MTBE reaction

Ethyl Tertiary-Butyl Ether. Similar to methanol in the MTBE reaction, ethanol can react with isobutylene to produce ETBE. Which alcohol is used to make the ether is highly dependent on the relative cost of the alcohols. To make ethanol more economically competitive with methanol, the federal tax credit for biomass-based ethanol used in fuel also appHes to ethanol used to make ETBE in the United States (24). 

The MTBE reaction is equilihrium limited. Higher temperatures increase the reaction rate, hut the conversion level is lower. Lower temperatures shift the equilihrium toward ether production, hut more catalyst... 

Figure 6. Response curves of n-pentane oxidation (a) and formation of maleic (b) or phthalic (c) anhydrides after stopping MTBE addition. (Conversion of n-pentane, and the amount of MA or PA formed are normalised to the values before MTBE addition). Experimental conditions n-pentane 1.5 vol.%, oxygen 20 vol.%, and 0.6 vol.% of MTBE reaction temperature 365 C, void section 7 cm. ti/2 is half time, namely the time necessary after stopping of MTBE to reach one half of its steady-state value before the MTBE was injected.

After having tested samples of polymer/carrier catalysts with varied polymer load and cross-linking in the MTBE reaction and successfully used the Raschig rings in a TAME column [25] we extended the application to other chemical syntheses. 

An important feature of packed DCRs is the need to pack the catalyst in a special way to ensure good flow, mass transfer, and contact characteristics. An example of this is the use of an ion-exchange resin catalyst (Amberlyst 15) in methyl tertiary butyl ether (MTBE) manufacture. The bed consists of bags made in the form of a cloth belt with narrow pockets sewn across it (Figure 14.3). The pockets are fllled with catalyst granules, and the belt is twisted into a helical form, referred to as a bale (see Smith, 1980, for details). Clearly, each pocket represents a closely packed bed of unconsolidated particles, and the pocket and the individual particle exhibit, respectively, their own distinctive macro- and microdiffusional features. This is broadly similar to the particle-pellet model of a catalyst pellet (see Chapter 8) but with distinctly different pellet behavior. Therefore, it is necessary to deflne an overall effectiveness factor that takes this unique feature into account. An attempt to do this was reported (Xu et al., 1995) for the MTBE reaction, for which... 

Figure 10.46. MTBE reaction chemistry. (Reproduced from (339) with permission.)...

The reaction is exothermic and requires a reactor design (adiabatic fixed bed with recycle or tubular) that keeps the temperature essentially constant. The equilibrium constant for this reaction is about 100 (Voloch et al, 1986). The reaction takes place below 100 C and at a high enough pressure to assure its occurrence in the liquid phase (about 200psig). In most commercial units, the catalyst is a cation exchange resin which limits the MTBE reaction temperatures due to catalyst stability constraints. The methanol/isobutene ratio is kept close to 1 to avoid the formation of oligomers at too low a methanol concentration or excess methanol in the C4 overhead stream. The upper limit in the methanol/isobutene ratio is due to the formation of an azeotrope with the C4 s in the distillation process which is about 4% methanol in the overhead stream. Typical yields from an MTBE unit at 1/1 methanol to isobutene ratio over a resin catalyst are shown in Table 22 (Miller and Piel, 1989). 

The reaction is also liquid phase, being carried out under somewhat more severe conditions than MTBE. Reaction rates are slower for TAME than for MTBE and equilibrium conversion are lower. Conventional single-stage iso-amylene conversion is 65% (Chase, 1984). Catalytic distillation can improve the conversion and selectivity by continuous removal of the product TAME as it is being formed and shifting the TAME equilibrium. The combination of fixed bed recycle process followed by a catalytic distillation is claimed to achieve yields of TAME as high as 90% (D Amico, 1990). 

Figure W8.1 MTBE reaction system without heat integration...

