Reaction cracking

For cracking reactions, combinations of zeolites, alumina, clay, and silica are used as the catalyst. These acidic materials, which contain both Br0nstead and Lewis acidic sites, initiate a complex set of carbonium- and carbenium ion-based reactions. Note that carbonium ions are protonated alkyl groups (e.g., C Hg ), while carbenium ions refer to alkyl cations (e.g., To enhance the acidic properties, rare 

In hydroprocessing units, cracking reactions - those that break carbon-to-carbon bonds - can be grouped into three main categories  

Step 1 of the dual mechanism involves adsorption of a paraffin molecule to a metal site, followed by reversible dehydrogenation to form an olefin. In Step 2, the olefin migrates to an acid site, where it reacts with a proton to form a carbenium ion. The carbenium ion can rearrange into a more-stable carbenium ion (Step 3), which explains why products from hydrocrackers are relatively rich in iso-paraffins. In Step 4, P-scission of the carbenium ion produces an olefin and a smaller carbenium ion. The olefin can undergo further cracking on an acid site, or it can react with hydrogen at a metal site 

In vacuum gas oils and other 650°F-plus (343°C-plus) material, more than half of the carbon atoms may be found in ring compounds - polyaromatic hydrocarbons (PAH), partially saturated PAH (naphthene-aromatics) and fully saturated naphthenes. According to Qader and McOmber and Lapinas, et al., ° hydrocracking converts complex ring compounds into light products by the following sequence of reactions  

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If inert material is to be added, then ease of separation is an important consideration. For example, steam is added as an inert to hydrocarbon cracking reactions and is an attractive material in this respect because it is easily separated from the hydrocarbon components by condensation. If the reaction does not involve any change in the number of moles, inert material has no effect on equilibrium conversion. 

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. 

The similarity of oxidation rates of different hydrocarbons in the higher temperature regions is probably related to the predominance of alkyl radical cracking reactions under these conditions (reaction 28). The products of such reactions would be similar for most common hydrocarbons (96). 

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

The second step is a -scission, the breaking of a carbon—carbon bond P to the charged carbon. The sum of the two reactions is the stoichiometry of the overall cracking reaction R H — RH + olefin. R+, a relatively stable carbenium ion such as the /-butyl cation, is a chain carrier. The role of the catalyst is to donate the proton to start the chain. This is a greatiy simplified representation. 

Pyrolysis. Pyrolysis of 1,2-dichloroethane in the temperature range of 340—515°C gives vinyl chloride, hydrogen chloride, and traces of acetylene (1,18) and 2-chlorobutadiene. Reaction rate is accelerated by chlorine (19), bromine, bromotrichloromethane, carbon tetrachloride (20), and other free-radical generators. Catalytic dehydrochlorination of 1,2-dichloroethane on activated alumina (3), metal carbonate, and sulfate salts (5) has been reported, and lasers have been used to initiate the cracking reaction, although not at a low enough temperature to show economic benefits. 

Cracking reactions are endothermic, 1.6—2.8 MJ/kg (700—1200 BTU/lb) of hydrocarbon converted, with heat supplied by firing fuel gas and/or fuel oil in side-wall or floor burners. Side-wall burners usually give uniform heat distribution, but the capacity of each burner is limited (0.1—1 MW) and hence 40 to 200 burners are required in a single furnace. With modem floor burners, also called hearth burners, uniform heat flux distribution can be obtained for coils as high as 10 m, and these are extensively used in newer designs. The capacity of these burners vary considerably (1—10 MW), and hence only a few burners are required. The selection of burners depends on the type of fuel (gas and/or liquid), source of combustion air (ambient, preheated, or gas turbine exhaust), and required NO levels. 

Obtain the energies of formaldehyde and 1,3,5-trioxane. What is AH n for trioxane cracking Is the process endothermic or exothermic Is ASrxn for the cracking reaction likely to be positive or negative Explain. Given the direction of the change in entropy, should one crack trioxane at higher or lower temperatures Explain (use equation 1). 

The distribution of the products obtained from this reaction depends upon the reaction temperature (Figure 5.1-4) and differs from those of other poly(ethene) recycling reactions in that aromatics and alkenes are not formed in significant concentrations. Another significant difference is that this ionic liquid reaction occurs at temperatures as low as 90 °C, whereas conventional catalytic reactions require much higher temperatures, typically 300-1000 °C [100]. A patent filed for the Secretary of State for Defence (UK) has reported a similar cracking reaction for lower molecular weight hydrocarbons in chloroaluminate(III) ionic liquids [101]. An... 

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... 

Hydrodealkylation. Hydrodealkylation is a cracking reaction of an aromatic side chain in presence of hydrogen. Like hydrocracking, the... 

The most important cracking reaction, however, is the carbon-carbon beta bond scission. A bond at a position beta to the positively-charged carbon breaks heterolytically, yielding an olefin and another carbocation. This can be represented by the following example ... 

The cracking reactions are principally bond breaking, and a substantial amount of energy is needed to drive the reaction toward olefin production. 

When liquid hydrocarbons such as a naphtha fraction or a gas oil are used to produce olefins, many other reactions occur. The main reaction, the cracking reaction, occurs by a free radical and beta scission of the C-C bonds. This could be represented as ... 

Steam cracking reactions are highly endothermic. Increasing temperature favors the formation of olefins, high molecular weight olefins, and aromatics. Optimum temperatures are usually selected to maximize olefin production and minimize formation of carhon deposits. 

Although olefins are intermediates in this reaction, the final product contains a very low olefin concentration. The overall reaction is endothermic due to the predominance of dehydrogenation and cracking. Methane and ethane are by-products from the cracking reaction. Table 6-1 shows the product yields obtained from the Cyclar process developed jointly by British Petroleum and UOP. ° A simplified flow scheme for the Cyclar process is shown in Figure 6-6. 

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