Refinery Overview

         The purpose of a refinery is to separate and upgrade crude oil to useful products like gasoline or diesel fuel.  In the diagram below, each block represents a refining technology that separates or reshapes the various crude oil components to be blended (mixed) in order to meet various product specs like octane for gasoline or sulfur content for diesel. 

                To understand what’s going on per a refinery flow diagram remember three things: 1) The crude enters the refinery through the Crude (Atmospheric) Column, 2) Any unit entitled Treating/Treater means sulfur/nitrogen/metals removal, 3)Any unit with Cracking in the title means breaking heavy material into lighter, more valuable products.  Given these rules of thumb and the unit glossary below you can understand the layout of any refinery.  For example, follow the heavy naphtha cut from the crude column (referred to as straight run naphtha because it comes directly from the crude unit) to a hydrotreater where organic sulfur and nitrogen are removed from the naphtha.  After hydrotreating, the naphtha goes to a catalytic reforming unit where it is reformed into higher octane material.  After reforming, the naphtha is directed to product blending where it is blended into the gasoline product pool.

http://en.wikipedia.org/wiki/Oil_refinery

 

Atmospheric Distillation

As we covered in the “Crash Course to Distillation” entry, the Crude Unit, also referred to as Atmospheric Column, is the first entry point for crude oil into the refinery.  In the Crude Unit, salts and other debris from the oil well or gained during transport are removed in the Desalters.  After desalting, the crude is heated to 700-800F in the Crude Furnace before entering the base of the Crude Column.  In the crude column, the crude is separated by distillation into various products: Fuel Gas, LPG, Naphthas, Kerosene/Jet Fuel, Distillate (Diesel), and Atmospheric Gas Oils.

Vacuum Distillation

Crude oil has a boiling range from -257.8F (boiling point of Methane) up to 1400-1500F.  The Crude Unit Furnace Outlet temperature is a maximum of 800 F; this means any 800-1500F range material cannot be vaporized and distilled in the Crude Unit.  Raising Crude Furnace outlet temperature to vaporize the entire boiling range of crude is impossible because these heavy components coke or polymerize (basically burn) before reaching their boiling points at atmospheric pressure.  Therefore, Atmospheric Gas Oil is directed to the Vacuum Tower, which operates under vacuum conditions (negative pressure), in order to distill heavy components without coking.  Typical Vac Tower products are Light Vacuum Gas Oil (LVGO-diesel range material), Heavy Vacuum Gas Oil (HVGO), and Vacuum Residue.

Gas Processing

Light components typically C4- and some naphtha from the crude units (and typically other units downstream of the Crude Tower) are directed to the Gas Processing Plant to separate these components into various light end products: fuel gas, LPG, butanes, and gasoline (naphtha).

Amine Treating

Many of the hydroprocessing and catalytic reforming processes have the byproduct hydrogen sulfide, H2S. H2S is a poisonous gas that is a danger to operating personnel as well as suppresses some refining unit operations.  As such, this gas is removed from refinery gas streams in an Amine Treating Unit.  Basically this unit works by allowing sour (H2S contaminated) gases to flow counter currently to amine rich caustic (basic) streams in an “Amine Contactor”.  The sour gas is essentially stripped of H2S which is carried away with the countercurrent caustic stream.

Merox Treating

In Merox Units, mercaptans, organic sulfur compounds in LPG, naphtha, and kerosene streams are removed.  First the feed is contacted with a countercurrent stream of caustic which captures the mercaptans.  The sweetened product stream then flows through a caustic settler to remove any remaining caustic followed by a salt bed to remove any water.  The caustic itself is then regenerated by contacting it with a liquid catalyst and oxygen to convert the mercaptans to disulfides and then later allowing them to settle out in a separator.

Claus Sulfur Plant

Sulfur Plants remove sulfur from H2S contaminated refinery streams by first burning the H2S and then sending it to a condenser which results in some elemental sulfur precipitating out.  In addition to elemental sulfur, SO2 , a combustion byproduct,  and unconverted H2S remain in the stream.  To complete the sulfur removal process, H2S and SO2 are passed over catalyst where they react to form elemental sulfur and water.

Hydrotreater

The purpose of a hydrotreater is to remove sulfur, nitrogen, oxygen compounds, organic halides (R-Cl), and metals compounds in order to 1) Prevent the poisoning (deactivation) of catalyst in downstream units and 2) meet environmental regulations for SOX and NOX.  Hydrotreaters work by passing hydrocarbon streams, e.g. naphtha, over a bed of catalyst in the presence of hydrogen. 

Isomerization

In isomerization, C4-C6 material is passed over a bed of catalyst in the presence of hydrogen.  The purpose of isomerization is to convert straight chain paraffins to branched paraffins which have higher octane for blending. 

