1.Introduction Methyl tert-butyl ether, 2-methoxy-2methylpropane (MTBE) [1634-04-4], Mr 88.15, was first synthesized (by the classical Williamson ether synthesis) and characterized in 1904 [1]. 2 Methyl Tert-Butyl Ether Extensive studies in the United States during World War II demonstrated the outstanding qualities of MTBE as a high-octane fuel component [2]. Even so, it was not until 1973 that the first commercial plant went on stream in Italy. Reduction of the lead content in gasoline in the mid-1970s led to a drastic increase in demand for octane enhancers, withMTBEbeing used increasingly in this role. Political decisions about gasoline quality (e.g., lower aromatics content, lower vapor pressure and a defined oxygen content) especially in the United States had led to a significant increase in MTBE consumption in the 90th. In 1997 the world wide MTBE production reached some 19×106 t [44]. After years with two-digit growth rates (1990 – 1995) the increase in MTBE consumption is expected to be less than 2% per year in the near future. Table 1. Selected physical properties of MTBE mp −108.6 ◦ C bp 55.3 ◦ C n 20 D 1.3692 Dielectric constant (20 ◦C) 4.5 Viscosity (20 ◦C) 0.36mP a · s Surface tension (20 ◦C) 20mNm−1 Specific heat (20 ◦C) 2.18 kJ kg−1 K−1 Heat of vaporization (at bp) 337 kJ/ kg−1 Heat of formation (25 ◦C) −314 kJ mol−1 Heat of combustion −34.88 MJ kg−1 Flash point (Abel – Pensky) −28 ◦C Ignition temperature (DIN 51 794) 460 ◦ C Explosion limits in air 1.65 – 8.4 vol% Critical data tcr 224.0 ◦C pcr 3.43MPa 2. Physical and Chemical Properties Physical Properties. Methyl tert-butyl ether is a colorless, readily mobile liquid with a characteristic terpene-like odor. Its most important physical properties are listed in Table 1: Vapor pressure, density, and solubility in water, as well as the composition and boiling points of azeotropes withwater and methanol, are given in Tables 2 and 3 [3]. Methyl tert-butyl ether has unlimited miscibility with all ordinary organic solvents and all hydrocarbons. Chemical Properties. Methyl tert-butyl ether is very stable under alkaline, neutral, and weakly acidic conditions. In the presence of strong acids, it is cleaved to methanol and isobutene. Depending on reaction conditions the latter can form isobutene oligomers. 3. Resources and Raw Materials MTBE is produced by the reaction of isobutene, contained in C4-fractions, and Methanol. At present, isobutene from the following sources is used as feedstock for MTBE production [45]: 1) isobutene in raffinate 1, which is formed as a coproduct of butadiene production from steam cracker C4 fractions (see Table 4 for typical composition). 2) pseudoraffinate 1, which is obtained by selective hydrogenation of butadiene in mixed C4 fractions from steam crackers (composition, see Table 4) 3) isobutene contained in the C4 fraction of fluid catalytic crackers (FCC –C4’s; for typical composition, see Table 4). FCC –C4 is used as feedstock for ca. 29%of MTBE production Table 2. Vapor pressure, density, and water miscibility of MTBE Temperature, Vapor pressure, ∗ Density, Miscibility ◦C kPa kg/m3 Water in MTBE, wt% MTBE in water, wt% 0 10.89 761.3 1.19 7.3 10 17.59 751.0 1.22 5.0 20 27.21 740.7 1.28 3.3 30 40.88 730.4 1.362.2 40 59.69 720.1 1.47 1.5 ∗ Antoine Parameters: A= 6.254909, B = 265.4, C = 242.517 log p=A−B / (C +ϑ); [ϑ] = ◦C Methyl Tert-Butyl Ether 3 4) isobutene from dehydrogenation of isobutane, which is obtained both from refineries and from field butanes after isomerization (35% of MTBE feedstocks); and 5) isobutene by dehydration of tert-butanol, a coproduct of propylene oxide synthesis (Halcon/ Arco process, see →Propylene Oxide) (ca. 15% of MTBE feedstocks) Table 3. Binary azeotropes with MTBE Azeotrope bp, ◦C MTBE content, wt% MTBE–water 52.696 ∗ MTBE–methanol 51.686 MTBE–methanol (1.0MPa) 130 68 MTBE–methanol (2.5MPa) 175 54 ∗ Condensate separates into two phases. Raffinate 1 and pseudoraffinate 1 provide isobutene for approximately 21% of world’s total MTBE production [46]. Table 4.Typical composition ofC4 hydrocarbon streams from steam crackers (raffinate 1) and fluid catalytic crackers (FCC –C4) Compound Raffinate 1, wt% Pseudoraffinate 1, wt% FCC –C4, wt% Isobutane 4 2 36 n-Butane 12 7.5 13 Isobutene 44 24 15 Butene-1 24 39 12 cis-Butene-2 68 9 trans-Butene-2 9 19 14 Butadiene-1,3 0.5 0.0 0.3 Balance 0.5 0.5 0.7 As demand for MTBE increases, the first source to be exploited will probably be free FCC–C4 reserves. Any further expansion – and this also holds for the raffinate 1 route – is thus tied to new cracker construction. Field butanes will grow more than proportionally in importance. The relative share of tert-butanol as an ). By contrast. However. [51]. Production Methyl tert-butyl ether can be obtained by the acid-catalyzed addition of methanol to isobutene [6]. [52]. STAR (Phillips Petroleum Co. To produce MTBE from tert-butanol [5]. isobutene must first be obtained by elimination of water from the alcohol before the olefin can be used for ether production. Suitable catalysts are solid acids such as bentonites [7]. 16×106 t/a. For the isobutane dehydrogenation. and the methanol demand of 26×106 t/a [48] would allow an additional increase of MTBE production of ca. the commercially available processes Oleflex (UOP). the second reactant in MTBE synthesis. and FBD – 4 (Snamprogetti SpA). Methanol (→Methanol). isobutane must be dehydrogenated. The reaction is weakly exothermic with a heat of reaction of −37. 