Cracking reactions coking

In theory, favorable conditions correspond to a pressure of 0.1.10 Pa and temperatures not exceeding 350 C However, cracking reactions (coke formadon) are excessive in this case, and the selecdvity of the operation is reduced. Hence the reactions producing aromatics must be activated selectively, and operations comlucted at a sufficiently hi partial pressure of hydrogen. 

As a result of the cracking reactions, coke is deposited on the catalyst, consequently the catalyst is poisoned and has to be regenerated. This exothermic regeneration process is carried out by circulating it to a fluidized bed regenerator, where under excess oxygen, the coke is burned off the catalyst at a temperature and pressure of about 1272 T (690 °C) and 34 psia (2.3 bara) respectively. The process conditions should ensure that nearly all carbon monoxide produced in the bed is converted to carbon dioxide. The carbon monoxide concentration in the stack gas should meet the following constraint Xco < 10" mol/mol. 

All modern refineries have conversion units, designed to transform black effluent streams into lighter products gas, gasoline, diesel fuel. Among these conversion units, coking processes take place by pyrolysis and push the cracking reaction so far that the residue from the operation is very heavy it is called coke . 

Cracking reactions are endothermic the energy balance is obtained by the production of coke that deposits on the catalyst and that is burned in the regenerator. 

Stabilized by hydrogen transfer. The stabilized free radicals undergo secondary cracking reactions as they come in contact with the hot coke. 

In the fluid coking process, part of the coke produced is used to provide the process heat. Cracking reactions occur inside the heater and the fluidized-bed reactor. The fluid coke is partially formed in the heater. Hot coke slurry from the heater is recycled to the fluid reactor to provide the heat required for the cracking reactions. Fluid coke is formed by spraying the hot feed on the already-formed coke particles. Reactor temperature is about 520°C, and the conversion into coke is immediate, with... 

Risers are normally designed for an outlet vapor velocity of 50 ft/sec to 75 ft/sec (15.2 to 22.8 m/sec). The average hydrocarbon residence time is about two seconds (based on outlet conditions). As a consequence of the cracking reactions, a hydrogen-deficient material called coke is deposited on the catalyst, reducing catalyst activity. 

Cracking, isomerization, and hydrogen transfer reactions account for the majority of cat cracking reactions. Other reactions play an important role in unit operation. Two prominent reactions are dehydrogenation and coking. 

Reactor temperature. An increase in the reactor temperature will also reduce delta coke by favoring cracking reactions over hydrogen transfer reactions. Hydrogen transfer reactions produce more coke than cracking reactions. 

Example 5.6 Hydrocarbon cracking reactions are endothermic, and many different techniques are used to supply heat to the system. The maximum inlet temperature is limited by problems of materials of construction or by undesirable side reactions such as coking. Consider an adiabatic reactor with inlet temperature Tm. Then T z) < T, and the temperature will gradually decline as the reaction proceeds. This decrease, with the consequent reduction in reaction rate, can be minimized by using a high proportion of inerts in the feed stream. 

In the 1970s more-active zeolite catalysts were developed so that the cracking reaction could be conducted in the transport riser. Recently, heavier crude feedstocks have resulted in higher coke production in the cracker. The extra coke causes higher temperatures in the regenerator than are desired. This has resulted in the addition of catalyst cooling to the regeneration step, as shown in Fig. 17-25. 

Nickel aluminate, a spinel, has long been known to trap nickel. Metals like arsenic(19), antimony(20-21) and bismuth(20) are known to passivate transition elements and can be used to decrease and coke make. Sulfur is also a known inhibitor for nickel therefore, higher sulfur-containing crudes may be a little less sensitive to nickel poisoning. In our work we also found that nickel at low concentrations is actually a slight promoter of the cracking reaction when incorporated into a molecular sieve (Figure 17). 

Sometime in the early twentieth century it was found that if the steel tubes in the furnace had certain kinds of dirt in them, the cracking reactions were faster and they produced less methane and coke. These clays were acting as catalysts, and they were soon made synthetically by precipitating silica and alumina solutions into aluminosilicate cracking catalysts. The tube fumace also evolved into a more efficient reactor, which performs catalytic cracking (FCC), which is now the workhorse reactor in petroleum... 

The problems in fixed bed cracking reactors are (1) heat must be supplied to heat the reactants to the desired temperature and overcome the endothermicities of the cracking reactions and (2) the reactor must be shut down periodically for coke removal. Both of these problems were overcome by the development in the 1940s and 1950s of a fluidized... 

The first reaction is the isomerization from a zero-octane molecule to an alkane with 100 octane the second is the dehydrocyclization of heptane to toluene with 120 octane, while the third is the rmdesired formation of coke. To reduce the rate of cracking and coke formation, the reactor is run with a high partial pressure of H2 that promotes the reverse reactions, especially the coke removal reaction. Modem catalytic reforming reactors operate at 500 to 550°C in typically a 20 1 mole excess of H2 at pressures of 20-50 atm. These reactions are fairly endothermic, and interstage heating between fixed-bed reactors or periodic withdrawal and heating of feed are used to maintain the desired temperatures as reaction proceeds. These reactors are sketched in Figure 2-16. 

The dry gas prodnced from the DCC process contains approximately 50% ethylene. The cracking reactions are endothermic, and compared to FCC, a higher coke make is required to satisfy the heat balance. 

The third type is the additional coke related with the feedstock quality. FCC feedstock contains a dissolved carbon, polynuclear aromatic compounds, called Conradson carbon residue (CCR ASTM D-189). It is deposited over the catalyst surface during cracking reactions. In the FCC unit, this material is part of the coke remaining in the catalyst. Some researchers have investigated cracking of heavy feedstock and observed that, in particular cases, the amount of Conradson carbon is linearly related with the carbon-hydrogen ratio of the feedstock [3]. 

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