Catalytic Reformer

Catalytic Reforming units accomplish two purposes: 1) increase the octane of naphtha feeds and 2) produce hydrogen to be used in other hydrogen consuming units in the refinery e.g. hydrotreaters. In fixed bed reformers, naphtha passes through multiple beds of reforming catalyst.  Several reactions occur that result in higher octane product molecules: straight chain paraffins become branched or form rings, naphthenes dehydrogenate to aromatics (generating hydrogen), and so on.  Octane increasing reactions compete with cracking reactions which consume hydrogen and reduce reformate yield.

Hydrocracker

Hydrocrackers provide both contaminant removal and upgrading of lower value products.  In the refinery above, heavy vacuum gas oil is directed to the hydrocracker where it typically first encounters a bed of hydrotreating catalyst where sulfur, nitrogen, oxygen, organic halide compounds and well as metals are removed in the presence of hydrogen.  After the treating section, several beds of hydrocracking catalyst follow in order to crack the treated HVGO to lighter material.  Hydrocrackers operate at incredibly high temperatures and pressures and are major consumers of hydrogen in refineries.

Alkylation

Alkylation Units allow the refiner to upgrade light end material to gasoline range material.  In the presence of Hydrofluoric Acid or Sulfuric Acid, isobutane is reacted with C3-C4 olefins to form C7-C8 naphtha range products.   

FCC Feed Treater

FCC Catalyst is very susceptible to metals poisoning, especially vanadium.  To prevent deactivation of FCC catalyst as well as removing sulfur and nitrogen to meet environmental specs, FCC feed is treated in a FCC Feed Treater.  The Cat Feed Hydrotreater works much like other hydrotreaters, gas oils pass through a series of catalyst beds at elevated temperatures and in the presence of hydrogen.

Fluid Catalytic Cracker (FCC)

FCC Units are referred to as the heart of the refinery because they are able to convert heavy gas oil material to more valuable gasoline, kerosene, and distillate products.  It does this by cracking the gas oil in the presence of a catalyst that selects for increased gasoline and distillate yields. FCC’s are considered the more complicated units in the refinery because instead of a fixed bed, the unit’s catalyst is fluidized and circulated throughout the reactor and regenerator sections.    

Delayed Coker

Delayed Coking is one of the thermal cracking (heat only-no catalyst) processes in the refinery.  Vacuum Tower Resid is heated in a furnace at incredibly high temperatures, typically 1000F, before entering large coke drums where the resid is allowed to coke or polymerize (burn).  Any material not burned in the drums rise as vapors and flow to the base of the coker fractionator where they are distilled to gas oil and lighter products.  The key to coking is that resid material is prevented from coking in the furnace tubes by achieving high velocities through the furnace tubes.  Once the drums are filled with coke, they are taken off line and the coke is cut out of the drums and sold as product depending on the grade of coke.

Asphalt Blowing

The Asphalt used to pave roads comes from the heavy vacuum residual material from the bottom of the vacuum tower.  This reside serves as a binder and is mixed with gravel and used in road construction.  Roads and highways face the stress of daily car use as well as changing weather conditions.  As such, the material used to pave roads must meet certain viscosity and strength tests in order to be used in asphalt.  To meet highway specification, asphalt is blown by exposing it to oxygen and heat in order to meet required viscosity specs in an Asphalt Blowing Unit.

What is Crude Oil?

So what is crude oil? Crude oil is a viscous, dark mixture of a variety of hydrocarbons of different shapes and sizes.  What’s a hydrocarbon?  Ok organic chemistry scholars out there-take a quick break-get coffee, run some errands, take a nap.  For the rest of us, hydrocarbons are molecules composed of carbon and hydrogen atoms…with, in the case of crude oil, a few miscellaneous atoms thrown in like nitrogen, oxygen, and sulfur, for example.   Sophomore/junior chemical engineering students and curve demolishing premeds originally encountered these molecules defined by chemistry professors the world over as alkanes, alkenes, cycloalkanes, and aromatics…you’ll be happy to know that the term aromatic is still used.  Unfortunately, the refining industry dumped the rest.  Alkanes are referred to as paraffins, alkenes as olefins, and cycloalkanes as naphthenes.  Examples of these are shown below to jog your memory coupled with a very cheesy overview of organic chemistry.  Ugh, bear with me.

Dabbling in Orgo

Paraffins

http://alevelnotes.com/Alkanes/138

Paraffins are straight chain or branched (iso-paraffins) molecules examples of which are butanes and isobutanes.  Paraffins are considered fully saturated molecules because they have single bonds between carbon atoms and each carbon’s remaining outer octet shell is completed by bonding with hydrogen.  This is unimportant other than the fact that this molecular arrangement makes paraffins more stable, and less reactive than other unsaturated molecules.

