Methanol partial pressure

The methanol transformations discussed precedingly can be modified to produce high amounts of light alkenes.437 454 474 475 The key to achieve this change is to prevent C2-C4 olefinic intermediates to participate in further transformations. Such decoupling of alkene formation and aromatization can be done by the use of small-pore zeolites or zeolites with reduced acidity. Reduced contact time and increased operating temperature, and dilution of methanol with water to decrease methanol partial pressure, are also necessary to achieve high alkene selectivities. This approach has led to the development of the MTO (methanol-to-olefin) process, which yields C2-C5 alkenes with about 80% selectivity. 

The reaction of tert-butyl alcohol and methanol to form MTBE is also catalyzed by heteropoly compounds (221-223). A relationship was found between the amount of pyridine sorbed in or on heteropoly compounds and tert-butyl alcohol conversion (221). The dependence of the rate on methanol partial pressure resembles that for the absorption of methanol in the bulk, suggesting pseudoliquid phase behavior (223). 

In the MTG process, durene is mostly formed by alkylation of lower molecular weight aromatics with methanol and/or ether. Low methanol partial pressures and high reaction temperatures tend to reduce the durene level, presumably by reducing the concentration overlap of methanol/ether with the aromatics formed. This overlap tends to increase with larger catalyst particles. As a consequence the early fixed-bed development work was conducted using 1 mm diameter extrudates. 

Observation of the CHX and COads species as a result of the interaction between methanol and a palladium surface indicates that the methanol decomposition occurs via two routes (1) scission of the C-O bond in methanol with the formation of CHX species (x = 0-3), and (2) methanol dehydrogenation giving COads- The contribution of CO bond scission with the formation of carbonaceous species is negligible under UHV conditions, especially at low exposures, but it increases strongly at higher pressures. These XPS results indicate unambiguously that the pathway of methanol C-O bond scission can take place on the atomically smooth surface of palladium, but that high methanol partial pressures are necessary to form a noticeable amount of carbon species produced by the methanol C-O bond scission. 

For completeness, we mention that SFG was able to detect the final product of CH3OFI decomposition (CO), whereas the frequency range of intermediate products such as formaldehyde was not accessible. Similarly, HP-XPS carried out with a laboratory X-ray gun cannot accurately differentiate CO from CH , 0. In contrast, the wider frequency range of PM-IRAS allowed the identification of, for example, CH2O (in addition to CO) at elevated methanol partial pressures on a strongly CH c-poisoned surface (177). As these PM-IRAS investigations are relevant to methanol oxidation, they are discussed below. 

The MTO reaction was investigated at 698 K at a weight hourly space velocity (WHSV) of 57-384 g/(gcat h) with a methanol partial pressure of 7.2 kPa. Because the catalyst underwent rapid deactivation, the MTO reaction was investigated by using 3-min interrupted pulses with GC analysis carried out after 2 min for each pulse (this is the integrated pulse method). The time between pulses was about 40 min, allowing for completion of the GC analysis of each pulse. The reactant... 

A typical uptake curve for methanol diffusion in SAPO-34 at 373 K and a methanol partial pressure of 0.75 kPa is shown in Fig. 13. The transient diffusion equation for a slab geometry (Eq. (7)) for sorbate uptake was found to give the best fit to the experimental results, although SAPO-34 appears to have a typical cubic shape. 

The sorption of methanol in SAPO-34 with crystal sizes of 0.25, 0.5, 2.5, and 25 pm was investigated in the temperature range from 350 to 400 K and methanol partial pressures from 0.75 to 7kPa. The steady-state diffusivity was found to decrease when the crystal size was decreased from 25 to 2.5 pm (Fig. 14). This result... 

Another synthesis process proposed to receive benefits from operating with monolith catalysts is the conversion of methanol for gasoline production [16,17J. The catalyst used was the ZSM-5 zeolite. However, rather than binding the catalyst onto the wall by use of a washcoat, it was uniformly crystallized on the cordierite honeycomb (62 cells/cm ) wall surfaces (up to 30% by weight), similar to the method described in the patent assigned to Lachman and Patil [18]. The effects of methanol partial pressure on conversion and temperature on hydrocarbon selectivity were determined. Three regimes of mass transfer resistances are experienced in this reaction reactant transfer to the reactor walls within the monolith channels through the laminar flow, diffusion resistance at the surface between zeolite crystals on the walls, and diffusion into the zeolite molecular-size pores to the active sites within the crystals, where the reaction rate limit is anticipated. 

It was pointed out that diffusion effects are less severe for thin zeolite layers on monolithic substrates than for pellets, such as were used for performance comparison. Methanol partial pressure variations led to results similar to other work in which it has been found that lower pressures favor olefin formation. As the temperature is raised, light... 