4. Catofin (ABB Lummus Crest.).7 kJ/mol. Some 25%of the present methanol output is consumed by MTBE. It has been shown recently. An excess of methanol not only increases the conversion of isobutene but also suppresses its dimerization and oligomerization. is produced at a typical purity of >99. the Butamer process is most commonly used. Dimerization of isobutene is the most important side reaction of MTBE . For the primary butane isomerization. the ER-model seems to be the most probable one. The kinetics of MTBE formation have been investigated intensively [9]. [54]. Inc. The same holds true for field butanes after isomerization of the n-butane fraction to isobutane. [47]. which amounts to 32×106 t/a. [49]. [50] and – commonly used in industrial world scale MTBE-production units – macroporous acidic ion-exchange resins [9]. A number of industrial processes have been established during the last decades [4]. are at present of industrial importance. is expected to decline because it is formed as a coproduct and thus linked to future propylene oxide demand. on the other hand. Because of equilibrium limitations only 92% conversion can be achieved with equimolar amounts of isobutene and methanol at 333 K. zeolites [8]. Raffinate 1 and FCC–C4 can be utilized directly in MTBE synthesis.isobutene source for MTBE production. The commonly used kinetic model has been developed by Rehfinger et al. that the observed reaction rates can be described sufficiently both by a kinetic model according to a Langmuir – Hinshelwood mechanism and by an Eley – Rideal (ER) approach [53].9% and is used directly for ether synthesis without further purification. Comparison of today’s methanol capacities. In most industrial plants.2. the pressure in the reaction section is chosen so that the reactants over the catalyst are entirely in the liquid phase over the catalyst bed. [46]. which must be removed. [58]. i. Design of the reaction section is largely determined by the heat of reaction. IFP [60]. If raffinate 1 is used as feedstock. In recent years. Residual butenes are mainly used for the manufacture of alkylate gasoline. whereas Arco employs recycle reactors. Process Description. and CDTECH (ABB Lummus Crest and Chemical Research Licensing) have been established [12]. 20×106 t/a are on stream [44]. A final isobutene conversion of ca. in addition to the Snamprogetti [57] and H¨uls (now Oxeno) processes [10]. [56].0 and 1. isobutene conversion of 95 – 97% is sufficient.. gives rise to a certain loss in selectivity for MTBE.4-trimethylpentene-1 and -2) in the “H¨uls-MTBE-Process”.synthesis [55]. In the three processes mentioned (Snamprogetti. At a molar methanol ex4 Methyl Tert-Butyl Ether cess as low as 10%. [59] processes developed by Arco [11]. Other industrial processes have been developed by DEA (formerly Deutsche Texaco) [13].e. between 1. or simply . Phillips Petroleum [60]. Snamprogetti and Oxeno prefer tubular reactors for reasons of process engineering.5MPa. an isobutene conversion of approximately 85%is achieved. Reaction Section. which also requires a recycle at higher isobutene concentration. More than 140 MTBE plants with a total installed capacity of ca. In the Snamprogetti. The heat of reaction depends on the isobutene concentration in the feedstock. and Arco processes. the selectivity for MTBE is practically 100 %. recycled to the cracker. however. only ca. H¨uls. 97% is achieved by a subsequent catalytic distillation. This process design. 100 – 200 ppm by weight of isobutene is converted to diisobutene (2. H¨uls. Most commercially available processes are comparable and consist of a reaction and a refining section. Shell (Netherlands) [14]. This enhances catalyst life time by reducing the polymerization of isobutene at the catalyst’s surface and promotes the selectivity for the MTBE formation. In the adiabatic reactors employed by IFP (Institute Franc¸ais du P´etrol) or CDTECH. The heat of reaction is removed by partial evaporation of the C4 hydrocarbons in the reactor as the reaction proceeds. For example. and Sumitomo [15]. Arco) adiabatic reactors are always employed with a FCC–C4 feedstock. All processes have in common the reaction of isobutene with a certain molar excess of methanol on a macroporous acidic ion exchanger at 50 – 90 ◦C. 98 %. 1). Two-stage Oxeno-MTBE-Process First stage: a) Multitubular reactor. e. This term does not distinguish between a homogeneous or heterogeneous catalyzed reaction in distillation columns. A more common term for this operation is RD. be recovered by distillation. it also results in isobutene conversions of ca. fixed-bed catalytic reactor. If they are to be utilized for other chemical purposes such as the production of polymergrade butene-1. Usually. This conversion corresponds to ca. From the viewpoint of reaction engineering. It can. The most important advantage of using CD for MTBE synthesis lies in the elimination of equilibrium limitation of isobutene conversion as a result of continuous removal of the reaction product MTBE from the reaction mixture.9% could be achieved in a two-stage process (see Fig. This high conversion can also be obtained by using highly sulfonated acidic resins in the reaction section followed by an additional catalytic distillation column in the refining section. the degree of isobutene conversion must be significantly increased. g) Methanol tower The partly reacted mixture from the reaction section – which is usually in chemical equilibrium – enters the CD column below the catalyst packing zone to ensure the separation of the high-boiling component MTBE from the feed stream. . c) First C4 distillation tower (debutanizer) Second stage: d) Secondary adiabatic reactor. where both catalytic reaction and distillation are carried out simultaneously in the debutanizer column. e) Second C4 distillation tower Methanol recovery: f) Methanol extraction. this excess methanol remains in the MTBE product. this column acts as a two-phase countercurrent flow. Refining Section.burned. Not only does this give MTBE with purities >99. 