World War II provided a driving force for developing catalytic technologies to increase the yield of distillate fuels. There was an extremely high demand for petroleum-derived products during the war. Such products or uses included paving runways, making toluene for explosives, manufacturing synthetic rubbers for tires, lubricants for firepower and machinery. Although, the majority of the petroleum was distilled to produce gasoline to fuel trucks, tanks, and airplanes. Thus, the previous thermal refinery processes were improved upon and more efficient catalytic refining processes were introduced. Such catalytic processes include catalytic cracking, catalytic reforming, alkylation, hydrotreating and polymerization. All these processes emphasized improving the yield and octane number of the gasoline produced. Catalytic cracking utilized fluidized bed for continuous regeneration which enabled production to be much more efficient than its thermal cracking counterpart. Furthermore, the process has useful byproducts that could be used as petrochemical feedstocks for other processes. The catalytic reforming process, which was based on the cyclic process, enabled increased production benzene for styrene, toluene for explosives, and aromatics for aviation fuel. In addition, a considerable quantity of hydrogen is produced as a byproduct of the catalytic reforming process. This hydrogen can be used for hydrotreating petroleum fractions to remove heteroatoms, in particular sulfur. Additional processes such as alkylation and polymerization utilized olefins and paraffinic reactions to increase the octane number of gasoline. Thus, the advent of such processes to meet the high demand of gasoline during World War 2 marked the beginning of the catalytic refinery period.

Refrences:

Course Website

Catalytic Refinery, 1940 – 1970, F. Self, E. Ekholm, and K. Bowers, Refining Overview – Petroleum, Processes and Products, AIChE CD-ROM, 2000

Wastewater treatment is considered an important supporting process in petroleum refining. The four types of refinery wastewater include cooling water, process water and steam, storm water, and sanitary sewage water. The refinery stages that produce the most wastewater are desalting, distillation, thermal and catalytic cracking, and coking. The most polluted wastewater is the process water that comes into direct contact with petroleum fractions; thus it cannot be simply treated in municipal wastewater treatment plants. Storm water is considered contaminated as a result of incidental exposure to pollutant sources and accidental spills during refinery reactions. The refinery’s cooling water and sanitary sewage water will probably not require much treatment before sending it to public water facilities where municipal wastewater is treated. Different wastewater streams are usually not mixed even if it reduces the load on treatment units. This is because different wastewater streams have different components and toxicities. The wastewater’s contamination level depends on its usage in the petroleum refinery. Generally, the pollutants in the streams include hydrocarbons, particularly toxic aromatic compounds such as benzene. Also, the wastewater streams include other heteroatom compounds mercaptans, amines, phenols, and cyanides, dissolved gases (H₂S and NH₃), and acids (H₂SO₄ and HF). Such components require much more treatment than municipal wastewater. In addition, environmental policies such as the Clean Water Act and Safe Drinking Water Act require refineries to effectively treat their wastewater.

Source:

Course Website

The objective of catalytic reforming is to convert low octane straight-run naphtha streams into high a high octane, low sulfur reformate which is used as the major blending product for gasoline. To get a higher octane number of gasoline, the aromatic and branched iso-paraffins concentrations have to be increased. Hydrogen is a useful by product produced by catalytic reforming. The hydrogen gas can be used for the hydro treating and hydrocracking processes. The naptha feedstock is usually hydro treated before reforming to protect the catalyst used. The majority of reforming catalysts used in the process contain either platinum, palladium, or bi/tri metallic formulations of platinum with Rhenium, Tin or Iridium supported on alumina. Limits on catalytic reforming capacity in American and European refineries have been placed to regulate the amount of benzene and aromatics sold in gasoline.

The main reactions of interest in catalytic reforming are the dehydrogenation of naphthenes to aromatics, isomerization of straight chain n-paraffins to branched iso-paraffins, dehydroisomerization of alkyl-C₅ naphthenes and dehydrocyclization of n-paraffins to aromatics. The aromatics in the feed should remain unchanged under the right conditions. In addition, varying the reactor conditions can control side reactions such as the hydrogenation of aromatics. Considering the reactions are endothermic, the most suitable reactor conditions would be high temperatures, low hydrogen pressures, low space velocity, and low H₂/HC ratio. It is important to note that the hydrogen pressure should be high enough to inhibit coke deposition on the catalyst surfaces.

Another objective in catalytic reforming is to inhibit the undesired reaction of hydrocracking in the reactor. Hydrocracking consumes hydrogen in the reaction, thus decreasing the yield of reformate. The reaction is favored at high temperatures and high hydrogen pressures; thus, hydrocracking in the reactor must be taken into consideration when choosing the optimum reactor conditions. Also, it is important to note that the balance between acidic and metallic sites must be controlled to catalyze specific reactions; for example, the platinum surface metals catalyze dehydrogenation reactions compared to the acidic alumina support sites that catalyze isomerization and cracking reactions.

In addition, the process’s coke deposition produced deactivates the coke; hence, the catalysts must be regenerated periodically to maintain a maximum yield. There are 3 types of catalyst regeneration for catalytic reforming and they are: semi-regenerative, cyclic and continuous.

