Vycor glass, coking results

Thermal reactions of acetylene, butadiene, and benzene result in the production of coke, liquid products, and various gaseous products at temperatures varying from 4500 to 800°C. The relative ratios of these products and the conversions of the feed hydrocarbon were significantly affected in many cases by the materials of construction and by the past history of the tubular reactor used. Higher conversions of acetylene and benzene occurred in the Incoloy 800 reactor than in either the aluminized Incoloy 800 or the Vycor glass reactor. Butadiene conversions were similar in all reactors. The coke that formed on Incoloy 800 from acetylene catalyzed additional coke formation. Methods are suggested for decreasing the rates of coke production in commercial pyrolysis furnaces. 

The results for Run 19 (Vycor glass reactor), Run 21 (alonized Incoloy 800 reactor), and Run 14 (coke-covered Incoloy 800 reactor) were similar to both the kinetics and type of products obtained. Although neither oxygen or hydrogen pretreatments were tried in Vycor glass or alonized Incoloy 800 reactors prior to acetylene pyrolyses, it is thought that such pretreatments would have little or no effect on acetylene reactions. This conclusion is based on such pretreatments prior to pyrolysis with other hydrocarbons in these two reactors. It has been concluded that all increases in acetylene conversions above those of Runs 14, 19, and 21 were in some way caused by surface reactions. Based on this assumption, surface reactions were of major importance in Runs 15, 18, and 23. 

Visual observation of the Vycor glass reactor immediately following the butadiene run at 700°C resulted in important information. The reactor was cut to permit inspection of the black deposits thought to be primarily coke. The last two-thirds of the reactor and a short section of the unheated tube that extended beyond the furnace were covered on the inner surface with a smooth layer of coke. This deposit, when viewed from the outside of the reactor, appeared as a black mirror. It is of special interest that the inlet section of the tubular reactor did not have any coke deposits. This section was the one that was subjected to increasing temperatures in the furnace. The start of the coke deposits occurred approximately in the section where maximum temperatures occurred during a run. Most of the deposits appeared to occur in the... 

Considerable information was obtained for ethane pyrolysis relative to coke deposition on and to decoking from the inner walls of a tubular reactor. Both phenomena are affected significantly by the materials of construction (Incoloy 800, stainless steel 304, stainless steel 410, Hastelloy X, or Vycor glass) of the pyrolysis tube and often by their past history. Based on results with a scanning electron microscope, several types of coke were formed. Cokes that formed on metal tubes contained metal particles. The energy of activation for coke formation is about 65 kcal/g mol. 

Table I presents results for six comparable pyrolysis runs made by using five laboratory reactors all runs were made with approximately 50% steam as diluent in the ethane feed. Conversions at the exit end of the reactor varied from 59% to 65%. Also, results reported for a commercial unit (11) are shown. Ethylene yields varied from about 78% to 89% in all cases except for run D44 made in the stainless steel 304 reactor. In that run, the ethylene yields were very low but production of CO, GOo, and net coke were much higher. Ethylene yields were highest in the run made in the Vycor glass reactor. In this run, coke formation was least of all runs, and no CO or C02 was detected in the product stream.

Data points for all runs of this investigation, including some up to four seconds residence time, all fell within the predicted limits, as shown in Figure 3. The results obtained when using the Vycor glass reactor were close to the zero coke line. With metal reactors, depending on the... 

Coking Results Temperature, precursor type, precursor partial pressure, and run duration all significantly affected the amount of coke for Incoloy 800 and aluminized Incoloy 800 coupons and Vycor glass. The... 

The second broad grouping of papers (Chapters 13-17) describes useful design modifications for commercial plants. Surface reactions resulting in both the formation of coke and carbon oxides, and the destruction of olefins and other desired products are described. In quartz or Vycor glass reactors, however, such surface reactions are relatively unimportant and often have not been considered. Yet such reactions can be most significant in metal reactors when steam is used as a diluent of the entering feedstock. Data obtained in metal reactors are of value to the designer of plant equipment. 

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