300 ppm per weight of residual isobutene in the spentC4 fraction. b) Adiabatic reactor. Unfortunately. In the Oxeno-MTBE process isobutene conversions of >99.g. Methyl Tert-Butyl Ether 5 Figure 1. Catalytic distillation (CD) or reactive distillation (RD) refers to a process.. Usually CD is defined as a process. as a methanol-poor minimum boiling azeotrope with MTBE [19]. in which a heterogeneous catalyst is located in a distillation column. nevertheless. The catalyst packing is installed in the upper mid portion of the column with normal distillation sections above and below. the spent C4 fraction is called raffinate 2. a higher excess of methanol in the feed stream can be used.7%(balance mainly tert-butanol produced by a reaction of isobutene with water dissolved in the feedstock). if the feedstock is a raffinate 1 stream. To overcome the conversion limit of 95 to 97% posed by the chemical equilibrium. The catalyst used in the CD columns is essentially the same as that employed in the reaction section. [61]. which contain pellets of strong acidic ion-exchange resin [63]. which is supported by a fiberglass cloth reinforced with stainless steel wire mesh. In this kind of process technology the catalytic section of the CD-column uses a conventionally structured. that since the debutanizer column must be taken out of operation during exchange of the catalytic packing. at least one process step is eliminated. CD-technology has the important advantage over conventional unit design (reaction section followed by debutanizing in a distillation column) of lower capital investment costs. wire-cloth distillation packing. so that an efficient and secure temperature control is possible. In the absence of methanol the exothermic dimerization of butenes take place . in the conventional design. a continued production can be performed at a lower feed rate. Beyond this. Therefore. in the two stageH¨uls-MTBE-Process). One evident disadvantage of the CD-process technology is the fact. An analogous catalytic packing system have been developed by Sulzer (commercialized as KataPak) [64] and Montz (commercialized as MultiPak) [65]. In the CD-technology it must also be ensured that a small excess of methanol exists along the reaction zone. the packing exhibits excellent separation performance combined with an efficient mass and heat transfer for chemical reaction. A CD-process has been developed also by UOP and H¨uls in 1992. In contrast.The CD-basedMTBEprocesswas first developed by CR&L using a patented catalyst support system (commercialized as CD MTBE or CD ETHEROL) [16]. also the whole MTBEunit must be shut down. It is an acidic ion-exchange resin. These fiberglass cloths are rolled to catalyst bales and stacked on sieve trays in the column. an efficient contact of reactants with catalyst pellets. the maximum temperature in the reaction zone is limited to the boiling point of the mixture. Therefore. Because in the CD-process both chemical reaction and distillation are carried out in the same 6 Methyl Tert-Butyl Ether device. the exothermic heat of reaction can be used to vaporize the reaction mixture. and instantaneous distillative removal of reactants. if a very high conversion of isobutene has to be achieved.g. utilizing Koch Engineering’s KataMax-technology [62] (commercialized as ETHERMAX process). where the reaction section usually consists of two or more fixed bed reactors (e. The benefit in using this type of packing is a very good distribution of liquid and vapor phase at a low pressure drop. while the catalyst is exchanged.. residual methanol. see Chapter 8. This 200 ppm residual methanol together with dimethyl ether. because of formation of an methanol-C4 azeotrope. also formed in trace amounts. running on raffinate I feedstock. Nevertheless. suitable precautions against groundwater contamination must be taken in loading and storage areas. The methanol content in the azeotrope depends on pressure and C4-composition. If methanol losses of ca. Investment costs (inside battery limits. see Section 11. were equipped with a CD-column. 200 ppm by weight are acceptable. excess methanol appears in the distillate.6MP a the azeotrope contains ca. Construction Materials. which have been built. Recently published complains about groundwater contamination and the reactions taken by public authorities in parts of the United States arose from careless storage of the product. as well as excess. IBL) for a 200 000 t/a MTBE plant (Oxeno-MTBEProcess) in the Federal Republic of Germany. Because MTBE is soluble in water. methanol recovery from the distillate with molecular sieves is also appropriate [17]. 