Aromatics paraffin/olefin conversion

Light hydrocarbons (Ci to C4) and aromatics (mainly Ce to Ce) were produced by ZSM-5 due to the the conversion of olefins and paraffins. Thus,these results provide evidence for cracking of olefins, paraffins and cyclization of olefins by ZSM-5 at 500 C. The steam deactivated ZSM-5 catalyst exhibited reduced olefin conversion and negligible paraffin conversion activity. 

Fig. 4 (right) Autocatalysis and retardation. Yields of paraffins, olefins, aromatics and "coke" as a function of duration of the experiment. Methanol conversion on HZSM5, 270 °C, Pqi oqu =2.5 bar, WHSV = 1 h. Differential coke yield abtained through internar standard). 

It is necessary, however, to maximize the intermediate olefin product at the expense of the aromatic/paraffin product which makes up the gasoline ( ). The olefin yield increases with increasing temperature and decreasing pressure and contact time. Judicious selection of process conditions result in high olefin selectivity and complete methanol conversion. The detailed effect of temperature, pressure, space velocity and catalyst silica/alumina ratio on conversion and selectivity has been reported earlier ( ). The distribution of products from a typical MTO experiment is compared to MTG in Figure 4. Propylene is the most abundant species produced at MTO conditions and greatly exceeds its equilibrium value as seen in the table below for 482 C. It is apparently the product of autocatalytic reaction (7) between ethylene and methanol (8). 

Selectivity results at constant 50% conversion are reported in Tables VI and VII for calcined and steamed zeolites, respectively. Product selectivities are divided into light gas (C1-C4), gasoline (C5-C12) and coke. The gasoline fraction is further divided into paraffin, olefin, naphthene and aromatic (PONA) components. 

For MTO, where the object is to optimize the olefins, we raise the temperature to about 500°C, which favors olefin formation. We also modify the catalyst to slow down the conversion of olefins to aromatics and paraffins. These changes produce a dramatic change in the reaction path. As Figure 17 shows, we have now decoupled the aromatics + paraffins plot from the olefins plot. 

The overall path of methanol conversion to hydrocarbons over ZSM-5 is illustrated in Fig. 2. Methanol and dimethyl ether (DME) form olefins, which are then converted to naphthenes, aromatics, and paraffins. Olefins initially react by oligomerization and methylation, and at increasing conversion olefins distribution is governed by kinetics. This effect, and the effects of process variables were summarized by Chang (ref. 14). The directional effects of process and catalyst variables on the MTO reaction are summarized in Table 3. 

Methanol Conversion. Methanol conversion reactions based on borosilicate catalysts have been studied extensively (10.15,24,28.33.52-54). During the conversion of methanol, the reaction proceeds through a number of steps, to yield dimethylether, then olefins, followed by paraffins and aromatics. The weaker acid sites of borosilicate molecular sieves relative to those of aluminosilicates require higher reaction temperatures to yield aromatics. The use of less forceful process conditions leads to the formation of olefins selectively, instead of a mixture of paraffins, olefins, and aromatics (10.28.53.54). 

Klyueva et al. have investigated the acidic properties of erionite modified by isomorphous substitution of B , Ga , and Fe " " by Si and Al . The incorporation of these elements in the aluminosilicate framework led to the generation of new acid centers. These acid centers have a lower concentration of aluminum cations than aluminosilicates, leading to s unples with lower acidity. Consequently, the rate of reactions involving hydrogen transfer, like olefin conversion into paraffins, was lower on isomorphous-substituted erionite samples. Table 5 shows that this enhanced the selectivity toward light olefins. The production of aromatics may... 

Sedran et al. revised the corrected model of Chang and found that it was able to predict effectively the product distribution (oxygenates, olefins, and aromatics/paraffins lumps) at various conversion levels and temperatures. 

The individual product components were separated from Ci to C12 by a capillary column and detected by FID. The products were divided into five groups, viz. gas oil (b.p. >221 °C), gasoline (C5-22I °C), LPG (C3-C4), dry gas (C1-C2) and coke. The gasoline fraction was further divided into paraffins, olefins, naphthenes and aromatics. The conversion was defined as the weight fraction of all products boiling below 221 °C plus coke. 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

The conversion proceeds through dimethyl ether as an intermediate and the products are paraffins, aromatics, cycloparaffins, and +olefins, all of which must involve alkylation reactions catalyzed by the strong acid function of the zeoHte. This technology represents a significant advancement in the potential for using coal as a raw material for gasoline and hydrocarbons. 

The fit of these equations to the data is very good, as seen in Fig. 18. These equations are valid to very small values of CO concentrations, where the reaction becomes first order with respect to CO. In a mixture of CO with oxygen, there should be a maximum in reaction rate when the CO concentration is at 0.2%, as shown in Fig. 19. When the oxidation of olefins and aromatics over a platinum loaded monolith is over 99% complete, the conversion of higher paraffins may be around 90% and the conversion of the intractable methane is only 10%. 

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