You drive up to the station, get gas (gasoline), pay for it (hopefully), and you're on your way. That's generally all the thinking we do about our gasoline supply, except when we're faced with price increases, which then sparks complaining. If you've pumped petrol (gasoline) in Europe, however, your fuel costs will be much greater. We wait until Lesson 8 to think about war/security issues that can also cause drastic price increases. Please watch the following introductory (0:33) video.
Transcript [Dr. Mathews parked his car at a gas station. He gets out of his car and walks over to the pump. As he is talking he puts the pump into the car's fuel hole, fills the tank, and replaces the pump to its holster.] Dr. Mathews: Today we are going to be talking about how we transform crude oil into the useful products we use. Mainly gasoline, as well as jet fuel, petroleum coke. Even medicines, plastics, and cosmetics. We use an awful lot of things in transforming this. There is a lot of requirements too: it has to be environmentally benign, it needs to be environmentally friendly, and of course, it has to make a profit for the industry, too. [Video ends]
This lesson covers what happens to crude oil between the time it gets pumped out of the ground and the time you use it as gasoline. We will look at the transformation of crude oil to gasoline, and the many other products that you use every day, probably without any knowledge that they came from the same source as gasoline.
Question: Do you agree that we should export crude oil to other nations?
Yes
No
ANSWER:
A few years ago this was a question about importing crude oil. We still import but are now a net exporter (how things change). The prosperity of the nation currently requires crude oil as it still powers transportation. As you will see we now are producing more domestic oil and natural gas production. We also produce alternative fuels as well. Tie that in with efficiency improvements in automobiles and we have more oil than we need. We do still import crude oil from Canada, select areas of the Middle East and other nations.
Crude oil is a highly variant natural resource. The quality ranges are similar to coal and depending on the maturation of the crude the quality can be high or low (younger crude's are of lower quality). One of the first indications of quality is color. The variations in oil color can be dramatic, and very indicative of the quality of that crude. Not all crude oil is black - higher quality oils can be golden or amber in color.
All the quality measures here are based on the ability to produce the desired products. In the U.S., about 50% of the oil is converted into gasoline. So an oil that produces a higher % of gasoline "cuts" is more desirable and have a higher quality oil. Take note, we have used much of the higher quality crude oil already! Now we need to use the lower quality oils too and the general trend is to use increasingly lower quality crudes. This quality reduction has an impact on how we refine the crude into the desired products.
Viscosity is the resistance to flow. Do not use the term "Thickness" which is a length measurement. The higher the viscosity the slower the liquid will flow and the lower the quality. We have many techniques for measuring viscosity, some of which are quite high-tech. Here is one of the simplest, utilizing one of the testing devices in one of our petroleum labs over in Hosler. Please watch the following (2:41) video.
[Video opens with Dr. Mathews standing with a viscometer.] Dr. Mathews: This is a Saybolt Viscometer. There are an awful large number of ways to measure viscosity. This is perhaps one of the simplest. I am going to pour this into a heated reservoir. [Dr. Mathews pours a beaker of Pennsylvania Crude Oil into the machine.] Dr. Mathews: And do the same with the other crude oil. [Dr. Mathews pours a beaker of Gulf of Mexico crude oil into the machine.] Dr. Mathews: The reason the reservoir is heated is because the temperature is a factor that influences viscosity. And now I am going to do a very simple experiment. What I am going to do is yank these chains and we are going to see how long of a time difference it is between the Pennsylvania crude on the left and the Gulf of Mexico crude on the right, to see how much more viscous the Gulf of Mexico crude is. So here we go. [Dr. Mathews pulls two chains which allow the oil to flow through the machine and starts a timer.] Dr. Mathews: As you can see the Pennsylvania crude oil, the higher quality crude oil, the old deep crude oil, is flowing out very rapidly. There is the same quantity in each reservoir and they are at the same temperature. Whereas the Gulf of Mexico, the blacker of the crude oils, is taking much longer to come out. Again, a very easy determination of the quality of the crude oil is the viscosity. [The Pennsylvania crude oil has finished flowing out of the machine.] Forty-two seconds for the Pennsylvania crude, and we are going to be here for a while for the Gulf of Mexico. [The Jeopardy theme plays as we wait for the Gulf of Mexico crude to finish flowing out. Dr. Mathews shows the stopwatch every once in a while. It ends at three minutes and seventeen seconds.] [Video ends.]
