Vycor surfaces, coke deposition

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... 

Formation and Removal oi Coke Deposited on Stainless Steel and Vycor Surfaces from Acetylene and Ethylene... 

Surface reactions including carbon (or coke) deposition (on the reactor surface) varied significantly in the reactors investigated. Table I and Figure 8 show carbon results for several runs in the 304 stainless steel, Incoloy, and Vycor reactors. 

To further clarify the role of geometrical effects> the following experiment was performed. Three layered pairs of Incoloy 800 coupons) each pair consisting of one coupon atop a second) were placed in three separate 2.5 cm. lengths of Vycor tubing. One pair was positioned near the furnace midpoint) and the other two pairs were positioned 5 cm. upstream and 3 cm. downstream of the midpoint, respectively. Coke was deposited from 0.05 atm. acetylene at 800°C. Upon completion of the 8 hour run, all three coupon pairs had to be pried apart to separate them. More coke remained on the top coupons than on the bottom coupons (Table 1) for each pair. The top surfaces of all three top coupons exhibited similar types of coke each was covered with a dense layer of spherical-... 

Surface deposits were noted In the Vycor reactor following the experimental runs. These dark brown or black deposits occurred primarily at the exit end of the reactor. These deposits were oily and tarry In nature, and were probably coke and condensed heavy hydrocarbons. 

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... 

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