Olefins

http://alevelnotes.com/Alkanes/138#/?id=139

Olefins are similar to paraffins except they have one or more unsaturated, double carbon bonds.  Per the discussion above, this means that olefins are typically more reactive than paraffins.    

Naphthenes

http://www.chemguide.co.uk/basicorg/conventions/draw.html

Naphthenes, or molecules formerly known as cycloalkanes, are fully saturated ring compounds.  They are also very stable not only because of they are saturated but because their ring structure allows them to balance charge around the molecule…too much?  Ok I’ll stop.

Aromatics

Toluene

http://www.pactox.com/library/article.php?articleID=21

Aromatic compounds are unsaturated ring compounds are fairly stable because of their ability to balance charge around the ring.  However, they are still more reactive than their sister naphthenes because they are unsaturated.

Enough of that!

Crude Oil Quality

So crude oil is composed of paraffin, olefin, naphthene, and aromatic hydrocarbon compounds of various sizes and shapes all lumped together.  Hmmm…lumped together, different sizes and shapes?  Does this mean there different types of crudes?  Yes! How does that affect the type of products that can come from crudes? So glad you asked! 

                Because of the variability in the composition of crudes, chemical engineers have come up with different ways of defining the quality or the degree of difficulty refineries have to create various products from gasoline and diesel fuels to petrochemical derivatives used in plastics and detergents.  Typically, crude oil quality can be superficially determined by examining API Gravity and sulfur content.  API Gravity describes the density of the crude.  Sulfur is important because it is related to both the quality of products that can be derived from the crude as well as the cost to upgrade that crude to meet environmental regulations.

                API Gravity varies inversely with the density or specific gravity of the crude.  So heavy Oil Sands Bitumen, rock like crude, may have an API Gravity of 10 where a sweet Nigerian Crude or West Texas Intermediate may be in the mid-40’s.  API Gravity is related to specific gravity by the formula below:

where SG is specific gravity or the density of a substance divided by 62.4 lbs/ft^3 or 1000 kg/m^3 (the density of water in English and Metric Units).

                A crude’s sulfur content determines whether it is described as sweet or sour. Sour is any crude with greater than .5 weight percent sulfur content.  High sulfur content is typical of heavier crudes.  Not only will these crudes have to undergo expensive sulfur removal processing but, because sulfur atoms are typically part of heavier hydrocarbon compounds, this means that the crude most likely has lower quality, heavier components that will need to be cracked into the smaller, higher quality lighter molecules that make up gasoline and diesel.  Remember, the lower the quality of a crude, the greater the degree of processing required, the higher the cost to refine a crude, therefore, the lower the price of the crude relative to others.

                In addition to the API Gravity and Sulfur Content of the crude, refiners examine the crude’s distillation curve to understand its value.  Another brief chemistry rule of thumb: the weight of a molecule is directly proportional to its boiling point.  Lighter, smaller molecules vaporize (go from the liquid to gas phase or boil) more easily than heavier molecules.  It’s like me when I gain a little weight; it’s a bit harder for me to get off the couch or up the stairs.

Crude Distillation Curve

http://pavementinteractive.org/index.php?title=Image:Distillation-curve.jpg

     Above is an example of a crude distillation curve.  It relates boiling point on the y-axis to percent volume of crude on the x-axis.  Butane and lighter products make up maybe 5% of this crude and boil below 60 degrees Fahrenheit.  Gasoline and Naphtha combined make up another 15% and Kerosene approximately 10%, with boiling ranges between 100-350F and 350-450F, respectively.  As you can see, lower value gas oils and residuum make up the bulk of this crude.  To be economical, the refinery upgrades this material to the lower boiling range, lighter products gasoline and kerosene (jet fuel).  Below is another view of the refinery product slate as it might be derived from a crude distillation column, the first unit in a refinery.

Crude Oil Assays

Crude Oil Assays are detailed laboratory analyses of crudes e.g. API Gravity, Sulfur, Nitrogen, and Metals Content, Distillation Curve, Viscosity, etc. Assays are typically described by crude boiling range.  See Chevron’s Bonny Light Assay properties below (see full assay).

Nigerian Bonny Light Assay Properties

Assays are used by the refinery’s planning and scheduling department to determine the most profitable product mix the refinery can produce for a given crude slate.  Refinery Planners input assays into LP (Linear Programming) Models which simulate how all of the refinery's or a group of refineries process units work together.  The LP Model is used to find the optimum operating mode of the refinery per the required feed and product qualities and prices and various operating constraints.  No modern refinery can be efficiently run without the use of an LP Model.