Catalytic cracking,

Thermal cracking processes can only convert up to 50% of the feed, as stated previously. Catalytic cracking was developed in order to improve the conversion level of the heavy feed. 

The use of a catalyst in the cracking reaction increases the yield of high-quality products under much less severe operating conditions than in thermal cracking. Several complex reactions are involved, but the principal mechanism by 

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Before we look at the mechanism of catalytic cracking on catalysts, it is appropriate to give a definition of a catalyst. Catalysis is derived from the Greek word katalysis meaning destruction or weakening. A catalyst is a substance that changes the rate of a chemical reaction but remains chemically unchanged at the end of the reaction. 

The reaction mechanism in a heterogeneous catalytic process is more complicated than the mechanism in a non-catalytic reaction. In the first place, there is the influence of many physical stages of the catalysis on the reaction itself and on the reaction velocity. The main stages of the catalytic reaction can be represented as follows  

Finally, as shown by Table 228. these light cuts themselves contain significant concentrations of oiefinic hydrocarbons. Moreover, they are the only effluents of catalytic cradcing that the reiiner may agree to make available for petrochemicals. 

On the whole, process performance, namely the product distribution, varies according to the operating conditions (space velocity, pressure, temperature, catalyst circulation 

TYPICAL ANALYSES OH LKiMT CUTS FROM CATALYTIC CRACKING 

Hydrogen Mcliumc. i thmtc. Ivtliylcnc i Propane. Propylene Inerts. . 1 

Cis 2-puntunu. . Trans 2-pcnlcnc 2 inclhyl I biitunu 3-melhyi I-butene 2 mclliy 2-bulencs Ci. 

Catalysts play an important role in the synthesis of fuels and chemicals as well as on the reaction systems hence, the catalytic cracking of polyolefins over solid acids needs to be explored. In this method a suitable catalyst is used to carry out the cracking reaction. The addition of a catalyst enhances the conversion and fuel quality, and lowers the reaction temperature and time. Reuse of catalysts and the use of effective catalysts in lesser quantities can optimise this option. It also enables an increased level of the cracking of plastics and a lower concentration of solid residue in the product. The cost should be further reduced to make the process more attractive from an economic perspective in order to solve the acute environmental problem of plastic waste disposal [2]. 

The catalyst causes a classical carbenium ion to be formed by acid catalyzed activation reactions. The classical carbenium ion is transformed into the key intermediate which can be described as a protonated cyclopropane structure. After some rearrangements cracking occurs. The formation of branched paraffins is very fortunate since branched paraffins have high octane numbers and the isobutane produced can be used in alkylation. The preferred products are those of which the formation proceeds via tertiary carbenium ions. Carbenium ions can also be generated by intermolecular hydride transfer reactions between alkane and carbenium ions that are not able to form tertiary carbenium ions (see Chapter 4, Section 4.4). Under more severe conditions lower paraffins can also be cracked. 

Extensive coke deposition takes place during catalytic cracking, resulting in loss of activity. Typically, the catalyst loses 90% of its activity within one second. An elegant solution has been found for this problem. The clue to this solution is a combination of a reactor in which cracking takes place with a reactor used for regeneration of the catalyst by burning the deposited coke. In this set-up coke is 

This overhead product is condensed by a compressor and separated into light gases and gasoline. 

FCC feedstock (380-580 °C) Brega Arabian light Arabian heavy 

The highest yield is obtained from polycyclic naphthenes, alkylsubstituted mono-cyclic aromatics and monocyclic naphthenes. Isoparaffins and normal paraffins give medium yields, whereas the gasoline yield from naphthalene derivatives is relatively low by comparison. 

The gas oil fractions obtained from the fractionation of the cat-cracker reaction product are rich in aromatics. They are therefore suitable as a basis for the recovery of polycyclic aromatics, e.g. naphthalene. Because of the high C/H ratio, they can also be used for the production of high-value carbon in the form of premium coke and as feedstock for the manufacture of carbon black (see Chapter 13). 

The aromatics content of the cat-cracker gasoline, which increases the octane rating, can also be influenced by the choice of catalyst. Table 3.18 shows the composition of a gasoline recovered by cat-cracking gas oil on a silica/alumina catalyst and a zeolite. With the zeolite, the formation of aromatics increases at the expense of olefin formation. 

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CeH3Me3. Liquid produced by catalytic cracking of methyl benzenes. Used to prepare trimellitic anhydride. 

Following certain refining processes like catalytic cracking, sizeable amounts of nitrogen can appear in light cuts and cause quality problems such as instability in storage, brown color, and gums. 