5. Environmental Protection The small amounts of byproducts (see Chap. 2 to 4 wt%methanol. process wastewater. can be separated from the butene fraction in a subsequent molecular sieve adsorption unit [18]. Because corrosive media are not used anywhere and do not result from catalyst disintegration. The byproducts. The ensuing sharp temperature rise (“hot spot”) will cause irreversible catalyst deactivation and catalyst damage. This methanol can completely be removed from the C4-stream in an extraction column using a countercurrent water wash. The catalyst is regenerable. at 0. Moreover under normal operating conditions. For emission control in the storage and handling of MTBE.. In the removal of nonconverted C4 hydrocarbons from MTBE by distillation. residues. since the mid-1980s most of the MTBE plants. Quality Specifications The usual purity of commercial MTBE is 98 – 99wt%. and gaseous emissions are not formed. Depending on the quality of the C4 feedstock . do not have a detrimental effect on octane number improvement through the use of MTBE. and no environmental problems arise from its disposal. e. amount to 14×106 DM.g. tert-butanol and diisobutenes. For the environmental significance of MTBE. The methanol can be recovered by distillation and is recycled to the reaction section.at high reaction rates. 6. the entire plant is fabricated from standard carbon steel. 6) in MTBE need not be removed if the product is to be used for gasoline. 0 wt% Water 50 – 1500 ppm by weight Total sulfur max.5 wt% Hydrocarbons (C5. [22] can be employed.). TCEP [1.. 3.5 – 1. Storage and Transportation Being noncorrosive. Methyl tert-butyl ether has a vapor pressure of 61 kPa at 40 ◦C. Carbowax 20M. and the existing distribution system can also be used for fuels containing MTBE. MTBE can be handled in the same way as fuel. 3 b. and other fuel-resistant plastics and rubbers can be used for seals. The product can be stored in nonpressurized containers. High purityMTBEis also available from Shell and ARCO.mixture. tert-butanol) 0. With alcohol-resistant foams. or polypropylene. the MTBE product may also contain C5 and C6 hydrocarbons. Extinguishing agents for fire fighting are powder.2). 10 ppm by weight Residue on evaporation max. copper.g. brass. Emission from storage facilities can be controlled or prevented by ordinary measures.2. 7. and alcohol-resistant foam such as Tutogen L (→Fire Extinguishing Agents. a high-purity MTBE (ether content>99. a higher application rate is necessary for MTBE than for pure hydrocarbons.1 – 1. preferably in capillary columns with a highly polar stationary phase. AtypicalMTBEcomposition adoptedworldwide for use in the fuel sector is given below: MTBE 98 – 99wt% Alcohols (methanol. see [23].1 .98%wt) Methyl Tert-Butyl Ether 7 is marketed under the trade name Driveron S by Oxeno. or DX1.3-tris-(2cyanoethoxy)propane]. For GC analysis of MTBE-containing fuels. Teflon. [24]. no. e. the use of vinylidene fluoride-hexafluoropropene copolymer (Viton) is not recommended. A removal of these components by distillation is not necessary in most cases. 3 b GGVSee/IMDG class 3. polyethylene. diisobutenes) 0. The usual safety precautions for highly inflammable liquids must be employed. [25]. an oxygen-specific detector (O-FID) [20] or a column combination technique [21]. Because water is miscible with MTBE (though only slightly). 10 ppm by weight For special applications (see Section 10. C6hydrocarbons. Transport classifications are as follows: ADR/RID II class 3. dry storage is required. no. 8. Buna-N. Chemical Analysis Pure MTBE is analyzed by gas chromatography.2. as well as aluminum. carbon dioxide. Name of substance: 33/2398 Methyl-tert-butylether ADNRII class 3. Chap. The ether has an indefinite storage life even in the presence of air. For other references on analysis of MTBE. Carbon steel can be used as container material. the Council Directive 85/536/EEC.UN no. Legal Aspects The principal reason for the current importance of MTBE is environmental legislation (especially in the United States. the lowest. Japan. 1985 [26].. with olefin-rich gasolines. the research octane number of the fraction with a boiling range <100 ◦C) is especially pronounced (see Fig. Sun Refining received a provisional waiver in 1988. This process has been driven especially by the CAA regulation in the United States. and R-100 ◦C-ON=77 . To reduce carbon monoxide emission and comply with provisions of the Clean Air Act. Because of the relatively low boiling point of MTBE. Uses 10. RON) and 92 – 120 (motor octane number. In the United States. blend octane numbers of 115 – 125 (research octane 8 Methyl Tert-Butyl Ether number. it is often the best way to satisfy their provisions. Depending on the composition of the base gasoline.e. In addition to many other favorable characteristics. the effect on the front-end octane number (FON or R-100 ◦C-ON. i. and Western Europe).7 wt% oxygen) [27]. paraffin-rich gasolines. As an Octane Enhancer (→Automotive Fuels) More than 95% of MTBE produced are used in the gasoline pool. 2398 ICAO Code/IATA-DGR II class 3 UN no. Although laws do not require the use of MTBE. Figure 2. permitting the use of up to 15 vol% MTBE (2. In the European Community. The lead and aromatics content also influences the blend octane numbers of MTBE. the current importance of MTBE is based primarily on its exceptionally good octane-enhancing properties when used as a gasoline blendstock [28]. 2) [29]. The highest blend values are obtained with saturated. These antiknock properties are especially important because the use of cheap but toxic alkyllead compounds has been restricted or banned by law. [31].S. 2398 9. Legislation has developed since oxygenates in fuel have become generally recognized.1. cities and states mandate the use of ca. In the 1980s – 1990s huge production capacities have been erected in order to cover the demand of MTBE. does not require special labeling at filling stations for fuels containing up to 15 vol%MTBE. December 5. the use of oxygenates in gasoline is governed by EPA waivers from the Clear Air Act. both on environmental grounds and to permit the use of exhaust catalytic converters. MON) can be achieved. MON= 81. Ranges of octane improvement by addition of MTBE to an unleaded gasoline with RON= 88. 