Surface reactions during pyrolysis

The present results clearly confirm the Importance and complexity of surface reactions during pyrolysis reactions. Obviously, the composition of the Inner surface of the reactor Is of Importance relative to the level and types of surface reactions. In addition, valuable new Information has been obtained concerning the role of coke In affecting more coke formation. Although the deposition of coke on the walls of a metal reactor decreases the activity of the reactor. It Is of Interest that the surface activities of coke-covered metal reactors always remained higher than those for the Vycor reactor. Lobo and Trimm (11) have Indicated that carbon without contaminants Is Inactive. Based on this finding, metal contaminants were presumably present In the coke formed. Other Investigators (10, 11) have found both nickel and Iron contamination of various cokes. Furthermore, coke Is sometimes reported to be autocatalytic In nature. The evidence that olefins and other hydrocarbons adsorbed on the surface and... 

How does the composition of the metal surface change during pyrolysis Surface reactions that have been identified on Incoloy 800 surfaces include oxidation, reduction, sulfidation, desulfidation, and coke formation (14). Do increased concentrations of nickel and chromium ever occur in the surface in view of the fact that iron is incorporated into the coke Tsai and Albright (14) found increased iron concentrations on inner surfaces of tubes used for pyrolyses. 

One way to produce a rapid heat transfer to the sample is to diminish the sample size [5]. This implies that the amount of heat required by the sample to reach a certain temperature is small and that the heat can be transferred rapidly. Typical sample sizes in analytical pyrolysis vary from a few ng to a few mg. A small sample size is, however, related to other effects, some advantageous and some not. Secondary reactions during pyrolysis are diminished for a small sample, but the contact with metal surfaces may... 

Figure 9. Approximate levels of two major surface reactions during ethane pyrolysis comparison of experimental results in metal reactors...

Termination reactions also occur when a free radical in the gas phase reacts or couples with a free radical on a solid surface. The solid coke formed as a by-product during pyrolysis is essentially pure carbon, which has numerous free radicals on its surface. The exothermic heats of such... 

I) and of Brown and Albright (2), who earlier studied surface reactions that occur during the pyrolysis of hydrocarbons. Such pyrolyses are used for commercial production of ethylene, other olefins, diolefins, and, to some extent, aromatics. Several important reactions occur on the inner surfaces of the high-alloy steel tubes used for pyrolyses. These surface reactions occur simultaneously and, to some extent, consecutively along with the gas-phase reactions that produce the desired products of... 

Thermal diffusivity measurements were performed on pellets by the use of a transient response technique. Pellets of cellulose or char were instrumented with thermocouples at known distances from the surface. Samples were then brought into contact with a hotplate whose surface temperature varied in an approximately sinusoidal manner. Samples of virgin cellulose could only be tested up to surface temperatures of about 500 K, to avoid pyrolysis. Char samples could be tested to temperatures up to those seen during pyrolysis (in excess of 750 K). Samples were covered by a bell jar, purged with nitrogen, in order to avoid reactions with oxygen. 

Tsai, T.C. Albright, L.F. Surface reactions occurring during pyrolysis of light paraffins. In Industrial and Laboratory Pyrolysis] Albright, L.F. Crynes, B.L., Eds. ACS Symposium Series, 32 American Chemical Society Washington, D.C., 1976 Chapter 16. 

For Sample CA36, our results indicate that 61% of the sulfur is evolved as H2S. On the surface, this appears inconsistent with Reaction (1). However, if 20% of the original sulfur is organic and is completely removed during pyrolysis, as suggested... 

It is also possible that the higher atom temperatures used in this study compared with those previously reported may have resulted in the hydrocarbon-producing reactions occurring closer to the surface of the carbon target. If indeed this is the case, the opportunity for reaction product pyrolysis to occur during diffusion out of the pores in the carbon target would be reduced. 

A recent study by Tsai and Albright (1975) clearly indicated some of the important surface reactions occuring in reactors constructed of different metals during the pyrolysis of light paraffins. These reactions include formation of carbon, removal of carbon, oxidation of metal surfaces, reduction of surface oxides, formation of metal sulfides, and destruction of metal sulfides. 

Tsai, C., and Albright L., "Surface Reactions Occurring During Pyrolysis of Light Paraffins," ACS Meeting, Philadelphia (April, 1975). 

Surface reactions producing coke, carbon oxides, and hydrogen occur during the pyrolysis of ethane, propane, and other hydrocarbons (1,2,3). Such surface reactions can be expected to be of relatively greater Importance when smaller diameter (and hence higher surface-to-volume (S/V) ratio) reactors are used. The relative Importance of surface reactions also Increase as the operating pressures for pyrolysis decrease lower pressures decrease the ratio of surface to mass of hydrocarbons In the reactor. As compared to commercial units, laboratory units use much smaller diameter reactors, and frequently they operate at lower pressures. Hence surface reactions are relatively more Important In laboratory units. This factor Is an Important, If not the major, reason why there are often significant differences In the products obtained for ethane pyrolysis In the laboratory as compared to commercial units. 

Small differences in the kinetics of pyrolysis may have occurred in different reactors see, for example, the results shown in Figures 1 and 3. Such differences could have been caused by one or more factors. One factor that likely is of some importance is small differences in the gas temperature heat transfer obviously depends to some extent on the materials of construction. Small differences in residence times at reaction conditions could also have occurred in the various reactors that did not have exactly the same internal volumes. In addition, surface reactions may affect the kinetics of the reactions. Some initiation or termination of free radicals may have occurred at the reactor surfaces. Probably at least some hydrogen free radicals were formed during coke formation. 

Preliminary investigations (3,4,5) have shown that surface reactions are important during the pyrolysis of propane at temperatures of commercial importance. Until now, however, experimental data were not available to make direct comparisons between reactors constructed of different materials or between propane and ethane relative to surface reactions. Such information has now been obtained in the present investigation. 

Clearly the activity of the surface relative to promotion of surface reactions differed depending on whether ethane or propane was used as a feedstock. This finding Is not surprising since Tsai and Albright (5) report that dynamic equilibria occur during pyrolysis relative to the following oxidation of surface with steam vs reduction of surface with gaseous components and carbon (or coke) deposition vs removal of coke with steam. The present results are, however, the first to show conclusively that surface activity depends on the feedstock surface activity must be related In some way to the levels of both surface oxides and surface carbon. 

Coke and carbon oxides, both undesirable by-products, are always formed to some extent In commercial pyrolysis units. The carbon oxides, are produced when part of the coke reacts with steam that Is used as a diluent with the hydrocarbon feedstock. Most, If not all, of these undesired products are formed by surface reactions that reduce the yields of olefins and other desired products. Coke also acts to Increase heat transfer resistances through the tube walls, and most pyrolysis units must be periodically shut down for decoking of the tubes. During decoking, pure steam or steam to which a small amount of oxygen (or air) Is added Is fed to the reactor, and the coke Is oxidized to produce carbon oxides. 

In the present Investigation, considerable Information has been obtained to clarify the role of the surface reactions that occur during pyrolysis. Reactions Investigated Include the formation and destruction of metal oxides and metal sulfides. Information has also been obtained relative to coking and decoking. 

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