Influence of Space Time

1 Influence of Space Time. Studies on the effect of space time on the methanol conversion and the hydrocarbon distribution clearly showed that Cj-Cg olefins are intermediates in the conversion of methanol to gasoline. This is illustrated in Table 8, which presents the yields of light olefins, aromatics and paraffins as a function of space time and methanol conversion. 

The lower part of Table 8 shows the distribution of light olefins within the hydrocarbon fraction. It is clear that at low space times the main hydrocarbons are olefins, but the yields are very low due to the low conversion. The yield of light olefins increases with space time and reaches a meiximum, indicative of their intermediate character. In the particular example of Table 8 this maximum (16.2 g/100 g MeOH fed) is reached at a conversion of 90%. At this point, however, the Cj-C olefins constitute only 45.7 wt% of the hydrocarbon fraction. An important feature of the methanol conversion on ZSM-5 is that, although the conversion is complete, a further increase of space time results in a continuing change of the hydrocarbon distribution. Therefore, care must be taken in the interpretation and comparison of results at 100% conversion. 

At very low space times the conversion of methanol or dimethylether to hydrocarbons is very slow, but in a narrow range of space times the rate accelerates rapidly as the concentration of hydrocarbons increases. This observation was interpreted as an indication of the autocatalytic nature of the reaction. According to Espinoza, the sudden increase in the conversion of methanol to hydrocarbons with space time is simply related to the required concentration of the intermediate dimethylether. With a dimethylether feed, the jump in conversion was not observed. In a batch reactor the methanol conversion also showed an induction period, whereas this was absent when dimethylether was fed. The induction period was not observed when propylene was co-fed with methanol. 

The autocatalytic effect was supported by experiments using a gas-recirculation system the methanol reaction was greatly 

Wu and Kaeding correlated the product distribution of the conversion of methanol on ZSM-5 catalysts with the a-value, a measure of the catalyst activity. Their results, at complete conversion of methanol, demonstrated that only catalysts with a lower than 30 are able to obtain high selectivities toward light olefins at high conversion. It was also advised that HZSM-5 catalysts with very high Si/Al-ratio (800) should be used to avoid diffusion disguise of the true intrinsic reaction path. °  

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