between the crude oil and the roof (except at very low liquid levels), evaporation effects are greatly reduced as well as the potential fire hazards.
 

Crude Distillation Unit

The atmospheric crude distillation unit is the first major processing unit in an oil refinery.  Typically, all the crude oil entering a refinery passes through this unit, where it is distilled into different components, called fractions or cuts, based on their boiling points.  These cuts are then routed to other parts of the refinery.

Before distillation, the crude oil first enters a desalter, where water, inorganic salts, trace metals and other impurities are removed.  The desalted crude feedstock then is heated to temperatures ranging from 650 to 750 degrees Fahrenheit and fed into a distillation, or fractionation, column, also called a tower.  All but the heaviest fractions flash into vapor, which rises in the tower, cooling as it goes up.  Heavy fuel oil and asphalt residuals are drawn from the bottom of the tower, while other major products are drawn off the tower at successively higher points, including lubricating oil, heating oil, kerosene, gasoline and uncondensed gases.  These cuts are subsequently routed to other parts of the refinery for additional processing or blending.

A vacuum distillation unit is often used to further refine heavy residuals after atmospheric distillation.  The vacuum distillation unit relies on the same principle as the atmospheric unit but employs a vacuum so the heavy components will boil at lower temperatures.  Both atmospheric and vacuum distillation are conducted at temperatures to avoid overly damaging the crude oil by cracking or coking.  The heaviest cuts often are sent to a delayed coking unit for further processing.
 

Delayed Coker

A delayed coking unit uses a process called thermal cracking to convert heavy residuals or bottoms into lighter, higher-valued products such as naphtha and diesel fuel, leaving behind petroleum coke as a residual product.  Cracking works by breaking complex hydrocarbon compounds into smaller molecules.

Heavy residual oils from the atmospheric and vacuum distillation columns are heated in a furnace to approximately 835 degrees Fahrenheit and then transferred to a large cylindrical vessel called a coke drum. Gas oil and lighter components separate from the liquid in a vapor phase, which is directed to a fractionation column where the fractions are drawn off.  The liquid products are then routed to the hydrotreater or hydrocracker for further processing.

The uncracked residual liquids that remain in the drum eventually form petroleum coke, a solid carbon material.  After water quenching, the top and bottom heads of the full coke drum are removed, and the coke is removed from the drum using mechanical or hydraulic methods.  Typically, coke drums operate in pairs so that one is filling while the other is being opened and decoked.  Petroleum coke can be used as a fuel and for the manufacture of electrodes, graphites and carbides.

Hydrocracking

Hydrocracking is a process that combines catalytic cracking in the presence of hydrogen.  It uses high pressure, high temperature, a catalyst and hydrogen to crack heavier feedstocks into lighter, more valuable products, including diesel and jet fuels, as well as naphtha for gasoline blending.  Products resulting from hydrocracking are nearly free of contaminants, as the process removes sulfur, nitrogen, oxygen and metals.

Hydrocracking is often a two-stage process.  Feedstock is mixed with hydrogen, heated and sent to a reactor vessel, where fixed-bed catalysts convert sulfur and nitrogen compounds and limited cracking occurs.  The hydrocarbon product is then cooled and partially condensed and sent to a separator, where the hydrogen is separated and recycled to the feedstock and the liquid is charged to a fractionation column. High-value fractions are drawn off and the bottoms are returned to a second reactor for further cracking under higher temperatures and pressures.  Like the first stage, the second-stage product is separated from the hydrogen and charged to the fractionator.

In addition to the liquid product, hydrocracking yields light gases that can be used as fuel for the refinery or as petrochemical feedstocks.

With heavier feedstocks, hydrocracking can improve the properties such that they become base lubricants.  Other hydrocrackers deal with very heavy components, like bitumen, which can contaminate the catalyst, requiring regular regeneration of the valuable catalyst.
 

Fluid Catalytic Cracker

A fluid catalytic cracking, or FCC, unit upgrades heavy distillates from the crude distillation unit into lighter, higher-valued products such as high octane gasoline, light fuel oils and liquefied petroleum gas.  It is one of the most widely used processes for increasing the ratio of light to heavy products from a refinery.

A catalyst is a material that assists a chemical reaction but is not itself chemically changed.  Catalysts used in refinery cracking units are typically solid materials, such as silica, alumina, clay and zeolites, that come in the form of powder, beads or pellets.

Fluid catalytic cracking uses a catalyst in the form of a very fine powder which flows like a liquid.  Heavy feedstock from the crude distillation unit or the delayed coker is preheated and sprayed into the base of a vertical sloped pipe called a riser where is contacts extremely hot fluidized catalyst at 1,230 to 1,400 degrees Fahrenheit.  The hot catalyst vaporizes the feed and facilitates the cracking reactions that break down the heavy hydrocarbons into lighter components.  The catalyst/hydrocarbon mixture flows through the riser and then is separated by cyclones in a reactor separation vessel.  The hydrocarbon stream is then routed to a fractionating column for separation into lighter products such as LPG, gasoline, light gas oil and heavy gas oil.