The viscosity process is a measure of quality because the chemical structure of the crude influences its flowability. Longer chain molecules, for example, are harder to flow than short chains because of non-bonding interactions. If you have had any chemistry you will recall ionic (type of bonding in salt crystals) and covalent bonding (the type of bonding between 2 carbon atoms). Those are bonding interactions. There are several non-bonding interactions that occur which attract (and repel) molecules. It is the relative strength of these non-bonding interactions that influence the resistance to flow.
For coal, we used the correct terminology, which was ultimate analysis. For crude, that terminology we use is Elemental Analysis. Crude oil is complex, it contains C, H, N, S, O, and metals too. But the bulk of the composition is C and H, the rest being the N, S, O, and metals. S is a good indication of the quality of the crude because as the oil is heated underground the weak S-C bond can break, producing H2S (hydrogen sulfide gas). So, older crudes - higher quality - will have lower S content. Higher S crudes also cost more to process as S is a catalyst poison it has to be removed or the extensive catalysts used in the petrochemical industry would be damaged, as would your catalytic converter. The atomic H/C ratio is also an indicator of quality (why?)
Element | Percentage |
---|---|
Carbon | 84 - 87% |
Hydrogen | 11 - 14% |
Sulphur | 0 - 6% |
Nitrogen | 0 - 1% |
Oxygen | 0 - 2% |
Hydrocarbons are molecules that contain only the elements of carbon and hydrogen. These are the bulk of the crude oil. We find 4 types of chemical structures of hydrocarbon in crude oil:
We have seen normal (for example n-heptane [1]) and branched (2,2,4 iso-octane) examples of the paraffins. They all have the same formula: CnH2n+2 (n is the number of carbon atoms). For example, in the cetane molecule above, to determine the molecular weight (Mw) you can count the carbons (x 12 the amu of a carbon atom) and count the hydrogen atoms (x 1 amu) and add the numbers together to obtain the molecular weight. Or you can use the formula:
Cetane has 16 carbon atoms (but if we used decane you would know how many carbons it contained, right?) so C16H(2 x 16)+2 OR C16H34 and the Mw is = (12 x 16) + (1 x 34) = 226 amu (atomic mass units).
The paraffins are the desired contents of the crude oil. Long chains (> 60 carbon atoms are wax) used to be used extensively for the production of candles. Now we use the shorter chains produce gasoline, diesel and jet fuel (and many other products). Note that each molecule might have many structural isomers, for example, a molecule containing 10 carbon atoms has 75 structural isomers. If an isomer is an unfamiliar term to you, I'd suggest looking it up online.
Aromatics are found in both crude oil and coal. In crude oil they are now undesirable because of soot production during combustion.
I took the soot picture above with a scanning electron microscope so we can see the very small (>1 micron) spherical soot particles. These spheres join together to form chains of spheres. To give you some idea of the scale: 80 microns is about the width of human hair. Take note that the aromatics have a much lower H/C ratio than the paraffins. The benzene ring contains double bonds (not shown). Aromatics can exist in complex structures containing many rings. The non-bonding interaction between these rings is strong and so pure compounds of 3 rings are solid at room temperature. The equivalent normal paraffin is a viscous liquid under the same conditions.
These are cyclo-paraffins and the example of cyclohexane above looks like a benzene molecule. There are no double bonds within the ring and so every carbon (in this example) has 2 hydrogen atoms bonded to it. Cyclohexane has an interesting boat or chair configuration. Can you see the differences?