2.7 wt% oxygen in fuel (mainly as MTBE) in winter months. 10. some U. and elastomers). Methyl tert-butyl ether itself can be used in a number of chemical reactions. The methanol obtained as a coproduct is recycled to MTBE synthesis. the fuel’s cloud point is significantly lowered. With regard to the hot-weather drivability of modern automobiles. addition of MTBE to fuel has other positive effects. see [37–42] . By reversal of its formation reaction. high ignition temperature. for example. isobutene production with MTBE as feedstock is employed by Exxon Chemical and Sumitomo. so that vapor emissions during automobile fueling and operation are reduced. MTBE can be cracked to isobutene and methanol on acidic catalysts at >100 ◦C [32]. and particulate carbon. the MTBE-cracking route and that involving the splitting of tert-butanol obtained by direct hydration of isobutene [33] will continue to displace the conventional sulfuric acid extraction process for the isolation (recovery) of pure isobutene from C4 streams [34]. MTBE is a good solvent for analytical use [42]. and narrow explosion limits. Although MTBE has a somewhat lower heat of combustion than gasoline. addition of up to 20 vol% neither impairs motor power nor increases fuel consumption. For other possible applications. 10. no difference exists between fuels containing MTBE and those without it [30]. lacquers. At present. for example. The MTBE-blended fuels are compatible with all materials used in automobile manufacture (e. the production of methacrolein and methycrylic acid [35] and of isoprene [36]. The fuel vapor pressure (Reid vapor pressure. metals in the carburetor. It does not necessitate any modifications to existing vehicles. in solvent dewaxing of hydrocarbon oils [42]. The preferred grade for this purpose is high-purity MTBE with a residual alcohol content <500 ppm by weight.. gaskets. It is also used as an extractant. or elsewhere.g. RVP) is decreased. Easier cold starting and prevention of carburetor icing are other advantages. Because of its negligible tendency to form peroxides. [43]. → Butenes. For economic and ecological reasons. Addition of MTBE cuts down exhaust emissions.Besides increasing the octane number.). polycyclic aromatics. Chap. . such as the product marketed byOxeno under the trade name Driveron S. The lack of acidic hydrogen atoms makes MTBE a suitable solvent for chemical reactions such as Grignard reactions. Other Uses Methyl tert-butyl ether is also used in the petrochemical industry. injection pump. particularly carbon monoxide. Production of isobutene by splitting MTBE is the only application that has been used on an industrial scale (for applications of isobutene. unburned hydrocarbons.2. Because of the miscibility of water with MTBE. 9. in respect of European Union regulations MTBE has been classified as a skin irritant. and are most probably not relevant to humans. such as the International Association of Research on Cancer (IARC. dermal LD50 >10 000 mg per kilogram bodyweight.Methyl Tert-Butyl Ether 9 11. For the oral route the no observed adverse effect level (NOAEL) in a 90 day test is 300 mg/kg bodyweight per day while it is 800 mL/m 3 or 2880 mg/m3 in 90 days inhalation tests. Toxicological Profile As regards its widespread application and the amounts used. but will be rapidly detoxified when formed in the body. Neither the structure of MTBE nor the comprehensive number of available in vitro and in vivo mutagenicity tests with MTBE and its metabolite tert-butanol provide any indication of possible genotoxic or mutagenic effects. Animal studies showed no sensitizing potential for neat MTBE. have been observed besides transient behavioral effects. Carcinogenicity. typical for organic solvents. Chronic Toxicity. Tests with different animal species on oral. and inhalation application routes indicate a low acute toxicity for MTBE (oral LD50 well above 2000 mg per kilogram bodyweight. MTBE was found to be only slightly irritant to the rabbit eye but causes reversible moderate to severe skin effects in rabbits. MTBE produces tumors in rats and mice at high concentrations. The absence of genotoxic potential is important when discussing possible carcinogenic effects. According to all of the toxicity data reviewed. Therefore available test data strongly suggest that with regard toMTBEitself and its metabolites no genotoxic activity should be assumed by MTBE exposure. formaldehyde. Genotoxicity. Toxicology and Occupational Health 11. inhalation LC50 values in the range from 85 – 142 mg/L for 4 h). At high concentrations anesthesia is the most immediate effect (CNS depression).1. The other metabolite. The tumors found have been formed via nongenotoxic mechanisms. Mutagenicity. The main effects observed in repeated dose toxicity studies via the oral and inhalation routes were CNS depression. MTBE ranges within the top 2% of all tested chemical substances. Acute Toxicity. No observed adverse effect concentrations (NOAEC) of 400 mL/m 3 by inhalation and 250 mg/kg bodyweight at oral dose are derived from the tests results. the European Union Working . Based on the available data several independent bodies. At