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.