MTBE Production

Isobutyl alcohol [78-83-1] forms a substantial fraction of the butanols produced by higher alcohol synthesis over modified copper—zinc oxide-based catalysts. Conceivably, separation of this alcohol and dehydration affords an alternative route to isobutjiene [115-11 -7] for methyl /-butyl ether [1624-04-4] (MTBE) production. MTBE is a rapidly growing constituent of reformulated gasoline, but its growth is likely to be limited by available suppHes of isobutylene. Thus higher alcohol synthesis provides a process capable of supplying all of the raw materials required for manufacture of this key fuel oxygenate (24) (see Ethers). 

Production. MTBE production capacity has grown steadily, usually at an annual rate of 10 to 20% per year. In 1980, world capacity was 30 thousand barrels per day (1.5 X 10 t/yr). By 1990, capacity was up to 180 thousand barrels per day (7 x 10 t/yr). Because of the requirements of the U.S. CAA, production capacity is expected to more than double from 1990 to 1995 (25). By 2000, MTBE may be the second largest organic chemical produced in the United States, second only to ethylene (26). 

The use of MTBE occurred quickly after the first MTBE plant was built in Italy in 1973. Its use then spread through Europe and by 1980, Europe was producing almost 90 million gallons per year. This reached 300 million gallons per year by the end of 1990. In the U.S. MTBE production began in the early 1980s and reached more than a billion gallons by 1987. 

Butane isomerization is usually carried out to have a source of isobutane which is often reacted with C3-C5 olefins to produce alkylate, a high octane blending gasoline [13]. An additional use for isobutane was to feed dehydrogenation units to make isobutene for methyl tert-butyl ether (MTBE) production, but since the phaseout of MTBE as an oxygenate additive for gasoline, this process has decHned in importance. Zeolitic catalysts have not yet been used industriaUy for this transformation though they have been heavily studied (Table 12.1). 

MTBE production capacity has gruwn steadily, usually at an annual rate of 10 to 20% per year. MTBE may be the second largest organic chemical produced in the United Slates, second only to ethylene. 

Arcopure High Purity MTBE Product Bulletin, ARCO Chemical Co., Newtown Square, Pa., 1990. 

Figure 9.21 Methods of integrating pervaporation membranes in the recovery of methanol from the MTBE production process [15]. Courtesy of Air Products and Chemicals, Inc., Allentown, PA...

The most important application of RD today seems to be the production of ethers such as methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether (TAME), which are widely used as modem gasoline components. Figure 7, upper part, shows a traditional process for MTBE production, which is a strongly exothermic reaction. The disadvantages of that process can be avoided if the reaction and separation take place within the same zone of the reactor (Figure 7, lower part). 

Sundmacher K, Hoffmann U. Activity evaluation of a catalytic distillation packing for MTBE production. Chem Eng Technol 1993 16 279-289. 

Description Crude C4 streams are converted into propylene and an isobutylene-rich stream in three IFP process steps (1) butadiene and C4 acetylenes selective hydrogenation and butenes hydroisomerization, (2) isobutylene removal via distillation or MTBE production and (3) metathesis (Meta-4). 

Institut Frangais du Petrole (IFP) has developed a process, where pervaporation of methanol is used to debottleneck MTBE production. In the debutanizer columns used in MTBE processing, the MTBE/ methanol azeotrope results in a concentration of methanol at a point midway between the feed tray and the... 

An unusual feature of the CD process for MTBE production is that it is recovered from the bottom of the CD column even though its normal boiling point (55°C) is less than the boiling point of methanol (64.5°C). This observation is attributed to the formation of a minimum boiling azeotrope from methanol and MTBE. Apparently, if sufficient quantities of MTBE were accumulated in the CD column, it would lift the methanol into the reaction zone of the column resulting in a higher methanol conversion. This unusual behavior is believed to be responsible for the multiple steady states observed in the MTBE synthesis shown in process simulation and optimization studies and verified experimentally. ... 

In the hydroperoxidation route, either a-methylbenzyl alcohol or t-butanol are the reaction coproducts both molecules are dehydrated to yield either a styrene monomer (2.5 ton per ton ofPO) or isobutene (2.1 ton per ton of PO), respectively. t-Butanol may also be the direct feed for MTBE production. Balancing the markets for PO and the coproducts has proven difficult with these routes, thus leading to a volatile economic performance over time. Furthermore, the hydroperoxidation routes require relatively large capital investments. Existing hydroperoxidation plants continue to operate and are incrementally improved however, future investments in these technologies will decline. 

Industrial experience has shown that this kind of plant, owing to the very high flexibility of the WCTR, can be switched easily from ETBE to MTBE production, and vice versa, without any reduction of feed rate and with few mechanical modifications. 

MTBE product purity Diisobutene (DIB) and terr-butyl alcohol are the primary byproducts in this process. In spite of these impurities, the... 

MTBE product purity is 97-99%. These byproducts, in small quantities, have no adverse effect on the MTBE product because they possess similar blending properties. 

In 1992, refiners began to choose a variety of routes to the synthesis of MTBE [51]. Valero Refining Marketing, in its MTBE synthesis plant, uses a butane/butylene mixture from the heavy oil cracker vapor recovery unit which on hydrogenation converts butadiene to butylene. This is then mixed with methanol in the MTBE synthesis unit, the MTBE product is separated and the butane/butene stream is charged to the alkylation unit. The butadiene is removed from the alkylation unit. This improves alkylate quality and reduces acid consumption. A block diagram of this unit is shown in Figure 3.29. 

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. 

Oxygenates can be produced from fossil fuels or from biomass. MTBE is produced by reaction of methanol with iso-butylene in the liquid phase over an acidic ion-exchanger resin catalyst at 100°C. MTBE production has increased at the rate of 10 to 20% per year. 

The FCC produces about 1.5 wt% on feed of isobutylene. Isobutylene is easily etherified with methanol into MTBE. MTBE has excellent octane numbers (RON = 117, MON = 101), but obviously, even if all i-C4 could be recovered from the FCC, the total MTBE product would be less than 2.5% of the gasoline blend. 

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