nonlethal concentrations local irritations. has a proven potential of DNA damage. WHO-agency). dermal. irritation of the respiratory and gastrointestinal tract and enlargement of the liver. service-station attendants and garage workers. compliance by workers with the short. Tests on reproductive toxicity and developmental development shows that MTBE is not significantly toxic to reproduction nor to fetal development.At 180 mg/m 3 no objective symptoms on the CNS and no signs of irritation of the respiratory tract were reported.2.19 mg/m3) as well as odor and taste thresholds in water (15 g/L and 40 g/L. invertebrates and fish are well above 100 mg/L.3.Group on Classification and Labelling of Dangerous Substances. distribution and handling of gasoline containing MTBE.1. and also for consumers filling 10 Methyl Tert-Butyl Ether MTBE containing gasoline in cars are significantly lower than the no effect levels in toxicity tests. As indicated by all reported human experiences there is no evidence of any adverse effect caused by low exposure levels. Although it is not reported. . at lowexposure levels effects on humans may be transient and subjectively caused by taste and odor. it is verified that the risk for environmental organisms is low. Because of the fact that the measured concentrations of MTBE in the environment are significantly lower than the no effect levels in the mentioned tests and the calculated no effect levels for sediment organisms and terrestrial organisms like earth worms. 12. Human Experience As the odor threshold of MTBE in air (0. 11. algae. MTBE production and use indicates no concern for human health. repeated exposure on skin may result in de-fatting of the skin. Based on the available data MTBE is not classifiable with respect to its carcinogenic potential for humans. Ecotoxicity Data The result of a large amount of acute aquatic toxicity test data available for organisms at different trophic levels indicates low toxicity to aquatic organisms. Occupational Health Since the exposure concentrations for workers involved in production of MTBE. and other bodies recently have not come to the conclusion that MTBE is a human carcinogen.and long-term occupational exposure limits and protection against skin contact is necessary to be protective against the described effects of MTBE. Reproductive Toxicity.053 mL/m3 = 0. 11. Health Risk Characterization. However. All measured EC50 and LC50 values for microorganisms. respectively) are very low. blending. The lowest recommended occupational exposure limits are 270 mg/m 3 (75 mL/m3) as the short-term and 90 mg/m3 (25 mL/m3) as the long-term value. Ecotoxicology 12. 13. The concentration of MTBE in groundwater and the extent of groundwater contamination depend largely on the volume and duration of a gasoline release and whether remediation has been initiated at the site. In many cases.2. minor on-going spill incidents. Removal of MTBE from Impacted Groundwater As a component of gasoline. with adapted bacteriaMTBEis readily biodegradable under aerobic conditions.Aquick response following an accidental release of an MTBE-blended gasoline is of great importance regardless of the selected remediation method since it decreases the size of the area requiring characterization and the volume of groundwater needing treatment thereby minimizing site characterization and remediation costs. it is easily washed into the groundwater. site-specific hydrogeological characteristics. ThereMTBEis readily degraded by photochemically produced hydroxyl radicals. accidental spills during tank refills. leaking fuel pipelines. source removal is critical for the success of remediation systems at MTBE-impacted sites. MTBE can enter subsurface environments through a number of release scenarios including leaking underground fuel tanks (LUFT). As MTBE is relatively easily soluble inwater (about 50 g/L) and will not adsorb on soil. As with any remedial effort for most contaminants. Biodegradation in contaminated soil and groundwater (remediation) is discussed in Chapter 13. dispensing equipment. The half-life for photodegradation in the air is about 3 – 7 days. Behavior in the Environment A calculation with McKay level 1 partitioning model estimates that in case of a spillage most of the MTBE (95 %) will distribute in the atmosphere.However.1. With inoculum of predominantly domestic sewage nearly no biodegradation occurs within 28 days inwater in a standard test. Aerobic and anaerobic degradation in water and groundwater depends strongly on the conditions. Of particular interest at MTBE-impacted sites is the behavior . and risk-based or regulatory-driven cleanup objectives.12. Overview of MTBE Remediation Technologies The selection of a remedial strategy at an MTBE-impacted site depends on the volume of contaminated soil and groundwater. Due to its physical-chemical properties and metabolism in organisms MTBE is not expected to bioaccumulate and this has been shown to be correct by a bioaccumulation test. gasoline retail stations and production terminals are sources of MTBE in groundwater. contaminant concentrations and mass. 13. and larger volume surface spills. .).27.g. Ex-Situ Phase Transfer Technologies 13. Air Stripping Air stripping is a physical process for removing volatile organic compounds from water. Contarary to early reports. high air-towater . off-gas treatment from air strippers) or waste disposal (e.2.g.1. The properties of MTBE suggest the following regarding its fate in subsurface environments: (1) MTBE does not readily sorb to soil particles in aquifers. emerging technologies. MTBE has a lower density and boiling point. spent granular activated carbon) may be required.2.. Chap. As compared with toluene. MTBE’s Henry’s constant is much lower than that of common groundwater contaminants such as toluene (0.1. Phase Transfer Technologies Phase transfer technologies move MTBE from the water phase to either the gas phase (e. and impact the success of remedial technologies. In general. but a higher vapor pressure and much higher water solubility. soil vapor extraction.. 