Dimethyl ether formation process

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 reaction occurs in the liquid phase at relatively low temperatures (about 50°C) in the presence of a solid acid catalyst. Few side reactions occur such as the hydration of isohutene to tertiary hutyl alcohol, and methanol dehydration and formation of dimethyl ether and water. However, only small amounts of these compounds are produced. Figure 5-8 is a simplified flow diagram of the BP Etherol process. 

Industrial development of homologation processes for dimethyl ether, methyl formate, methyl acetate and acetic acid is likely only if they are integrated into a comprehensive plant for the production of C2 derivatives from syn-gas (Scheme 5). 

A complex and comprehensive simulation of the formation of dimethyl ether, the first intermediate in the MTG process, has been reported.649... 

The effect of coke deposition on the MTO reaction is complex. Coke deposition influences either the formation of dimethyl ether (DME) or the DME conversion inside the pores during MTO. However, the effect of coke deposition on the dimethyl ether conversion to light olefins (the DTO process) catalyzed by SAPO-34 is much simpler and can allow us to focus on the effect of intracrystalline coke on the olefin formation from DME. 

The new oxidation process has one important limitation. Allylic or benzylic alcohols are not oxidized but instead replacement of hydroxyl by chlorine is observed. Still another reaction may occur in polar media thus, methylthiomethyl ether formation becomes pronounced when methylene chloride-dimethyl sulfoxide is used as solvent. 

The formation of alkyl nitrites, CD3OD, and dimethyl ether as well as the appearance of the alkoxyl radicals indicates the importance of the primary process 72, whereas the low HD content of the hydrogen formed from CH OD seems to favor reaction 73, which... 

Hydrogenation of C02 occurs on a number of solid catalysts with Cu0/Zn0/Al203, methanol can be prepared but the equilibrium yields are less than 40%. The use of hybrid catalysts, containing solid acids, improves the yields by partial dehydration of the methanol to dimethyl ether.56 In situ IR spectroscopy has been used to identify catalytic intermediates in some processes. With Cu/ZrO/Si02, surface bound formate, gem-diolate and methoxide species could be observed before the final hydrolysis to methanol.57 Lithium salt-promoted Rh/Si02 catalysts increased the ethanol content of reduction mixtures from C02 as compared to the unpromoted reactions, but the main product was methane.58... 

The second stage in the process is required because the MTBE formation is an equilibrium reaction. The temperature needed ( 100°C) to achieve a sufficiently high rate of conversion means a decrease in isobutene equilibrium conversion (XiB = 0.9 at 65°C, Xjb = -0.75 at 100°C). The main side reaction in the MTBE process is the dimerization of isobutene towards di-isobutene (two isomers). Side reactions with essentially no significance are the formation of f-butyl alcohol (due to the presence of water as feed impurity), the formation of dimethyl ether from methyl alcohol, and the oligomerization of isobutene towards tri- and tetramers. A (three stage) process is also in operation which tolerates butadiene. The butadiene/ methyl alcohol reaction is faster than that of the n-butenes but consider-... 

Direct continuous reaction of SOs with benzene, highly successful in the case of dimethyl ether as described above, is not practical because of high sulfone formation. Indirect continuous reaction with SOs by a procedure stated to yield no sulfone has, however, been achieved by the method developed by Dennis and Bull. This process is based upon an observation made by the former that, in the presence of sulfuric acid, benzene will dissolve 2-3 per cent of its own volume of benzenesulfonic acid. This process is also designed to operate in continuous countercurrent flow in a cascade system, benzene being introduced at the bottom and a benzene solution of the sulfonic acid overflowing from the top. Concentrated sulfuric acid is added continuously at the top, and spent sulfuric acid (77 per cent) is removed at the bottom of the reactor. The spent acid may be fortified to original strength with SO3 for reuse, and the benzene is recycled after the product sulfonic acid has been extracted from it with water. This procedure is, in theory, the most efficient possible, since benzenesulfonic acid is, in... 

But the series of reactions needed to bring about the conversion is much more complex. Indeed, the mechanisms are not yet agreed for all the stages in the process. The first stage is the dehydration of methanol to methoxymethane (dimethyl ether), but there is much controversy over the mechanism of the initial carbon-carbon bond formation. However, once ethene has been formed (presumably via a slow step), well-known fast reactions take over, analogous to those seen when zeolites are used in petroleum refining. 

The kinetic scheme proposed for the MTG process is shown in Table 2, together with the corresponding kinetic constants for each reaction step. Its main characteristics are [8] 1) methanol (M) and dimethyl ether (D) are taken into account separately because it has been proven that they have a different reactivity 2) a step of light olefin formation by cracking of the gasoline lump is considered 3) the attenuating effect of water in the reaction medium on the kinetics is considered. On the basis of this kinetic scheme, the reaction rate equations in eq. (1), n, are ... 

Dimethylether. Several strategies for the production of dimethyl ether (DME) are described, e.g. direct synthesis from syngas according to equation (8.5) or via dehydration of methanol according to equation (8.6). From a mechanistic point of view direct synthesis proceeds also via methanol formation and subsequent release of water but without procedural isolation of methanol. The process can also be designed to yield both methanol and DME. Established methanol catalysts are employed for methanol formation and typical dehydration catalysts are solid-acid catalysts, e.g. alumina, silica-, phosphorus- or boron-modified alumina, zeolite, (sili-co)aluminophosphates, tungsten-zirconia or sulfated-zirconia. " ... 

Dimethyl ether ((CH3)20, DME) is also expected to be a clean energy source with large calorific value and excellent transportation properties, almost same as LPG Industrially, DME is generally produced in a two-step process namely, MeOH formation and its dehydration. It should be pointed out that its equilibrium yield is far beyond that of MeOH. Therefore the use of a bifiinctional catalytic system that is, a combination of a MeOH synthesis component with a dehydration partner can avoid the equilibrium limit of MeOH. 

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