Selectivity, pyrolysis

Assuming that pyrolysis selectivity Is not affected by SC water, rate constants of pyrolysis at various SC water densities were calculated with naphthalenes as pilot compounds and the "dry experiment as the reference pyrolysis rate. The rate constant of hydrolysis ki, 2 subsequently followed as the difference of total ether conversion rate constant and pyrolysis rate constants. 

To achieve pyrolysis, selective chemical treatment has to be accomplished and Lewis [81] lists the major reactions which can be used in the pyrolysis of aromatic hydrocarbons ... 

A hydrocarbon system undergoing pyrolysis is a most complex mixture of molecules and free radicals which react simultaneously with one another in a multitude of ways. Based on established theories supported by experimental data, the production of olefins and diolefins — that is, pyrolysis selectivity towards olefins and diolefins — has been found to be favored by short residence times and low hydrocarbon partial pressures (2,3,4). Pyrolysis reactor selectivity has been expressed as a function of the following two parameters ... 

The bulk residence time, 6b, which is the more commonly used parameter to correlate pyrolysis selectivity, expresses only the length of time that a mass of gas spends in the pyrolysis coil. Bulk residence time is not adequate to represent the combined effects of temperature and chemical reaction which take place in the pyrolysis coil as witness by the failure to correlate the available experimental data as a function of bulk residence time especially when data from pyrolysis coils of different configurations and, therefore, different temperature profiles are considered.(3)... 

Pilot plant, prototype and commercial data on pyrolysis selectivity collected by Lummus have been correlated as a function of the average residence time and the average hydrocarbon partial presure. A method of correlation which has proved highly successful is illustrated in... 

Figure 1. Each sloped line represents the loci of all possible combinations of average residence times and hydrocarbon partial pressures which are consistent with a fixed pyrolysis yield pattern, i.e., constant pyrolysis selectivity lines. For liquid feedstocks, the methane-to-ethylene ratio found in the pyrolysis reactor effluent has been used as a good overall indicator of pyrolysis reactor selectivity. Low methane-to-ethylene ratios correspond to a high total yield of ethylene, propylene, butadiene and butylenes. Consequently, the yields of methane, ethane, aromatics and fuel oil are reduced. TL refore, each constant pyrolysis selectivity line shown in Figure 1 is identified with a fixed methane-to-ethylene ratio. This specific selectivity chart applies to a Kuwait heavy naphtha which is pyrolyzed to achieve a constant degree of feedstock dehydrogenation, i.e., a constant hydrogen content in the effluent liquid products, which in this case corresponds to the limiting cracking severity.

The results of this correlation work have shown that, while the average residence time and hydrocarbon partial pressure are both key factors in determining pyrolysis selectivity, the average hydrocarbon partial pressure is somewhat more important than previously realized by investigators. 

Two commercial pyrolysis coils of different geometry were designed to operate on a constant pyrolysis selectivity line. These coils will be referred to in this work as Coil 1 and Coil 2. Pyrolysis Coil 1 consisted of... 

Identical pyrolysis selectivity toward the production of olefins, i.e., same pyrolysis selectivity line as shown in Figure 3, at the same feestock conversion. 

These results consitute one of many confirmations of the slope of the constant pyrolysis selectivity lines previously discussed. 

The two commercial coil designs just discussed have identical pyrolysis selectivity despite their differences in geometrical and process characteristics. The axial gas temperature and partial pressure profiles constitute the major differences. The effect of axial temperature and axial partial pressure profiles on pyrolysis reactor selectivity are taken into account in the definitions of average residence time and hydrocarbon partial pressure. Therefore, when pyrolysis coils of different geometries and thus different temperature and partial pressure profiles are com-... 

The pyrolysis selectivity chart shown in Figure 3 and confirmed by the experimental results of this pyrolysis study is also supported by kinetic considerations. 

Pyrolysis selectivity is a function of the relative rates of the primary reactions (olefins forming reactions) to the secondary reactions (olefins consuming reactions). 

For two pyrolysis coils of different configurations and operating at a different combination of average residence times and hydrocarbon partial pressures, the pyrolysis selectivity is considered identical when ... 

The application of Equation (15) to pyrolysis coils operating at average residence times in the millisecond region leads to the important conclusion that the constant pyrolysis selectivity lines are not straight. A typical pyrolysis selectivity chart extended into the millisecond region is schematically shown in Figure 9. 

The above analysis of the factors affecting pyrolysis selectivity now permits the selection of a locus or conditions (i.e., a selectivity line) which will achieve a given pyrolysis selectivity and thus, a certain yield structure. Since this constant selectivity line covers a wide range of combinations of average residence times and average hydrocarbon partial pressures, the question remains what other factors should be considered by the designer to select a point on this selectivity line which is optimum for large-scale commercial production of olefins ... 

The effect of increased pressure drop per unit length has been demonstrated in the pyrolysis selectivity experiment described above. This experiment showed that the shorter residence time coil, i.e. Coil 1, because of the higher pressure drop, and therefore higher average hydrocarbon partial pressure, achieved the same selectivity as the larger diameter, longer residence time, lower partial pressure coil. 

In summary, not only will a large tube coil coke at a lower rate, but also the effect of whatever coke does form is less significant in terms of continued pyrolysis selectivity. 

A given pyrolysis selectivity can be achieved by designing for an average residence time/average hydrocarbon partial pressure combination which can be anywhere on a locus of points which describe a line of constant selectivity. 

It has been experimentally shown that the pyrolysis selectivity of two coils of different geometry, tube diameters and temperature profiles can be identical provided their average residence times and hydrocarbon partial pressures fall on the same selectivity line. The same conclusions have also been drawn from kinetic considerations. This approach has been used to extend the pyrolysis selectivity lines into the millisecond region. 

The pyrolysis selectivity correlation and the coking models have been combined with momentum heat and mass transfer models to design pyrolysis coils. This application has led to the SRT III pyrolysis reactors which emphasizes low hydrocarbon partial pressure by employing a coil design with large diameter outlet tubes. 

Fernandez-Baujin, J.M. Factors Affecting Pyrolysis Selectivity," Safety and Reliability of Large Single Train Ethylene Plants, Unpublished Report, New York, May, 1974. 

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