In a similar manner to coal, as the source rock is buried deeper, the temperature increases with increasing depth. Thus, looking at quality indicators allows for a classification system similar to that of coal rank.
Because "old deep" oil provides the highest quantity of gasoline, it is the higher quality crude oil.
Most graphs you are used to seeing or plotting have just 2 axes. This works fine if you're just comparing 2 components, but as you see below, we're comparing 3 general classifications for crude oil compound types. It is the ratio of these compound types (aromatics, paraffins, and naphthenes) that impacts the quality of the crude (in addition to S content, especially when the S is within the aromatic portion, which makes it much harder to remove during refining). So, to plot 3 items on a single graph we use ternary diagrams like the one you see above. At the three apexes, the composition would either be pure (100%) aromatics, pure naphthenes, or pure paraffins (clockwise from top). Along any of the borderlines of this triangle, you're looking at a mixture of just 2 of these components (aromatics – naphthenes or naphthenenes, paraffins or paraffins - aromatics). At any point within the triangle, the crude contains all three components, in varying degrees.
Take the example of 50% aromatics to begin with. To plot this point on the graph, you'd create a drop a horizontal line about halfway between the apex (100 %) and the base of the triangle opposite of that apex (0%), representing 50%. You repeat this process to locate the other %'s of the compound types on the graph, and the point you're after is the convergence of those three lines. Thus, the center of the triangle is: 33%, 33%, and 33% of aromatics, naphthalenes, and paraffins– crude oil that would generally fall into the "old shallow" classification. Here is a dynamic example [2].
We move crude oil and the finished products (gasoline for example) via a variety of methods: pipeline, tanker, and the multi-wheeled big (trucking) rigs. We are concerned that the transportation be performed safely, and without spillage.
By far the best method of transporting a fluid is by a pipeline. Some of these pipelines are very long, such as the 800-mile Trans-Alaskan pipeline, which carries 17% of the domestic production of crude oil. These pipelines are expensive, however, a cost of $8 billion at 1977 rates! The pipeline is cleaned periodically with "pigs" (which are mechanical devices that can travel inside the pipe to remove any wax buildup from the inside of the wall - other pigs check for corrosion etc.) Perhaps we will build a new pipeline (go and take a quick look at this project: Keystone [3]) to bring an improved (upgraded) tar sand obtained crude oil from Canada all the way to Texas.
Keeping these pipelines functioning properly [4] is no small feat.
Unfortunately, the pipeline ends in Valdez (because it is a relatively deep port, good for tankers, and is free of ice most of the year). Thus, to get the Alaskan crude oil from the state of Alaska to the markets in the rest of the United States requires tankers to carry the fuel the ocean leg of the journey. Generally, this is to the refinery operations on the West Coast (we in the North East get our crude oil from exotic locations such a Nigeria, Saudi Arabia, Venezuela, etc but also from even more exotic locations such as Warren, or Oil City, etc. in Pennsylvania! In 1989 the Exxon Valdez ran aground leaking 11 million gallons of crude oil. This was the worst spill in US history; it resulted in legislation that addressed the transportation of crude oil into US territorial waters (more on the Oil Pollution Act of 1990, later in the lecture).
The US produces a great quantity of crude oil (but it provides only about 55 % of our needs). Our production is only exceeded by that of Russia and Saudi Arabia (normally we are 2nd). Much of it arrives in the country via tanker. Those tankers operating in US territorial waters now need to be double hulled (by 2015) as a strategy to reduce large oil spills. Remember these tankers can be huge.
There are only a few travel "lanes" for the international trade of crude oil. Much of the transportation is via tanker or via pipelines (The World is not Enough-James Bond Movie, splendid). This has major security implications for the safe delivery of a very valuable commodity. The map below shows the important oil flow bottlenecks.
