The Gasoline Refinement Process
Gasoline is a product that has to be refined in order that it be available for use. The refinement process is complicated, it starts with distillation and it is followed by more complicated chemical techniques to bread down the complex hydrocarbons.
The 20th century can easily be called the age of petroleum. Oil and gasoline make up our economic lifeblood. Without oil and gas our world would look very different. A 42-gallon of crude oil is used to produce gasoline and other petroleum products. Around 1900, only 11 gallons of gas were produced from each barrel of oil. Today, over 25 gallons of gas are produced from the same barrel. More than half! Oil is a geologic product but gas is a refined product. To get gas out of oil, a thermal and distillation process is used to produce the product. There are also government regulations that need to be adhered to. Oil is not the only product from which we can distill gasoline, other products like coal, shale oil, tar sands and even recycled plastics and rubber tires can be used. But the down side of these processes is that they are complicated and expensive. Consequently crude oil is the most economical source for the distillation and refinement of gasoline.
There are three general steps in the refinement of gasoline, the first is the isolation of gas that is naturally present in crude. Second, a thermal process must be used so that the petroleum molecules, the hydrocarbons, are rearranged to make gasoline production easier. Finally different refining steps, called streams, are used to blend petroleum products into finished gasoline.
The Raw Product:
Petroleum has a variety of chemical consistencies. It can be thin and light-colored or it can be thick and black. The thin crudes have high gravities whereas thick crudes have low gravities. Their molecular size makes them one or the other. Low-gravity crudes have small carbon chains; high-gravity crudes have long carbon chains. This chemical makeup means that when the petroleum is heated to a boiling point, the lighter crude percolates to the top faster than heavier crude because the smaller carbon chains separate faster than do long carbon chains.
But the composition of the heavy low-gravity crudes contains more natural gasoline and their low sulfur and low nitrogen contents makes them easy to refine. Modern refining techniques can turn low-gravity crude petroleum into high-gravity crude. However, refining low-gravity crudes requires complex and expensive equipment as well as additional steps and more energy. This makes it a costly operation. This price difference between the two types of crudes is reflected in the refining cost difference. Here is how gasoline is refined.
Stage 1: Isolation of gas from petroleum: Distillation
All petroleum crudes are composed primarily of three hydrocarbon classes: paraffin, naphthen, and aromatic. Each class contains a very broad range of molecular weights because the carbon chain is different. This makes the boiling point for each one different so the product produced is different.
Distillation is a process that involves boiling the crude to remove the lighter elements; it is immediately followed by condensation to capture the vapor in isolation from the remaining crude. Early distillation techniques used a process called batch processing whereby each product was assigned to a specific temperature range. Petroleum was heated to the specific boiling point and then the vapor was captured in a condenser to bring the distilled vapor back into the desired liquid product. It was condensed and captured and placed in a separate funnel. For a different product a separate heating process occurred. So each product was produced separately in a batch.
But the batch process was inefficient. Each batch effort took time, and lost energy. It was costly in terms of labor hours spent as well as production facilities that were tied up. A better process had to be developed.
A continuous process evolved that made the distillation process cheaper and efficient. In continuous process distillation a pump is used to provide the crude flow and a furnace to heat the crude. A distillation column is provided to separate the different boiling products or cuts. In the distillation column, the first vapor to emerge is propane and butane. Straight run gasoline, kerosene, and diesel fuel cuts are the next products to be isolated. Finally at the bottom are the hydrocarbons with boiling points higher than diesel fuel. These are called atmospheric bottoms.
Stage 2: The hydrocarbon chemical re-arrangement process.
Distillation was good up to a point. But because of the boiling points used there was a lot of waste. Some unusual applications were discovered like road paving and sealing. But the problem was that the demand for gasoline was higher than the supply. Even improving the efficiency of the existing distillation processes did not yield enough commercial product.
The refining industry discovered a process whereby high-boiling point hydrocarbons could be broken down into lower boiling point hydrocarbons. This process is called cracking. Cracking is a way to increase gasoline production since it now uses the remaining discarded crude from the distillation process. The first non-distillation process is thermal cracking. It came into use in 1913. Here, the hydrocarbon is subjected to a very high temperature to break the carbon molecule chain and leave a carbon molecule product that is easier to distill at lower temperatures.
Catalytic cracking came next. Instead of heat, a catalytic agent is used to change the molecular hydrocarbon chain. A catalyst is a chemical that causes a reaction to take place; sometimes it is used to speed a reaction, or to force a reaction to occur. Catalytic cracking is an important process because it can produce a higher quality gasoline than thermal cracking. But, the cracking process continued to evolve.
The next cracking procedure developed was fluid catalytic cracking (FCC). In this process, a repeat of the catalytic cracking procedure occurs again and again. This practice is called fluidizing. The catalytic cracking is continuously recycled from the reaction section to the regeneration section. Fluid catalytic cracking is at the heart of modern petroleum refining.
Stage 3: Blending different Streams to produce the finished gasoline product.
It may be said that gasoline is as much a government created product as it is a petroleum refining process product. This is due to government regulations that treat gasoline that comes out of the refinery as a product that must meet additional social policy conditions. Many of these policies were introduced for the first time in the 1970's. For instance, gasoline must not contain lead, it must have a high-octane (no-engine knock) rating, and it must meet air pollution regulations. To comply with the myriad of government regulations, a gasoline must be blended with up to five or more refining streams. The streams can be broken down into two major categories, atmospheric distillation or vacuum distillation. The streams are:
- Coker light gasoline hydrotreater, which includes isomerization and/or benzene saturation. This yields a straight run or hydrocracked light gasoline blend.
- Heavy gasoline hydrotreater, which includes reformer and which yields a reformate gasoline.
The Vacuum Distillation
- The hydrocracker stream, which produces polymerized and alkylation gasolines.
- The Fluid catalytic cracker (FCC) hydrotreater stream can produce FCC light or heavy gasolines as well at Mtbe/Tame/Etbe gasolines as well as the alkylate blend.
- The coker stream can mix with FCC stream to produce FCC light or heavy gasoline blends. These blends make the commercial product that we know.
These streams are used to produce commercially and governmentally sellable products. They meet different government regulations for quality and pollution control. Usually the final gasoline blends are the result of combining different streams to meet quality or governmental regulations.
The gasoline refining business has been the product of some intense chemistry research over the last 90 years. But it is not enough to have a good product, it must meet social policy regulations. The process is now remarkably efficient; but major oil refiners are still looking for ways to get more gas from a barrel of oil.