Isobutene, MTBE from

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. 

The acidity of a clay can be either of the Brpnsted (H+ donor) or Lewis (electron pair acceptor) type. Even at temperatures below 100 °C, tertiary carbocation intermediates can be generated on clays with high Brpnsted acidity through protonation of the C=C double bond in secondary alkenes, as in the clay-catalyzed formation of MTBE from methanol and isobutene ... 

Figure 1 Examples of industrial processes employing reactive distillation (a) methyl ferf-butyl ether (MTBE) from isobutene and methanol (b) cumene via alkylation of benzene with propylene (c) ethylene glycol via hydration of ethylene oxide.

Diphenyl carbonate from dimethyl carbonate and phenol Dibutyl phthalate from butanol and phthalic acid Ethyl acetate from ethanol and butyl acetate Recovery of acetic acid and methanol from methyl acetate by-product of vinyl acetate production Nylon 6,6 prepolymer from adipic acid and hexamethylenediamine MTBE from isobutene and methanol TAME from pentenes and methanol Separation of close boiling 3- and 4-picoline by complexation with organic acids Separation of close-boiling meta and para xylenes by formation of tert-butyl meta-xyxlene Cumene from propylene and benzene General process for the alkylation of aromatics with olefins Production of specific higher and lower alkenes from butenes... 

The catalytic distillation process of Smith [15], by providing for the fixing of the catalyst in a reactive section of a column between nonreactive stripping and rectification sections, and thereby for the continuous removal of MTBE from the reactants, boosts the conversion of isobutene to well in excess of 99%. The concept is still more economically attractive when OCFS are employed to secure the catalyst in the reactive section— DeGarmo et al. [16]—due to their significantly higher mass transfer efficiency. 

Iso-butane is a highly demanded chemical in the refinery industry for the production of alkylates (by alkylation with butenes), and methyl tert-butyl ether (MTBE) (from isobutene and methanol), both important additives for reformulated gasolines. n-Butane isomerization is performed over platinum supported on chlorinated alumina. The chlorine compound which is continuously supplied to the feed in order to maintain the activity [1] is harmful to the environment. 

CDTech uses catalytic distillation to convert isobutene and methanol to MTBE, where the simultaneous reaction and fractionation of MTBE reactants and products takes place [51], A block diagram of this process is shown in Figure 3.31. The C4 feed from catalytic crackers undergoes fractionation to extract deleterious nitrogen compounds. It is then mixed with methanol in a BP reactor where 90% of the equilibrium reaction takes place. The reactor effluent is fed to the catalytic distillation (CD) tower where an overall isobutene conversion of 97% is achieved. The catalyst used is a conventional ion-exchange resin. This process selectively removes MTBE from the product to overcome the chemical equilibrium limitation of the reversible reaction. The MTBE product stream is further fiactionated to remove pentanes, which are sent to gasoline blending, whereas the raffinate from the catalytic distillation tower is washed with water and then fractionated to recover the methanol. 

Most industrial processes of this kind use strong-acid ion exchangers for reactions catalyzed by hydrogen ions. A large-scale example is the synthesis of methyl tert. -butyl ether (MTBE) from methanol and isobutene as anti-knock gasoline additive [34,35],... 

ETBE (ethanol tertiary butyl ether, CgH, 0, density = 760 kg/m, LHV = 36 MJ/kg) is a better ingredient than bioethanol because it is not so volatile, not so corrosive, and has less affinity for water. ETBE-15 fuel is a blend of gasoline with 15% in volume of ETBE. ETBE is obtained by catal5dic reaction of bioethanol with isobutene (45%/55% in weight), noting that isobutene comes from petroleum. The other gasoline-substitute ether, MTBE (methanol tertiary butyl ether, (CH3)3-CO-CH3), is a full petroleum derivate (65% isobutene, 35% methanol). 

Fig. 2.8 shows an example of process simulation using the mass-transfer model and kinetic starting points for the reaction. The simulation and experiment results are shown for the preparation of MTBE from isobutene and methanol. This is an... 

Fig. 3.3. Azeotropes, distillation border, and chemical equilibrium of the terna system isobutene-MTBE-methanol at a pressure of 500 kPa ([18], reprinted from Chem. Eng. Sci., Vol 57, Beckmann et al.. Pages 1525-1530, Copyright 2002, with permission from Elsevier Science)...

In Fig. 5.28a experimental and simulated rates for the synthesis of MTBE from methanol and isobutene are depicted, which show that the rate expression (5.63) is valid for the MTBE synthesis [45]. Fig. 5.28b illustrates its validity for the ETBE synthesis from ethanol and isobutene [41] compared with experimental data reported by Francoisse and Thyrion [47]. In analogous manner this rate approach can be applied to the synthesis of the fuel ether TAME from methanol and isoam-lyenes [43, 46]. Activity-based rate expressions were also applied for other reactions carried out in strongly non-ideal liquid mixtures, for example for butyl acetate synthesis [48] and for dimethyl ether synthesis [49]. 