13.1.g. As a consequence.018 – 0. govern transport Methyl Tert-Butyl Ether 11 mechanisms. 2. In addition to single technologies traditionally used at gasoline-contaminated sites. 0.2. 13. bioremediation and phytoremediation. multi-phase extraction. Consequently. Thus. a common gasoline component and groundwater contaminant (→Toluene. most of the technologies used historically at gasolinecontaminated sites are effective at removing MTBE from groundwater. advanced oxidation or granular activated carbon systems. and (3) MTBE strongly partitions into the aqueous phase and preferentially remains in water.g. granular activated carbon) without chemically transforming it. additional treatment steps to destroy MTBE (e. phase partitioning and biological interactions. [66]). In-situ technology options include air sparging. The properties of MTBE greatly influence its fate in the environment.026 vs. air stripping) or the solid phase (e. Key physiochemical properties of MTBE were presented in Tables 2 and 3. modifications to existing technologies or the use of a treatment train involving more than one technology can greatly improve MTBE removal efficiency and/or reduce life cycle remediation costs.of MTBE relative to other gasoline components in subsurface environments. (2) MTBE volatilizes readily from non-aqueous phase liquid (NAPL) sources. MTBE can be highly mobile in subsurface environments relative to other gasoline constituents. the effectiveness of air stripping increases with a compound’s Henry’s constant and the operating air-to-water ratio. Successful technologies include groundwater extraction followed by air stripping..1. GAC can be used to treat low flows of water from private wells with low MTBE concentrations.ratios (100 to 250) are needed to remove MTBE from water. One of the limitations of resins is the experimentally observed reduction in MTBE sorption due to the presence of other gasoline constituents. Because state and/or local air quality regulations sometimes required stripper off-gas treatment (granular activated carbon. Background water quality and co-contaminant concentrations influence MTBE removal efficiencies. High concentrations of natural organic matter (NOM) and other gasoline constituents in influent water streams compete with MTBE for GAC sorption sites thereby increasing GAC usage rates and process costs. Granular Activated Carbon (GAC) Although MTBE has a low affinity for sorption to solids.2.3. thermal and catalytic oxidation. Evaluation of MTBE performance data from eight case studies of low profile and packed tower air strippers showed that air stripping has the potential to be more widely used in both drinking water and remediation applications [68]. 13. In addition to synthetic resins.2. such as membranes and . This is analogous to the effects of NOM and BTEX on GAC sorption as discussed above.1. For each of the studies evaluated. toluene. One of the advantages of using synthetic resins is the ease of regeneration relative to GAC. For treatment of higher concentrations of MTBE. MTBE removal efficiencies exceeded 90 %.2. [69]. 12 Methyl Tert-Butyl Ether 13. process costs can increase significantly. Air stripping has been used to successfully remove MTBE at concentrations that are typically associated with groundwater contamination from LUFTs [67]. several types of activated carbon including coconut shell or coal-based GAC can be used to remove low concentrations of MTBE in water (200 g/L) cost-effectively [67]. GAC can be used as a polishing step in a treatment train following air stripping or chemical oxidation [69]. Other technologies. ethylbenzene. advanced oxidation). such as benzene. and xylene (BTEX) compounds and tertbutanol (TBA. The use of synthetic resins is costly relative toGACbut resins can be designed to achieve a higher degree of selectivity and trend to have a longer lifetime than GAC. in contaminated water.1. Other Ex-Situ Phase Transfer Technologies Results from laboratory and pilot scale studies suggest that synthetic resins have the potential for successful remediation of MTBE-impacted sites [67]. tert-butylalcohol). other sorbents such as high silica zeolites have been shown to be effective in removing MTBE from contaminated water [70]. and pilot-scale studies have been conducted. Soil Vapor Extraction (SVE) and Multi-Phase Extraction (MPE) Soil vapor extraction (SVE) is commonly used for the remediation of vadose zones at gasolinecontaminated sites. The injection of air has the additional advantage of introducing oxygen to subsurface environments thereby potentially enhancing aerobic contaminant biodegradation rates. Results from a number of field studies revealed that MTBE groundwater concentrations at gasoline-contaminated sites were reduced by 88 to 99% after two years of air sparging [79]. 