Aluminum clusters react more rapidly with methanoP " than either Dj or DjO. Alj, A1 4, and AI22 are the least reactive clusters. As a result of sequential addition reactions the observed product peaks are A1 (CH30H) with m = 1, 2, and 3. At low extent of reaction, products with m = 1 are obtained in the highest yield. The peak reactivity, occuring at Ali -i, is about 500 times greater than that of the most reactive cluster (Al ) toward D2 - As the methanol partial pressure is increased, the mass spectra become very complicated. 

Figure 5.26. Absorption coefficient a of various gases in methanol (partial pressure lbar).195...

Kinetics of the reaction were determined by varying the partial pressures of oxygen, water, and methanol as well as the temperature. Other partial pressures were kept nearly constant nitrogen was the diluent. Kinetic observations also were similar as previously reported( 0 as is Illustrated in Figures T, 8 and 9 for different phases. The methanol reaction rate was nearly independent of the oxygen partial pressure, except at very low oxygen pressures in the reactor in which case the catalyst begins to be reduced. It was shown previously(6) that a reduced catalyst is much less active. The reaction rate has a positive dependence on methanol partial pressure, but the reaction is inhibited by the addition of water. Water does however increase selectivity to formaldehyde at the expense of dimethoxymethane, methylformate and dimethylether. 

Figure 8. Reaction Rate of Methanol versus Methanol Partial Pressure for FeAl(Mo04)3.

Methanol oxidation experiments were carried out in order to determine if methanol was an intermediate in the production of formaldehyde from methane. To this end a methanol saturator was placed upstream of the reactor. The saturator was submerged in an ice/acetone bath (at -16 to - 20 °C) keeping the saturated methanol partial pressure at 5 kPa. This was approximately equivalent to the total carbon containing products generated during standard reaction conditions. The gas feed stream to the saturator consisted of 81 kPa helium and 20 kPa air. The flow rate was varied from 6.25 - 100 ml min. ... 

Table 11 Influence of methanol partial pressure on selectivity...

Methanol and Wood Conversion Product Classes. Methanol has been used in this screening work to ascertain catalyst activity. The methanol relative product distribution on an active, pure catalyst is shown in Figure lA. (Table II gives the identification of the ions observed). No methanol (m/z 31 and 32) breakthrough was observed, and the first formed product, dimethyl ether (m/z 45 and 46), has been consumed to form a mixture of C2 to Cg olefins and toluene, xylene, and trimethyl-benzene. Note the lack of benzene and alkanes. With lower space velocities and higher methanol partial pressure, the alkenes are known to disproportionate to branched alkanes and to form more aromatics (11). The absence of products above m/z 120 indicates the well-known shape selectivity of the catalyst. 

Figure 2 shows a comparison of the results obtained with the one and two dimensional models when diffusional limitations in the solid pellets (heterogeneous models) were considered. These curves correspond to the operating condition of = 10 kg/s and for three different inlet methanol partial pressures - 1 atm,... 

Predictions of Temperature and Methanol Partial Pressure Unidimensional Homogeneous and Heterogeneous Models, Wc = 15 kg/s. 

Figure 14-14. Solubility of carbon dioxide in methanol, partial pressure of carbon dioxide = 1 atm. Data of Heitert (1956)...

The autocatalytic nature of the early stages of reaction is clearly evident in Figure 13 [23], which contains a plot of C2C5 olefin selectivity versus contact time at a very low methanol partial pressure. Characteristic sigmoid trajectories are shown by the C3C5 olefins ethylene increases only slowly. This is consistent with a chain-growth mechanism whereby olefins are homologized by Cl addition, with ethylene as the "first" olefin. 

The discussion of MTG kinetic effects just presented is generally applicable to MTO. Olefin selectivity is improved by decreasing methanol partial pressure, increasing temperature, and increasing zeoUte Si02/Al203. An additional effect, that of varying zeolite crystallite size, was reported by Howden et al [61], who found that when the crystallite size was reduced from 30 to 3 pm, ethylene selectivity increased. This was attributed to enhanced diffusivity of li t products, which reduces their opportunity for further reaction. 

CHjOH). This means that a comparison of the catalysts in a fixed-bed reactor at different conversion levels or at different methanol flow rates and equal conversion may potentially mean different temperatures and is relatively tricky. That is a reason why most of the publications use a highly diluted methanol feed [29,92,93]. The dilution media acts as a heat vector in this case to smooth over the adiabatic temperature increase. However, the presence of dilution media greatly decreases methanol partial pressure. The latter masks mechanistic details and may change reaction pathways. Some important studies also highlight the role of diffusion, adsorption equilibrium, role of water, and the role of surface species, which could hardly be seen with diluted feed [24,28,60,119]. 

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