Coking, transitions

End effects were likely important for Incoloy 800 coupons. Relatively coke—free surface metal) enriched in both chromium and titanium> was visible at the upstream and/or downstream ends of several Incoloy 800 coupons subjected to 0.05 atm. acetylene at 800°C. Numerous small craters> or holes> were observed on the metal. A few isolated filaments protruded from the surface> and sparse amounts of globular coke were also detected at the ends. The bulk of the coke deposit) which consisted of filaments intermixed with larger amounts of globular coke (Figure 8A)> occurred near the midsection of the coupon. A matrix of smooth and rather solid carbon was visible at the metal to coke transition regions the smooth matrix lay under the globular coke. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

The transition resistance between the surface of the metal and the electrolyte with uncoated iron anodes in coke backfill, the transition resistance is usually low. With metals in soil, it can be increased by films of grease, paint, rust or deposits. It contains in addition an electrochemical polarization resistance that depends on the current [see Eq. (2-35)]. 

Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects (a) formation of an otherwise possible product is blocked because it cannot fit into the pores, and (b) formation of the product is blocked not by (a) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (a) is involved in zeolite catalyzed reactions which favor a para-disubstituted benzene over the ortho and meso. The low rate of deactivation observed in some reactions of hydrocarbons on some zeoUtes has been ascribed to (b) inhibition of bimolecular steps forming coke. 

It should be noted here that since the original work done at Mobil was completed, there have been new developments published in the literature. Ishida and Wen (1968) analytically solved a special case for the transition region when the reaction rate does not depend on the local solids (coke) concentration. Wen (1968) has also numerically solved the more general problem for certain kinetic forms, and Amundson and co-workers have done much work on the diffusion and reaction in the boundry layer about a carbon particle (Caram and Amundson, 1977). We will not attempt to review the literature or compare the more accurate numerical solutions with our ad hoc approximation technique. However, we note that our technique was simple, fit the experimental and commercial data extremely well, and provided us with valuable insight and understanding... 

C. D. Prater, J. Wei, V. W. Weekman, Jr., and B. Gross, A Reaction Engineering Case History Coke Burning in Thermofor Catalytic Cracking Regenerators Costei D. Denson, Stripping Operations in Polymer Processing Robert C. Reid, Rapid Phase Transitions from Liquid to Vapor John H. Seinfeld, Atmospheric Diffusion Theory... 

Nickel aluminate, a spinel, has long been known to trap nickel. Metals like arsenic(19), antimony(20-21) and bismuth(20) are known to passivate transition elements and can be used to decrease and coke make. Sulfur is also a known inhibitor for nickel therefore, higher sulfur-containing crudes may be a little less sensitive to nickel poisoning. In our work we also found that nickel at low concentrations is actually a slight promoter of the cracking reaction when incorporated into a molecular sieve (Figure 17). 

The oxidation of carbon on the catalyst surface proceeds through formation of solid surface oxides that decompose to CO and CO2 as primary products. Previous studies have shown the CO2/CO ratio at the catalyst surface is a function of temperature (Arthur s Ratio) and is typically 1.0 for FCC catalyst and conditions [3]. However, the CO exiting the bum site can be further oxidized to CO2 at a rate dependent on temperature, CO, O2, H2O, active metals on the catalyst, and even the presence of the catalyst itself. Also, transition metal oxides have been found to increase the coke... 

The catalytic isomerization of 1-methylnaphthalene and all lation of 2-methylnaphtha-lene with methanol were studied at ambient pressure in a flow-type fixed bed reactor. Acid zeolites with a Spaciousness Index between ca. 2 and 16 were found to be excellent isomerization catalysts which completely suppress the undesired disproportionation into nwhthalene and dimethylnaphthalenes due to transition state shape selectivity. Examples are HZSM-12, H-EU-1 and H-Beta. Optimum catalysts for the shape selective methylation of 2-methylnaphthalene are HZSM-5 and HZSM-li. All experimental finding concerning this reaction can be readily accounted for by conventional product shape selectivity combined with coke selectivation, so there is no need for invoking shape selectivity effects at the external surface or "nest effects", at variance with recent pubhcations from other groups. 

Catalysis of 12-membered zeolites, H-mordenlte (HM), HY, and HL was studied In the alkylation of biphenyl. The para-selectlvltles were up to 70% for Isopropylblphenyl (IPBP), and 80% for dllsopropylblphenyl (DIBP) In HM catalyzed Isopropylatlon. Catalysis of HY and HL zeolites was nonselectlve. These differences depend on differences In pore structure of zeolites. Catalysis of HM to give the least bulky Isomer Is controlled shape-selectlvely by sterlc restriction of the transition state and by the entrance of IPBP Isomers. Alkylation with HY and HL Is controlled by the electron density of reactant molecule and by the stability of product molecules because these zeolites have enough space for the transition state to allow all IPBP and DIBP isomers. Dealuminatlon of HM decreased coke deposition to enhance shape selective alkylation of biphenyl. 

The results described above suggest that the methylation of naphthalene over MFI- metallosilicates occurs inside crystalline pores by a restricted transition-state mechanism, not with unordered sites at or near external sites. Weaker acid sites preferentially form 2,6-DMN and favor a decease in coke formation. 

---