Paraffins, methanol conversion

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). 

The reaction mechanism for the conversion of methanol to hydrocarbons over molecular sieve catalysts has been extensively investigated over the past 25 years. It is widely accepted that methanol conversion initially proceeds through equilibration with DME. Early work with ZSM-5 showed that light olefins are then the initial hydrocarbon products, followed by heavier olefins, paraffins and aromatics (Figure 12.5) (2). 

On the other hand, it was proposed that acid catalyzed reactions such as skeletal isomerization of paraffin [2], hydrocracking of hydrocarbons [3] or methanol conversion to hydrocarbon [4] over metal supported acid catalysts were promoted by spillover hydrogen (proton) on the acid catalysts. Hydrogen spillover phenomenon from noble metal to other component at room temperature has been reported in many cases [5]. Recently Masai et al. [6] and Steinberg et al. [7] showed that the physical mixtures of protonated zeolite and R/AI2O3 showed high hydrocracking activities of paraffins and skeletal isomerization to some extent. 

Methanol, which is relatively easily derived from methane and is also readily purified, avoids some of the contamination problems of the higher, n-paraffin carbon sources. Also methanol is easily put into aqueous solution at any desired concentration. This is the basis of Id s process to manufacture protein from methanol using Methylophilus methylotrophus, on a scale of 30,000-50,000 tonne/year [66] (Fig. 16.9). Recombinant DNA technology was employed to raise the efficiency of methanol conversion by this organism. The dry product trade named Pruteen contains 72% protein and is suitable for animal feed supplementation. 

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. 

Since the measurement of on-line catalyst activity is difficult, we found it convenient to follow an on-line "reaction index" (RI), which is a selectivity ratio. The complex MTO reaction scheme can be presented schematically as A —> B —> C, where A represents methanol and DME, B - olefins, and C -aromatics and paraffins (Fig. 3). One particularly useful RI is the propane/propene ratio. Propene is the primary light olefin and propane represents paraffins. The propane/propene RI can be easily monitored by an on-line GC. We found that hydrocarbon selectivities correlate well with RI. For fixed hydrodynamics, it also correlates well with methanol conversion. 

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). 

Methanol Conversion to Olefins. - Chabazite, erionite, zeolite T, and ZK-5 have been used by Chang et al. for the conversion of methanol into olefins. The C2-C4 olefin concentration in the hydrocarbon fraction was always less than 60 wt% at 100% methanol conversion. It follows from Table 3 that the hydrocarbon fraction becomes richer in Cj-C olefins as the conversion of methanol decreases. That is because the conversion of olefins to paraffins is lower. Hydrocarbon fractions with more than 80 wt% of Cj-C olefins were attained with a dealuminated H-erionite, but the conversion of methanol was very low. 

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. 

Romannikov et al. studied the catalytic properties of beryllium-silicates with a zeolite-type structure. Methanol conversion yielded primarily olefins, while on the isostructural aluminum-silicate catalyst paraffins and aromatics were obtained. The methanol conversion was considerably higher on the [AlJ-ZSM-5 than on the [Be]-ZSM-5, except in one example (Table 12). 

Marchi and Froment observed that on dealuminated mordenite the selectivity changed with time on stream. The yield of C2-C4 olefins increased, while the yields of paraffins and aromatics dropped, even if the methanol conversion was maintained at 100% (see Figure 10). Coke would already be formed at early stages of the methanol conversion and would cover the strong acid sites, thus reducing the conversion of olefins to paraffins and aromatics. 

Hierarchical (or mesoporous) zeolites became the focus of the review by Christensen et al. [7]. The main reason behind the development of hierarchical zeolites is to achieve heterogeneous catalysts with an improved porous structure and thereby enhanced performance in alkylation of benzene with alkenes, alkylation, and acylation of other compounds, methanol conversion into hydrocarbons, aromatization processes, isomerization of paraffins, cracking of diverse substrates and raw materials (naphtha, aromatic compounds, hexadecane, vacuum gas oil, and some polymers), and hydrotreating. The reactions that are of interest from the point of view of fine chemicals synthesis occurring on hierarchical zeohtes include aldol condensation, esterification, acetalization, olefin epoxidation, and Beckmarm rearrangement. 

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 direct conversion deals with the straight hydrogenation of carbon monoxide to paraffins, olefins and heteroatom (oxygen, nitrogen) containing products. The indirect conversion invokes intermediates such as methanol, methyl formate and formaldehyde. The latter ones in a consecutive reaction can yield a variety of desired chemicals. For instance, acetic acid can be synthesized directly from CO/H2, but for reasons of selectivity the carbonylation of methanol is by far the best commercial process. 

The initial dehydration reaction is sufficiently fast to form an equilibrium mixture of methanol, dimethyl ether, and water. These oxygenates dehydrate further to give light olefins. They in turn polymerize and cyclize to form a variety of paraffins, aromatics, and cycloparaffins. The above reaction path is illustrated further by Figure 3 in terms of product selectivity measured in an isothermal laboratory reactor over a wide range of space velocities. ( 3) The rate limiting step is the conversion of oxygenates to olefins, a reaction step that appears to be autocatalytic. In the absence of olefins, this rate is slow but it is accelerated as the concentration of olefins increases. 

The conversion of methanol into olefins is similar to the commercially proven methanol to gasoline (MTG) which was commercialised using natural gas as the feedstock in New Zealand. The variant generally uses similar catalysts to produce light olefins only, rather than the iso-paraffins and aromatics of the MTG process. This leads to the prospect of coal or gas conversion into resins (solids). These high value products may be easier to transport and sell than liquid fuels Figure 11.6 illustrates the basic unit operations for the process. 

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