Inactive coke

The results of this investigation and particularly of those with butadiene strongly suggest that at least portions of the inactive coke formed during pyrolyses involve the following sequence of events (a) production in the gas phase of unsaturated hydrocarbons, (b) chemical condensation or polymerization of unsaturated hydrocarbons to produce rather heavy hydrocarbons, (c) physical condensation of these heavy hydrocarbons as liquids on the reactor walls or in the transfer line exchangers, and (d) decomposition of the liquids to coke (or tars) and hydrogen. This sequence of events is essentially identical to the one proposed by Lahaye et al. (12) for coke production from cyclohexane, toluene, or n-hexane. 

Although more information is needed to determine details concerning factors that favor inactive coke formation, relatively high levels of surface sulfides probably promote formation of such coke. On the other hand, metal oxides on the surface likely favor production of active coke. Sulfiding the reactor tube immediately upon completion of the decoking step would form metal sulfides. An aluminized surface, such as provided by the alonized Incoloy 800 reactor, also has been found to be an effective way to prevent the production of active coke. Quite possibly, the initial type of coke formed on the just-cleaned tube would have an important effect on the length of time a reactor tube could be used in a commercial plant before decoking would be required. 

When the catalyst was exposed to i-butanol at 500°C, the coke formation was relatively fast. 1.5 wt% coke was deposited during 10 minutes. The formation of olefins from methanol was lower for this precoked sample. This is probably due to n-butene formation, originating from i-butene isomerization at 500°C. n-Butene probably forms inactive coke in the pores or at the pore openings, which results in significant deactivation. 

Therefore, the coke in the MTO and DTO should be divided in two categories Inactive coke formed from olefins having a deactivating effect and active coke formed from oxygenates having a promoting effect. The activity at different coke contents depends on the ratio of active to inactive coke. 

The coke, which was recorded as the total mass increase in the TEOM reactor, can be divided into active coke formed from oxygenates and inactive coke formed from olefins. The active coke promotes the conversion to olefins and could in fact represent the true surface intermediate of the reaction. 

The two limiting cases for the distribution of deactivated catalyst sites are representative of some of the situations that can be encountered in industrial practice. The formation of coke deposits on some relatively inactive cracking catalysts would be expected to occur uniformly throughout the catalyst pore structure. In other situations the coke may deposit as a peripheral shell that thickens with time on-stream. Poisoning by trace constituents of the feed stream often falls in the pore-mouth category. 

The amount of coke decreased drastically above 0.25M. Since U.D. is closely related to the amount of framework aluminum, the good correlation between the amount of coke and U.D. suggests that coke forms on Br nsted acid sites in zeolite. The removal of framework aluminum corresponding to BrjJnsted acid may be effective for decreasing coke formation. Furthermore, this result also indicates that the active iron cluster is inactive for coke formation in spite of high activity for toluene disproportionation. 

The activity and selectivity of catalyst HZSM5-1 was constant over 1 hour on stream although the formation of a linear hydrocarbon species at the catalyst was noticed. The IR spectra suggest that this species is a linear aliphatic hydrocarbon (coke precursor) increasing in concentration with time on stream [13], It is concluded that this species is adsorbed at the (catalytically inactive) Si-OH groups of the catalyst. During our measurements, the catalytically active Si-OH-AI groups were not blocked by this surface species and the product selectivity was not altered... 

Based on the results of this run, the coke formed on the Incoloy 800 surface is quite inactive. Probably even lower conversions would have occurred if the run had continued longer and if more coke had been allowed to form on the surface. Unfortunately, no attempt was made to inspect or analyze the coke formed from benzene. It would be of special interest to determine how much metal was incorporated in the coke formed. 

The consideration of coke evolution. Its increase can contribute to deactivation by pore blocking. Due to the nature of the coke precursors (irreversibly adsorbed growing polymer chains), it is postulated that these chains in pores of high radius become partially inactive. 

Toxicity of coke molecules number of strong acid sites that one coke molecule renders inactive. 

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