The gas oil cut from catalytic cracking called Light Cycle Oil (LCO), is characterized by a very low cetane number (about 20), high contents in aromatics, sulfur and nitrogen, all of which strongly limit its addition to the diesel fuel pool to a maximum of 5 to 10%. 

In the 1970 s, heavy fuel came mainly from atmospheric distillation residue. Nowadays a very large proportion of this product is vacuum distilled and the distillate obtained is fed to conversion units such as catalytic cracking, visbreaking and cokers. These produce lighter products —gas and gasoline— but also very heavy components, that are viscous and have high contaminant levels, that are subsequently incorporated in the fuels. 

H2S is found with the reservoir gas and dissolved in the crude (< 50 ppm by weight), but it is formed during refining operations such as catalytic cracking, hydrodesulfurization, and thermal cracking or by thermal decomposition of sulfur[Pg.322]

In addition, salts deactivate reforming and catalytic cracking catalysts. 

The main feedstock for catalytic reforming is heavy gasoline (80 to 180°C) available from primary distillation. If necessary, reforming also converts byproduct gasoline from processes such as visbreaking, coking, hydroconversion and heart cuts from catalytic cracking. 

The feedstock usually comes from catalytic cracking, sometimes from steam cracking. The reaction products are Cy-Cg isoparaffins. The byproducts are the C3-C4 n-paraffins which do not react. 

Feedstocks come mainly from catalytic cracking. The catalyst system is sensitive to contaminants such as dienes and acetylenes or polar compounds such as water, oxygenates, basic nitrogen, organic sulfur, and chlorinated compounds, which usually require upstream treatment. 

Proportions correspond to the material balance for catalytic cracking in Figure 10.3 showing streams (l)(2)(3)(4) and (5). 

Catalytic cracking is a key refining process along with catalytic reforming and alkylation for the production of gasoline. Operating at low pressure and in the gas phase, it uses the catalyst as a solid heat transfer medium. The reaction temperature is 500-540°C and residence time is on the order of one second. 

Products of conversion from catalytic cracking are largely olefinic for light fractions and strongly aromatic for the heavy fractions. 

Figure 10.8 presents a variant of the FCC process, the RCC (Residue Catalytic Cracking) capable of processing heavier feedstocks (atmospheric residue or a mixture of atmospheric residue and vacuum distillate) provided that certain restrictions be taken into account (Heinrich et al., 1993). 

In a single stage, without liquid recycle, the conversion can be optimized between 60 and 90%. The very paraffinic residue is used to make lubricant oil bases of high viscosity index in the range of 150 N to 350 N the residue can also be used as feedstock to steam cracking plants providing ethylene and propylene yields equal to those from paraffinic naphthas, or as additional feedstock to catalytic cracking units. 

Mild hydrocracking prepares the feedstock for catalytic cracking or for the conventional lubricant production scheme. 

The feedstocks in question are primary distillation streams and some conversion products from catalytic cracking, coking, visbreaking, and residue conversion units. 

For gas oil from catalytic cracking (LCO), reducing the aromatics content to 20 wt. % results in a chemical hydrogen consumption of 3.4 wt % and a cetane number of 40. 

Mercaptans are naturally present in crude oil (Chapters 1 and 8), or they result from the decomposition of other sulfur compounds during thermai or catalytic cracking operations. 

Fractions treated by this process are light products from the primary distillation LPG to Kerosene, or light products from thermal and catalytic cracking (visbreaking, coking, FCC). 

Acid gases are mainly hydrogen sulfide (H2S) originating essentially from hydrotreating units off-gas. Smaller quantities are also produced in thermal and catalytic cracking units. 

Contaminated water comes from primary distillation (desalting), hydrotreating, thermal cracking and catalytic cracking units. 

Hydrocracking is a major process for the production of diesel motor fuel catalytic cracking is its counterpart for the gasoline production. 

The question then lies in the selection of more appropriate feedstocks for these two processes. The cost of hydrocracking leads to selecting feedstocks that are the easiest to convert as for catalytic cracking, its flexibility and extensive capabilities lead to selection of heavier feedstocks. 

Furthermore, the major problem of reducing aromatics is focused around gasoline production. Catalytic reforming could decrease in capacity and severity. Catalytic cracking will have to be oriented towards light olefins production. Etherification, alkylation and oligomerization units will undergo capacity increases. 

Ecole Nationale Superieure du Petrole et des Moteurs Formation Industrie end point (or FBP - final boiling point) electrostatic precipitation ethyl tertiary butyl ether European Union extra-urban driving cycle volume fraction distilled at 70-100-180-210°C Fachausschuss Mineralol-und-Brennstoff-Normung fluid catalytic cracking Food and Drug Administration front end octane number fluorescent indicator adsorption flame ionization detector... 

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