Catalysts

The rate of reaction for some chemical process may be sped up in one of two ways. Either temperature can be increased or the mechanism can be altered so as to lower the activation energy of the reaction. The latter method may be can be done with the implementation of a catalyst. A catalyst is an intermediate in a chemical reaction that participates in the activated complex for the rate limiting step. There are three main features of catalysts:

1. The catalyst allows a reaction to proceed via an alternative mechanism.

2. For every step in the mechanism in which the catalyst appears as a product, there is another step in which the catalyst is a reactant.

3. The catalyst increases the rate of reaction by lowering the activation energy of the reaction.

Catalysis in Fluid Catalytic Cracking

One very important example of the use of catalysis is fluid catalytic cracking. This process has been fundamental in the last few decades in meeting the high demands for high octane liquid fuels, such as gasoline. In fact, without FCC, methods of transportation such as cars and buses in their present forms would not be sustainable due to such high costs in fuel. From this fact alone, it is obvious that FCC is an essential process that has an important impact on many areas, including industry, transportation, and multiple aspects of everyday life.

When petroleum is collected from the stores in the earth, its raw form contains many different hydrocarbon compounds. These hydrocarbons can contain anywhere from five to forty carbon atoms. Historically, this small fact has caused a bit of a dilemma for the energy industry. Hydrocarbons with less carbons in the chain are much more valuable than the long hydrocarbons with carbon chains of twenty or more.

Figure \(\PageIndex{1}\) Content of crude oil after distillation but before FCC

These more compact hydrocarbons are especially valuable in a culture highly dependent on cars buses, jet planes, and other such aspects of life that require a compact and potent source of fuel.^[1] Due to this imbalance in demand and the excess of heavier hydrocarbons, the process of fluid catalytic cracking was implemented.

Generally speaking, a longer carbon chain will have a significantly higher boiling point. This basis forms the means by which the various hydrocarbons in petroleum can be distinguished and separated. As the goal of FCC is to break down the larger hydrocarbons into smaller, more valuable compounds, the portion of the crude oil that is used in FCC (the feedstock) is all those hydrocarbons with an initial boiling point of roughly 340 degrees Celsius or higher. In an equivalent sense, the feedstock contains all hydrocarbons with a molecular weight ranging from roughly 200 to 600 grams per mole. In fluid catalytic cracking, these hydrocarbons are vaporized and broken at high temperatures in the presence of a special catalyst. The purpose of the catalyst is to make the process more efficient and thus economically viable.^[2]

Catalyst Specifications

The most efficient form for the FCC process is highly desired and has been avidly searched for. One factor that effects the overall efficiency very significantly is the substance that is used as the catalyst. After many years of trial and error, a set of criteria for an efficient FCC catalyst has been established. The best form is a fine powder with a density ranging from 0.80 to 0.96 g/ml and an average particle size of 60-100 microns. The catalyst should be highly reactive, stable at high temperatures, and retain large pore sizes. All FCC catalysts contain a crystalline zeolite. In essence, the zeolite component acts as a molecular sieve that only allows a certain size range of hydrocarbons to enter its lattice.

Figure \(\PageIndex{2}\) Diagram of a standard FCC unit

The catalytic activity sites are provided by the matrix component of the catalyst, which contains amorphous alumina that are capable of cracking the larger feedstock molecules. Other components are present in order to ensure that the catalyst maintains strength and stability. Due to the nature of the catalyst components, it is highly important that the catalyst is not introduced to feedstock with metal contaminants. Even concentrations in the range of a few ppm of these contaminants can have detrimental effects on the performance of the FCC catalyst.^[3] Once an efficient catalyst is found, it is used extensively as a main ingredient. For every one kilogram of feedstock in the processing unit, there are five kilograms of catalyst compound added. Thus, a comparable processing unit might circulate 55,900 metric tons of catalyst each day.^[4]

Fluid catalytic cracking is a form of applied catalysis that has become essential to the energy industry. Its ability to recycle large hydrocarbons present in petroleum into more valuable and smaller hydrocarbons has allowed for demand imbalances to be reconciled. During the year 2007, in the United States alone, over 5,300,000 barrels of crude oil were processed using FCC.^[5] This astounding data proves the importance of fluid catalytic cracking to industry, and the entire process could not be implemented without a well devised incorporation of catalysis.

From ChemPRIME: 18.10:Catalysis

References

1. ↑ www.americanheritage.com/arti...005_3_36.shtml
2. ↑ http://en.Wikipedia.org/wiki/Fluid_catalytic_cracking
3. ↑ http://en.Wikipedia.org/wiki/Fluid_catalytic_cracking
4. ↑ http://www.chmltech.com/reactors/fcc.pps
5. ↑ tonto.eia.doe.gov/dnav/pet/hi...s=mcrccus2&f=A

Contributors and Attributions

• Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.