Other reactions have been developed to further increase the octane number. For instance, alkylation is a process whereby light iso-paraffins are combined with C₃–C₄ olefins, to produce a mixture of higher molecular weight iso-paraffins. Alkylation requires a strong acid catalyst such as sulfuric acid and hydrofluoric acid. However, the latter is preferred because it is less sensitive to temperature fluctuations. Furthermore, the polymerization process combines propenes and butenes to produce olefins, which contribute to a higher octane number, but the process has been largely replaced by alkylation.

Sources:

Course Website

Following the development of a fixed- bed (Houdry process, 1936) and a moving-bed (Thermafor Catalytic Cracking, 1941) catalytic cracking process, fluid-bed catalytic cracking (FCC, 1942) became the most widely used process worldwide because of the improved thermal efficiency of the process and the high product selectivity achieved, particularly after the introduction of crystalline zeolites as catalysts in the 1960s.

Catalytic Cracking processes were developed during the Second World War and became widely used ever since because of their improved thermal efficiency and the high product selectivity to produce gasoline with a higher octane number. Also, catalysts and additives are very significant for the selectivity and flexibility of catalytic cracking; the introduction of zeolite catalysts in 1965 has had a huge impact on the industry processes.

In 1936, the first full scale industrial catalytic process was developed. It was known as the Houdry Catalytic cracking process, which used much less expensive catalysts, such as clay, natural alumina and silica particles. For this process, the gas oil feed stock must be heated to high temperatures and is fed to a fixed-bed reactor containing the catalyst particles. The product stream is sent to the separator to produce gas, gasoline, light cycle oil (LCO), and heavy cycle (HCO) products. A problem that faced this process was the deactivation of the catalyst bed because of coking. Thus, the process included swing reactors to switch between periodically to overcome this issue. After switching, the reactor is stripped with stream to remove the liquid products from the catalyst bed. The coke is then burnt off to reactivate the catalyst bed. A small percentage of the heat generated by burning off the coke could be used to supply heat for catalytic cracking, however the thermal efficiency of this reaction is considered low.

However, more efficient catalytic cracking processes have been developed. Such process include thermafor catalytic cracking and fluid catalytic cracking. Thermofor cracking utilizes a moving catalyst bed. In addition, catalyst particles used in this process are synthetic and thus have consistent and homogenous properties. This process is a slightly more thermal efficient process than the Houdry process. In comparison, the fluid catalytic cracking utilizes a fluidized catalyst bed. This catalytic cracking process is considered the most thermal efficient process. Also, this process enables the production of large quantities of light distillates known as crackate without the addition of hydrogen by burning off the coke.

Catalytic hydrocracking is relatively a recent addition to the petroleum industry. The main reasons for its development are the increasing demand for light and middle distillates, the large quantities of hydrogen as a by-product from catalytic reforming, and the limits imposed on sulfur and aromatics content in motor fuel. Hydrocracking is able to process a wide range of feedstocks. Its main process objective is to decrease the molecular weight and boiling point of the heavy oils from a mostly aromatic feedstock. Through the hydrocracking process, it is possible to convert an aromatic compound to a paraffin compound with out rejecting any carbon. The catalysts used for such reactions include platinum and palladium metals, however, care must be taken as they can be easily poisoned by sulfur.

Source:

Course Website

Wikipedia

Thermal cracking is a process that produces short straight chain paraffin from longer straight chains found in gas oils and other heavier crude oil fractions. The chemistry of thermal cracking involves free radicals that are reactive species with unpaired electrons but have a neutral electronic charge. It is the free radical chemistry that is responsible for producing gasoline with a relatively low octane number.

A Russian engineer named Vladimir Shukov introduced the first thermal cracking method in the Russian Empire in 1891. However, it was much later in 1912 that William Merriam Burton and Robert E. Humphreys designed a similar thermal cracking process which operated under temperature conditions of 700 to 750 °F and an absolute pressure of 90 psi. The advantage of the system they developed was that both the condenser and the boiler were continuously kept under pressure. A few years later in 1921, an employee at the Universal Oil Products Company, C.P. Dubbs, developed a more advanced technique which operated at higher temperatures of 750–860 °F. The design became known as the Dubbs process and was extensively used until the early 1940s.

Modern day techniques of thermal processing include visbreaking and coking. Visbreaking is a mild fom of thermal cracking whereby the viscosity of the heavy crude oil residue is lowered significantly without affecting the boiling point range. Temperatures of about 950° F are used in the distillation column. Visbreaking mostly depends on temperature and time of the reaction. Coking is a severe form of thermal cracking that is used to convert heavy residuals into lighter more useful products and distillates. The most common coking techniques include delayed coking, fluid coking and flexi coking.

Sources:

Wikipedia: http://en.wikipedia.org/wiki/Cracking_(chemistry)

Course Webpage: https://www.e-education.psu.edu/fsc432/content/chemistry-thermal-cracking

Set Laboratories: http://www.setlaboratories.com/therm/tabid/107/Default.aspx