The used catalyst is sent to a stripper where it is contacted by steam to remove any remaining hydrocarbons and then to a regenerator, where the combustion residue is burned off and catalyst activity is restored.  The regenerated catalyst then flows to the base of the riser, and the cycle is repeated.

Since the catalyst is always flowing and is subjected to extreme temperatures, it may be damaged.  Much of the process attempts to trap any catalyst from escaping the reactor/regenerator and recovering the valuable material.
 

Naptha Hydrotreater

Hydrotreating, also known as hydrodesulfurization, is a process that removes contaminants such as sulfur, nitrogen, oxygen and metals from liquid petroleum fractions.  As the fractions move through a refinery, these impurities can damage equipment, catalysts and the quality of the finished products.  In addition, to improve air quality, many countries have imposed limits on the amount of sulfur in transportation fuels, and hydrotreating enables refiners to make products meeting these requirements.  Hydrotreating also converts some hydrocarbons to saturated compounds, which can change certain properties.

Hydrotreating takes place under high pressure and temperature conditions with catalyst and hydrogen present.  Pressurized feedstock is combined with hydrogen-rich gas, heated to the point of vaporization, and then passed through a fixed-bed of catalyst where several reactions occur:  hydrogen combines with sulfur to form hydrogen sulfide, nitrogen compounds are converted to ammonia, any metals in the feedstock may be deposited on the catalyst, and saturated hydrocarbons are created.  After cooling, the liquid/gas mixture is separated, and the hydrogen sulfide gas is routed to the sulfur recovery plant for further processing.  The desulfurized liquid products are blended or used as feedstock for downstream processes like the catalytic reformer and FCC unit.

In addition to removing sulfur from gasoline and diesel fuel, hydrotreating can be used to improve the burning characteristics of middle distillates such as kerosene.

Catalytic Reformer

Catalytic reforming is a process that converts low-octane naphthas into high-octane gasoline blending components called reformates.  Reforming also produces high-purity hydrogen that can be used for hydrotreating and other refining processes. 

The reforming process literally reshapes, or reforms, the molecules in the feedstock in the presence of hydrogen and a catalyst that contains platinum and often another noble metal such as rhenium.  The reaction requires a continuous supply of process heat to maintain reaction temperature in the catalyst beds, so the process is usually done with three or more reactors in a series with furnaces in between.

The naphtha feedstock, sourced from the crude distillation unit and the hydrotreater, is mixed with hydrogen, vaporized and passed through an alternating series of furnaces and reactors.  The liquid-gas mixture from the final reactor is cooled and sent to a separator to remove the hydrogen gas.  The liquid product from the bottom of the separator is sent to a fractionating column where reformate is drawn from the bottom and light ends from the top are sent to the refinery's saturate gas plant.

Since the catalyst is very expensive, the process conditions are carefully controlled and catalyst is often regenerated before it suffers much damage.
 

Diesel Hydrotreater

Hydrotreating, also known as hydrodesulfurization, is a process that removes contaminants such as sulfur, nitrogen, oxygen and metals from liquid petroleum fractions.  As the fractions move through a refinery, these impurities can damage equipment, catalysts and the quality of the finished products.  In addition, to improve air quality, many countries have imposed limits on the amount of sulfur in transportation fuels, and hydrotreating enables refiners to make products meeting these requirements.  Hydrotreating also converts some hydrocarbons to saturated compounds, which can change certain properties.

Hydrotreating takes place under high pressure and temperature conditions with catalyst and hydrogen present.  Pressurized feedstock is combined with hydrogen-rich gas, heated to the point of partial vaporization, and then passed through a fixed-bed of catalyst where several reactions occur:  hydrogen combines with sulfur to form hydrogen sulfide, nitrogen compounds are converted to ammonia, any metals in the feedstock may be deposited on the catalyst, and saturated hydrocarbons are created.  After cooling, the liquid/gas mixture is separated, and the hydrogen sulfide gas is routed to the sulfur recovery plant for further processing.  The desulfurized liquid products are blended or used as feedstock for downstream processes like the catalytic reformer and FCC unit.

In addition to removing sulfur from gasoline and diesel fuel, hydrotreating can be used to improve the burning characteristics of middle distillates such as kerosene.

Hydrogen Plant

In many large refineries, high-purity hydrogen is required for the hydrocracking and  hydrotreating operations.  Hydrogen is produced as a by-product of several refinery processes, especially catalytic reforming, but this is often not enough to meet the total refinery demand.  As a result, hydrogen must either be manufactured on site or acquired from external sources.