It is not just transportation of crude oil, or its products, but storage also. We produce (extract) a lot of oil, and we also store a lot of oil and crude oil products. Safety is a concern around all the flammable liquids. Spills inland can be just a devastating as those affecting the coastline. Regulations also require that the retaining walls, which surround the tank, are sufficient to retain the liquid in the event of a failure.
What you don't see in the image is that the tanks are in a very large depression in the ground (a bit like an empty swimming pool). Should the tanks break, the oil would be retained in the "swimming pool" by the retaining walls.
Oil products have also been leaking into the ground from the storage of gasoline at gasoline stations. Recall the MTBE issues. When you buy a house one of the things the homeowners have to reveal is if there is a storage tank on the property. It is not good news if there is one, as often they need to be removed. You also accept liability if it does leak at a later date.
Road crashes also leak crude oil products like gasoline. The fire trucks carry long buoyant absorbent socks (similar to booms) to prevent gasoline and diesel spills from further contaminating the waterways.
The influence of any spill on the surrounding wildlife depends on the nature and the size of the spill, as well as the ability of the wildlife to avoid the area. Gasoline, for example, will eventually evaporate; diesel and most of the other fuel oils, however, will not evaporate completely.
Oil will seep to the surface and form tar pits, or on the ocean form a small oil slick. These events are natural and occur every day. However, we move large quantities of crude oil and nearly as much in various products (gasoline, jet fuel, etc.) Some spillage due to transportation is going to occur. When it does there are different approaches to cleaning the spill. It comes down to three general approaches: Contain and remove, disperse with chemicals, or do nothing (it will eventually disperse). There are also "spills" as a result of war (First Gulf War), and from drilling, notable the Gulf of Mexico drilling disaster associated with BP's Deepwater Horizon operation [5]. Here is a good article on the relief well that finally stopped the spill. [6] After reading this page you should know how spills are treated and prevented.
The role of the refinery is very simple. Make a profit for the shareholders and produce an environmentally responsible product.
The method of making a profit is to carefully follow the supply and demand curve for their products.
Products from a refinery are the obvious: gasoline (46%), diesel, jet fuel, & fuel oil and the less obvious (to some of us): asphalt, coke (for the aluminum, iron, and steel industries), chemicals, plastics, & lubricants (including motor oil).
The demand for these products will be dependent on the weather (fuel oil), economy, driving habits (Americans are driving further, and more in the summer), military conflict (jet fuel, etc during the Gulf war and other conflicts), and other suppliers. The quality that a barrel of crude oil produces will also be dependent on the quality of the crude oil, which can be highly variable.
The first thing to do is clean up the crude oil and take out the water from the oil. An interesting feature of this water contamination is that it contains salt. This is a very corrosive liquid (salt water) and needs to be removed prior to any other processing steps.
Distillation is the heart of the refinery operation. It is the location where the crude oil is separated into many "cuts". Often the distillation tower is very noticeable, as it tends to be one of the taller structures at the refinery. The crude oil is separated into certain "cuts" depending on the volatility of the compounds. This occurs as a continual process: crude oil arrives, is stored and sent for separation via the distillation tower. The cuts are blended, or altered to increase the quality or the quantity of the more desirable products.
Crude oil is very complex. Some crude oils will contain over 1,000,000 separate compounds. Different isomers, length molecules, sized molecules will be present. It is very difficult and expensive to separate the compounds into pure cuts, so we don't even try. We are content to separate the molecules into an initial series of cuts.
Do you know what factors influence the desirability of different products [8]?
Below is a very simplified view of the distillation process. If you find that this topic keenly interests you, then you should consider the 3 credit, 400-level class our department offers just on this subject alone. The processes and products are explained in more detail below the image! Place your mouse over the green text on the image for more information.