Acid catalyzed addition of alcohols to alkenes is sometimes used Indeed before Its use as a gasoline additive was curtailed billions of pounds of tert butyl methyl ether (MTBE) was prepared by the reaction... 

Methyl /-Butyl Ether. MTBE is produced by reaction of isobutene and methanol on acid ion-exchange resins. The supply of isobutene, obtained from hydrocarbon cracking units or by dehydration of tert-huty alcohol, is limited relative to that of methanol. The cost to produce MTBE from by-product isobutene has been estimated to be between 0.13 to 0.16/L ( 0.50—0.60/gal) (90). Direct production of isobutene by dehydrogenation of isobutane or isomerization of mixed butenes are expensive processes that have seen less commercial use in the United States. 

Isomerization. Isomerization is a catalytic process which converts normal paraffins to isoparaffins. The feed is usually light virgin naphtha and the catalyst platinum on an alumina or zeoflte base. Octanes may be increased by over 30 numbers when normal pentane and normal hexane are isomerized. Another beneficial reaction that occurs is that any benzene in the feed is converted to cyclohexane. Although isomerization produces high quahty blendstocks, it is also used to produce feeds for alkylation and etherification processes. Normal butane, which is generally in excess in the refinery slate because of RVP concerns, can be isomerized and then converted to alkylate or to methyl tert-huty ether (MTBE) with a small increase in octane and a large decrease in RVP. 

The second new zeoHte-basedhquid-phase process was developed by Chemical Research licensing Company (CR L). The process is based on the concept of catalytic distillation, ie, reaction and separation in the same vessel. The concept has been appHed commercially for the production of MTBE (48—51) but has not yet been appHed commercially for the production of ethylbenzene. 

The / f/-butanol (TBA) coproduct is purified for further use as a gasoline additive. Upon reaction with methanol, methyl tert-huty ether (MTBE) is produced. Alternatively the TBA is dehydrated to isobutylene which is further hydrogenated to isobutane for recycle ia the propylene oxide process. 

In commercial extraction operations, the fractions that contain butadiene, isobutene, and 1- and 2-butenes usually first go through a butadiene extraction unit in which the butadiene is removed. This may be followed by isobutylene removal via reaction between isobutylene and methanol to form methyl /-butyl ether [1634-04-4] (MTBE). The butenes are then distilled from the MTBE. 1-Butene may then be separated from 2-butene by distillation. 

Esterification and etherification may be catalyzed by mineral acids or BF3. The reaction of isobutylene with methanol to make MTBE is catalyzed by a sulfonated ion exchange resin. 

This reaction showed a remarkable selectivity with respect to the solvent and base used. In THF and EtOAc using TEA as the base, a 1 1 mixture of the allylic carbonate and bisacylated products is obtained, but when CH2CI2 is used as solvent, the reaction favors the allylic alcohol by a factor of 97 3 (mono/bis). In THF or MTBE, the use of TMEDA as the base also results in a 97 3 mono/bis ratio. ... 

Heterogeneous catalytic systems offer the advantage that separation of the products from the catalyst is usually not a problem. The reacting fluid passes through a catalyst-filled reactor m the steady state, and the reaction products can be separated by standard methods. A recent innovation called catalytic distillation combines both the catalytic reaction and the separation process in the same vessel. This combination decreases the number of unit operations involved in a chemical process and has been used to make gasoline additives such as MTBE (methyl tertiai-y butyl ether). 

TAME can also be produced by the reaction of methanol with iso-amylenes. The reaction conditions are similar to those used with MTBE, except the temperature is a little higher ... 

The reaction between isobutylene and methyl and ethyl alcohols is an addition reaction catalyzed by a heterogeneous sulfonated polystyrene resin. When methanol is used a 98% yield of methyl-ter-butyl ether MTBE is obtained ... 

Ethyl-ter-butyl ether (ETBE) is also produced by the reaction of ethanol and isobutylene under similar conditions with a heterogeneous acidic ion-exchange resin catalyst (similar to that with MTBE) ... 

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