EGEE 101
Energy and the Environment

Refinery Operations

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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.

 A refinery reflected in the water in Tampa Florida
A refinery lit up at night in Tampa, Fl. We only have about 100 refineries in the US. Similar to our lack of new construction with utilities (electricity generation), we have not built any new refineries in years. We are at risk of running out of capacity and having to import finished products such as gasoline (assuming other locations will have excess capacity).
Credit: NETL

Major Refinery Components

Separation

Desalination

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 Tower

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.

A distillation tower
A distillation tower, again with me in the lower bottom right to give you a sense of scale.
Credit: EIA

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?

Refinery Products - Click for text description. This will expand to provide more information.
Dr. Mathews: The role of a refinery is to make money for the share holders. It is a billion dollar operation to build one of these refineries and keep it processing. We only have about a hundred of them in the United States and they have to meet a variety of demands and a multitude of products. Now the market for some of these products are very large, such as gasoline. The market for some other products will be much smaller, for example candle wax. However, what they are going to produce depends very much on the quality of the crude oil that comes in. For example, sulfur-rich crude oil is going to take much more work to clean up. That is why they tend to be a lower quality and cheaper. If you are getting lower quality or less mature crude oils then you are going to get less of a gasoline fraction and perhaps more asphalt cuts or other cuts. So what is the desired products depends much on the quality of your crude oil. Because it is what it is naturally going to give you and how much work you are willing to put in to meet the product demand. The bottom line is you have to make money. If you can make more money by selling gasoline, such as in the summer months when Americans drive the most, then that is what you will do. You will increase your gasoline output. In the winter, when you can probably make more money selling fuel oil because there is a higher a demand for things like home heating, which we have already discussed, then that is what you will do. Instead of producing as much gasoline you are going to produce fuel oil. Or in times of the Gulf War, certain refineries will be producing a great deal of military jet fuel. So depending on the weather, depending on the holidays coming up, and driving seasons, and the demand for air travel, which of course diminished somewhat after September eleventh. These all effect what is the desired product coming out of the refinery. And it is variant, so realize what is coming out in America is not necessarily the same for Europe. For example in Europe, they have a great deal of a number of diesel vehicles, so the demand there for more diesel and less gasoline. So all of these things come into play.

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 distillation process at a glance. Place the mouse over the green text on the image for more information.
Credit: EIA

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" provides more good information on this topic.

Vacuum Distillation Towers

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.

Chemical Processes

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).

Cracking

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.

  Example of cracking.
Example of Cracking
Credit: JPM

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 - Revisited

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)

Lead Audio - Click for text description. This will expand to provide more information.
Dr. Mathews: Lead, or more specifically tetraethyl lead, was added to gasoline to increase the octane number. It's palely branched molecule has a very high octane number. And so it is useful in helping prevent knocking. The secondary use is that the lead would vaporize in the cylinder, and some of it would go out the exhaust, but some of it would help coat the inside of the cylinders. And so a number of cars took advantage of that and used lead as a natural lubrication. Unfortunately, the lead would leave through the tailpipe and would generally concentrate in the brains of young children. Lead in the air would be breathed in and unfortunately does not exit the body. And so high levels of lead, highly detrimental, and actually reduced brain capacity in small children and young children. And so one of the reasons for getting rid of it was the fact that it is detrimental to your health. We have the same problem in houses built before something like 1974 where another lead compound was added to paint for a couple of reasons. One is the pigments in it, you could get some nice bright whites and also that the bugs would not like to eat it very much because of that lead content. The main reason though that lead was taken out of gasoline was that the lead would have coated any catalyst in the catalytic converter, preventing it from working. So prior to the invention of catalysts, catalytic converters for your vehicle, the lead had to be essentially taken out, or actually not added, to the gasoline.

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.