The crude oil is heated before entering the distillation tower. In the tower, the more volatile compounds will turn into gases and flow up the tower, and those compounds that have higher volatilization temperatures will remain behind and get hotter. Thus, the top of the tower will have the lower temperatures and the compounds that have the lower boiling points (temperatures). The bottom of the tower will have the less volatile compounds and have the hotter temperature. To ensure good separation there are lots of stages (also called trays) that the volatile compounds may pass on the way up the tower. When the volatiles are cool enough they will turn back into the liquid form. It is the liquid in the trays that will make up the initial cut. A better quality crude oil will yield more of the lighter cuts than the denser cuts. Unfortunately, even the good quality crude oils will not give a 45% gasoline cut from the crude oil which is what is desired (average for the year), thus other processing steps are required to increase the yield. As always, "How stuff works [9]" provides more good information on this topic.
Some refineries will also operate a vacuum distillation unit to increase the more useful products from the remnants of the atmospheric distillation tower. By lowering the pressure it becomes easier for certain compounds to enter the vapor stage at lower temperatures.
Often the gasoline fraction produced by the initial cut in the distillation tower will not be of sufficient quantity or quality for the market and so chemical processing is required to increase the product yield and to ensure appropriate quality and compliance with environmental regulations (which in turn is dependent on market and country location; California, for example, has more stringent requirements than central Pennsylvania).
In the past, the longer chain molecules were highly prized for the production of waxes, and while they're still prized for specialty lubricants, the market is not as large as the gasoline market, so some of the long chain molecules will be "cracked" to produce the smaller molecules that are of appropriate length for gasoline production.
This cracking can be achieved through high temperatures and high pressures or through combined catalysts and temperatures with high pressures. The "ends" of the molecule require capping hydrogen atoms so to achieve this one fragment forms a carbon to carbon double bond that we chemists call an alkene (we call paraffin alkanes).
Gasoline quality is often indicated by the octane number. 2,2,4-trimethylpentane is assigned an octane number of 100 (it contains 8 carbons hence the name "iso" octane, compounds can have octane numbers higher than 100), heptane an octane number of 0. The octane number of gasoline indicates the fuel has the same combustion performance in an engine as a certain blend of 2,2,4-trimethylpentane and heptane (i.e., an octane value of 80 has fuel characteristics similar to a blend of 80% 2,2,4-trimethylpentane and 20% heptane). The higher the octane number the less likely the fuel is to "knock", i.e. buying a higher octane number gasoline indicates a better quality fuel.
Lead (tetraethyl lead to be precise) [10]
As you'll recall from Lesson 5, was also used as an octane enhancer but has been banned from most of our gasoline back in 1970.
The quality of the gasoline can be increased by reforming, which is either altering the shape (isomerisation) or altering the composition of the molecules. Essentially, the quality of gasoline can be increased by increasing the branched chain producing higher octane numbers.
Click on the image below to open The American Petroleum Institutes Refining portion of "Adventures in Energy". Go through all of the Refining Oil pages.
Mouseover the content for more information. When finished here complete the L06 quiz.
Accessible Version (word document) [12]
After looking at this map, please take the L06 quiz.
Links
[1] http://youtu.be/i3ctRVPVTC4
[2] https://courseware.e-education.psu.edu/courses/egee101/flash/ternary.html
[3] https://www.nrdc.org/stories/what-keystone-pipeline
[4] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson05/L05_pipelineLeak.mp3
[5] http://www.bp.com/en/global/corporate/gulf-of-mexico-restoration/deepwater-horizon-accident-and-response.html
[6] http://beyondeconomics.org/2010/07/05/gulf-oil-spill/
[7] https://www.e-education.psu.edu/egee101/node/767
[8] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson05/desired_products.mp3
[9] http://science.howstuffworks.com/environmental/energy/oil-refining.htm
[10] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson05/lead.mp3
[11] http://www.adventuresinenergy.org/Refining-Oil/index.html
[12] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson06/Lesson%206%20Coverage%20Map.docx