Thermal cracking of ethane

This agrees with the overall rate data, which is first order. However, estimates of the concentrations of the ethyl and hydrogen radicals, as found from the steady-state approximation and the free radical rate expressions, indicate that the former is the larger, and thus that the alternate termination reaction (f) would be more appropriate. Unfortunately, this is (jifi), and leads to an incorrect order of one-half. There are also other predictions of temperature coefficients of reaction and foreign gas effects that are not in agreement with the experiment. This is an illustration of how carefully one must cteck all the implications of an assumed mechanism. 

By assuming that the unimolecular initiation step was in the second-order range, Kiichler and Theile [35] developed an alternate free radical result using termination Eq. (f). From Table 1.4-1 this (pp) termination for second-order initiation again leads to the proper overall first-order reaction rate  

The ratio of ethyl to hydrogen radicals can be found from the rate expression for hydrogen radicals  

At moderate pressures, this expression gives larger ethyl than hydrogen radical concentrations, and is consistent with the use of termination (f). At lower pressures, the relative amount of hydrogen radicals is larger, and increases the importance of termination (e). This (fifi) step then leads to an overall order of, which is what is experimentally observed at low pressures. 

Other possible terminations are (g) and (hX The first would require a third body, because hydrr en is an uncomplicated radical, yielding a case with f-orda- reaction. This is usually not observed, however, because of the slowness of ternary reactions. Case h could be (/l )—second orda—or orda 

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The most important commercial use of ethane and propane is in the production of ethylene (qv) by way of high temperature (ca 1000 K) thermal cracking. In the United States, ca 60% of the ethylene is produced by thermal cracking of ethane or ethane/propane mixtures. Large ethylene plants have been built in Saudi Arabia, Iran, and England based on ethane recovery from natural gas in these locations. Ethane cracking units have been installed in AustraHa, Qatar, Romania, and Erance, among others. 

Cracked Gas Drying. Ethylene and propylene are two of the most important petrochemical raw materials today. They are manufactured by a thermal cracking of ethane, propane, or naphtha. One of the important separation-purification steps in the production of ethylene and propylene is removal of water before low temperature separation. Although alumina has been the most commonly used desiccant in drying cracked gas in the past, 3A molecular sieve adsorbents have an overall economic advantage 32), and many cracked gas plants are using the 3A molecular sieves today. 

Ethylene is manufactured on a very large scale 1 by the thermal cracking of ethane in the gas phase ... 

Methane-based commercial production of ethylene via oxidative coupling has been investigated, but to date the lower per pass conversions required for acceptable ethylene selectivities combined with purified oxygen costs make this process noncompetitive with thermal cracking of ethane from natural gas liquids. 

As an example, thermal cracking of ethane will be examined here in detail. 

Example 9.2. Thermal cracking of ethane. Thermal cracking of ethane at temperatures in the range of 800 to 1200 K and ambient or lower pressures yields mainly ethene and hydrogen ... 

Thermal cracking of ethane is an excellent example of an intricate mechanism that leads to a kinetic behavior obeying a simple, first-order rate law in good approximation over a fairly wide range of conditions. It also serves to show how easily such a deceptively simple rate law is misinterpreted. Moreover, the example illustrates an important general point ... 

Examples include the hydrogen-bromide reaction, thermal cracking of ethane and n-butane, oxidation of cyclohexane, and the hydrogen-oxygen reaction. 

The thermal cracking of ethane is believed to occur by the reaction sequence given in Part (a). The specific reaction rates are given as a function of temperature ... 

Example 7-2 PSSH Applied to Thermal Cracking of Ethane Professional Referenite Shelf... 

Thermal cracking of ethane, propane, butane, naphthas, gas oils, and/or vacuum gas oils is the main process employed for the production of ethylene and propylene butadiene and benzene, toluene, and xylenes (BTX) are also produced. Thermal cracking of these hydrocarbons is also called pyrolysis of hydrocarbons. Ethylene is the organic chemical produced worldwide in the largest amoimts and has been called keystone to the petrochemical industry. This technology is well documented in the literature. Somewhat similar thermal cracking processes are used to produce vinyl chloride monomer (VCM) from ethylene dichloride (EDQ, styrene from ethylbenzene, and allyl chloride from propylene dichloride (PDC). Production of charcoal and coke from wood and coal is actually a pyrolysis process, but it is not discussed here. 

Ethylene. Ethylene is the petrochemical produced in the largest quantity in the United States. Earliest commercial production began about 1936 with the thermal cracking of ethane and/or propane (this was accomplished by companies such as Dow Chemical and Union Carbide, which are primarily chemical companies) ... 

For the thermal cracking of ethane in a tubular reactor, the following data were obtained for the rate coefficient at different reference temperatures ... 

When the density of the reaction mixture varies with the conversion, p, in Eq. 9.1-8 has to also account for this. This is illustrated in the example on the thermal cracking of ethane, later in this chapter. Pressure drop equations for packed beds will be discussed in Chapter 11. 

Example 93-2 Design of a NomsothernuU Tulmlar Reactor for Thermal Cracking of Ethane... 

Table 1 Molecular reaction scheme and kinetic parameters for the thermal cracking of ethane... 

Table 2 summarizes the results obtained when the oxygen transport rate was held constant by adjusting cell resistance and the methane feed rate varied. As the residence time at 8(X) C increases the ethylene/ethane ratio increases, although the selectivity to C2 s decreases. This is consistent with the hypothesis of thermal cracking of ethane following the oxidative coupling of methane. Similar results have been observed with heterogeneous catalysts. 

Fig. 34.9 shows the results for hydrogen extraction from thermal cracking of ethane at 800 °C. Extraction rates were close to the theoretical calculated from Faraday s law, but a large part of the voltage required resulted from ohmic resistance of the proton conductor. If the ohmic resistance is excluded, however, the voltage required is fairly low as indicated with dotted lines in Fig. 34.9b. This means that, if a very thin and non-porous film of the proton conductor were available, it could be put to practical use as an electrochemical hydrogen separator.

Koyama, H. and Dranoff, J. S., Modeling the Thermal Cracking of Ethane and Propane in a Non-Isothermal Vertical Pneumatic Transport Reactor , Ind. Eng. Chem. Res., 31, 2,265(1992). 

MOLECULAR REACTION SCHEME AND KINETIC PARAMETERS FOR THE THERMAL CRACKING OF ETHANE ... 

Polyethylene (PE) is currently the most common packaging plastic in use. It is produced from the polymerization of ethylene which in turn is obtained as a petrochemical product by thermal cracking of ethane and propane. Depending on the polymerization process conditions, different types of PE can be obtained as listed in Table 4. The density ranges given in Table 4 differ in value across different literature sources, and are dependent on the synthesis techniques used for polymerization as well as on the polymerization conditions. 

Chain reactions are very common in chemistry. One well-known chain reaction is the formation of NO in car engines. Here the initiation is the thermal decomposition of an O2 molecule at high temperature, and the thermal cracking of ethane,... 

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