In many of the processes where ion exchange resin catalysts have proved valuable some additional factor has played a role in allowing technology to evolve, and in some instances the selectivity achieved is not well understood, but is accepted. In the case of pure isobutene production from cleavage of MTBE (see above) the sulphonic acid resin used is specifically designed to minimise other known side reactions (Figure 6.30). These are... 

The first system is the production of MTBE from the reaction of methanol with isobutene. The second is the production of ETBE from the reaction of ethanol with isobutene. 

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. 

Methyl-te/t-butyl ether, a gasoline additive, is made from isobutene and methanol with distillation in a bed of acidic ion-exchange resin catalyst. The MTBE goes to the bottom with purity above 99 percent and unreacted materials overhead. 

In a typical process, the conversion of isobutene in the reactor stage is 97 per cent. The product is separated from the unreacted methanol and any C4 s by distillation. The essentially pure, liquid, MTBE leaves the base of the distillation column and is sent to storage. The methanol and C4 s leave the top of the column as vapour and pass to a column where the methanol is separated by absorption in water. The C4 s leave the top of the absorption column, saturated with water, and are used as a fuel gas. The methanol is separated from the water solvent by distillation and recycled to the reactor stage. The water, which leaves the base of the column, is... 

Pervaporation can also be used to unload a distillation column, thereby reducing energy consumption and operating cost and increasing throughput. The example shown in Figure 9.20(c) is for the recovery of pure methanol by pervaporation of a side stream from a column separating a methanol/isobutene/methyl tertiary butyl ether (MTBE) feed mixture [14,15]. 

The process shown in Figure 9.21 was first developed by Separex, using cellulose acetate membranes. The separation factor for methanol from MTBE is high (>1000) because the membrane material, cellulose acetate, is relatively glassy and hydrophilic. Thus, both the mobility selectivity term and the sorption term in Equation (9.5) significantly favor permeation of the smaller molecule, methanol, because methanol is more polar than MTBE or isobutene, the other feed components. These membranes are reported to work well for feed methanol concentrations up to 6%. Above this concentration, the membrane is plasticized, and selectivity is lost. More recently, Sulzer (GFT) has also studied this separation using their plasma-polymerized membrane [56],... 

Figure 4.6 illustrates the PSPS and the chemical equilibrium surface. The PSPS has a hyperbola-type shape and passes through all pure component vertices and the stoichiometric pole n. It intersects the isobutene-MeOH edge and the MeOH-MTBE edge at two points, which are nonreactive binary azeotropes. From Fig. 4.6 one can also see that there exists no reactive azeotrope in this system. All the bifurcation branches and the pure component vertices, as discussed by Venimadhavan et al. [7], are located on the PSPS. 

Fig. 4.8(b)). At Damkohler numbers Dac> 0.085 and Dar> 0.166, pure isobutene and pure MeOH are feasible top and bottom products, respectively. At Dar< 0.166, both pure MeOH and a kinetic azeotrope (i.e., the mixture on the branch from MTBE to the pinch point) are possible bottom products, while another kinetic azeotrope (i.e., the mixture on the branch between isobutene and the nonreactive azeotrope isobutene-MeOH) is the possible top product. 

MTBE is currently synthesized industrially from methanol and isobutene over an acidic ion-exchange resin, mostly Amberlyst 15 which is in fact a macroreticular cation-exchange resin [1,2]. ETBE which is obtained by reaction of isobutene with ethanol, is also an attractive octane enhancer for gasoline [3]. Although the commercial catalyst is very efficient, it suffers from several drawbacks such as thermal instability, acid leaching from the resin... 

Alkylate is a gasoline blending component with exceptional antiknock properties, which seems to avoid the legislative pressure. Alkylate consists exclusively of isoalkanes and is obtained from the C3-C4 cut of the FCC units. In many instances, isobutene from the C3-C4 fraction is transformed selectively with methanol into methyl tert-butyl ether (MTBE). Therefore, a mixture of 1-butene and 2-butene is used for alkylation purposes. The other reactant is isobutane. The major constituents of the alkylate are 2,2,3-, 2,2,4-, 2,3,3- and 2,3,4-trimethyl pentane (TMP). Besides, the alkylate contains other C8 isoalkanes, such as dimethyl hexane (DMH), 3-ethyl 2-methyl pentane, methyl heptane and ethyl hexane, and even isoalkanes with carbon numbers that are not multiples of 4. 

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