For those refineries that manufacture hydrogen on site, the most common process used to produce hydrogen is steam methane reforming.  In this process, a gaseous hydrocarbon feedstock -- often natural gas or methane -- is pretreated for sulfur removal, mixed with steam and introduced to a reforming furnace, where it passes through tubes containing a nickel base catalyst.  The reformed gas, which now consists of steam, hydrogen, carbon monoxide and carbon dioxide, is cooled and then passed through a shift converter containing an iron catalyst.  Here, the carbon monoxide generated in the reformer is converted with the addition of steam to carbon dioxide and more hydrogen.  The effluent from the shift converter goes next to a pressure swing adsorption unit, where carbon oxides and water are removed and high-purity hydrogen is the final product.

Kerosene Hydrotreater

Hydrotreating, also known as hydrodesulfurization, is a process that removes contaminants such as sulfur, nitrogen, oxygen and metals from liquid petroleum fractions.  As the fractions move through a refinery, these impurities can damage equipment, catalysts and the quality of the finished products.  In addition, to improve air quality, many countries have imposed limits on the amount of sulfur in transportation fuels, and hydrotreating enables refiners to make products meeting these requirements.  Hydrotreating also converts some hydrocarbons to saturated compounds, which can change certain properties.

Hydrotreating takes place under high pressure and temperature conditions with catalyst and hydrogen present.  Pressurized feedstock is combined with hydrogen-rich gas, heated to the point of vaporization, and then passed through a fixed-bed of catalyst where several reactions occur:  hydrogen combines with sulfur to form hydrogen sulfide, nitrogen compounds are converted to ammonia, any metals in the feedstock may be deposited on the catalyst, and saturated hydrocarbons are created.  After cooling, the liquid/gas mixture is separated, and the hydrogen sulfide gas is routed to the sulfur recovery plant for further processing.  The desulfurized liquid products are blended or used as feedstock for downstream processes like the catalytic reformer and FCC unit.

In addition to removing sulfur from gasoline and diesel fuel, hydrotreating can be used to improve the burning characteristics of middle distillates such as kerosene.
 

Sulfur Recovery Unit

Crude oil can contain anywhere from 1% to 5% sulfur by weight, typically with sulfur imbedded in large complex molecules.  This sulfur can be released during distillation, cracking, coking and hydrotreating processes.  In addition, all of the combustion units in a refinery, such as boilers and furnaces, will produce sulfur dioxide if there is sulfur in the fuel.  Also, many of the water streams throughout the refinery contain sulfur compounds that must be removed prior to discharge.  The sulfur recovery facilities in a refinery are used to remove sulfur compounds from these liquid and gas streams.

Most sulfur recover facilities include units for gas and liquids treating, sour water treating, sulfur recovery, tail gas treating and incineration.  Removal of hydrogen sulfide from hydrocarbon streams is typically achieved by absorption using a solvent, or amine.  Hydrogen sulfide and other acid gases from the amine treating unit are sent to the sulfur recovery unit.

The sour water treating unit, which usually includes a sour water stripper, removes hydrogen sulfide, ammonia and other contaminants from various sour water streams using steam.  The sour gas is sent to the sulfur recovery unit and the stripped sour water is sent to the water treatment plant.

The sulfur recovery unit converts hydrogen sulfide to elemental sulfur using both thermal and catalytic conversion reactions in what is known as the Claus process.  Ammonia in the sour gas is destroyed as well.  Effluent gas from the sulfur recovery unit is sent to the tail gas treating unit, where nearly all of the remaining sulfur is recovered.  Any residual sulfur-containing gases are sent to a thermal incineration unit, where all sulfur species are converted to allowable limits of sulfur dioxide before release to the atmosphere.

Since sulfur emission standards are very strict, the sulfur recovery processes must be very reliable and are sometimes configured with redundant units to make sure plant operations are not disrupted.
 

Product Blending & Storage

The product blending and storage area in a refinery is where the product streams from various process units, and appropriate additives, are mixed together to provide fuels that meet customer and government specifications.  This area includes short-term storage capacity and facilities for bulk loading of products to trucks, barges, ship or railcars for transportation.

With more and more specialized fuel blends that are required to meet environmental mandates or to accommodate seasonal temperature variations, the blending and storage area has become an increasingly important part of the refinery.  Many refineries use sophisticated monitoring and control systems as part of their blending operation.

In addition to storing finished products after blending, refineries use flat-bottom tanks to store the raw crude oil coming in to the refinery for processing.  Also, refineries generally have facilities for storing intermediate stocks or unfinished material.  Intermediate storage allows the refinery to run more smoothly and provides emergency storage for upsets.

Most storage tanks are made out of steel plates that are formed into shape and welded together.  Some of the tanks have what is called a floating roof, which means the roof of the tank moves up and down as the amount of liquid stored in the tank changes.  This helps reduce emissions and fire hazards.

Other flat-bottom tanks have a fixed roof, which is welded into place on top of the tank and does not move.  Some tanks also are refrigerated in order to store liquefied gases such as butane, propane, ethylene and ammonia at low temperatures.  Refineries also use spherical pressure vessels to store liquefied petroleum gas and other liquids under pressure.