13.2. Results from another laboratory study showed that 85% of MTBE in water equilibrated with gasoline was successfully removed using air sparging [78]. .2. if the site hydrogeology is conducive to successful use of IAS. Effectiveness with MTBE can be enhanced if the remediation system is operated under high vacuum. Laboratory air sparging column studies in different soils showed that the rate ofMTBEvolatilization is very rapid in sandy soils [77].solvent extraction. In order to remove MTBE from both saturated and unsaturated zones. is effective in removingMTBE from contaminated aquifers [75] and is routinely used at many sites.3.2. 13.2. Emerging phase transfer technologies include steam injection/ vacuum extraction and six-phase heating.. The success of SVE has been documented at MTBE-impacted sites when implemented either as the sole technology or in conjunction with other groundwater treatment technologies [74–76]. can be used to removeMTBE from contaminated groundwater [71].2. soil porosity is satisfactory. The combination of SVE and groundwater extraction.g. 13. known as multiphase extraction.2. multiphase extraction (MPE). e. [72]. In-Situ Air Sparging (IAS) In-situ air sparging involves the injection of air below the groundwater table to transfer contaminants from the aqueous phase into the gaseous phase. Phytoremediation occurs by plant uptake of the contaminants followed by translocation and accumulation into plant tissues. As a general rule. 13. in-situ air sparging (IAS) and phytroremediation. this technology could be very successful at removingMTBE from groundwater. In-Situ Phase Transfer Technologies Demonstrated in-situ phase transfer technologies include soil vapor extraction (SVE). other remediation technologies can be used in combination with SVE. [73] but only a few bench.2. Phytoremediation Phytoremediation involves the use of plants or trees to either remove or degrade groundwater contaminants.1. MTBE recovery rates were shown to increase with increased air injection rates and water saturation levels.2.2. 13. AOP systems can be expensive and can lead to the formation and accumulation of toxic or undesirable byproducts. In addition. Ex-Situ Transformation Technologies 13. Advanced Oxidation Processes (AOPs) (→Water.. Some of these compounds are either more toxic than MTBE or more resistant to oxidation [87]. and TBA. 10.0 [81]. Phytoremediation is most effective for contaminants with log Kow values in the range of 0.3. poplar trees [80].3. Common problematic byproducts include bromate due to the reaction of ozone with bromide.1. Fenton’s reaction (hydrogen peroxide in combination with iron(II)salts). Several plant and tree species have been recently reported to uptake MTBE including alfalfa plants [82]. Transformation or Destruction Technologies The underlying principle of transformation or destruction technologies is the transformation of MTBE in water to another chemical form either partially (e. Transformation or destruction technologies can either be implemented ex-situ following groundwater extraction (e. In laboratory studies.3. Although the use of AOPs is effective at treating high concentrations of MTBE.. AOPs are believed to be most efficient for groundwater with MTBE concentrations ranging between 0.1. Chap. Since MTBE has a log Kow of 1. [83].2. especially at sites where the contaminated groundwater is shallow.or transpiration through leaves. phytoremediation is expected to Methyl Tert-Butyl Ether 13 be an effective technology at MTBE-impacted sites.g. acetone) or completely (carbon dioxide). acetone and formaldehyde from the use of UV–H2O2 [91]. tert-butyl ether (TBE). AOP technologies have been tested for MTBE destruction in both laboratory and pilot-scale studies.1. [88].4.1 and 80 mg/L [87]. poplar trees were shown to remove 30% of the MTBE in test solutions at concentrations of 300 to 1600 g/L in a week [81]. Results of several laboratory studies suggest that phytoremediation has the potential to be effective for the removal of MTBE from groundwater. in-situ bioremediation).5 to 3. peroxide –UV. the breakdown of contaminants can take place either within plant tissues or in the rhizosphere [80].3. ozone –UV. 13.g. some fullscale applications have been reported [86–91]. In some cases.) Advanced oxidation processes involve the use of ozone-peroxide. in-situ chemical oxidation. biological treatment) or in-situ (e.. high-energy electron beam irradiation (E-beam) or sonicationhydrodynamic cavitation. chemical oxidation.g. Most of these processes are characterized by the production of hydroxyl radicals. [84] and Eucalyptus trees [84]. . [85]. membrane and trickling filter bioreactors. 13. Recent laboratory and field studies.3.6mg/L with an average removal rate of 96. porous pot. ozone. In one study. Such systems show great promise for biologically treating MTBE contaminated water streams. Several types of ex-situ reactors can be used for MTBE treatment including fluidized bed. such as MTBE. however. In a growth bioreactor microorganisms were able to reduce MTBE concentrations from 2400 mg/L to 1.1. MTBE concentrations dropped from 6000 g/L to below detection limits in less than a year . [97] and anaerobic [98–102] conditions. have shown that a number of bacterial and fungal cultures from various environmental sources are capable of degrading MTBE under aerobic [96]. Common oxidants include peroxide. Most ISCO technologies involve the injection of oxidants into groundwater to produce hydroxyl radicals. Biological Treatment Alkyl ethers. In-Situ Chemical Oxidation In-situ chemical oxidation (ISCO) is based on the same principles of advanced oxidation. permanganate.2% over the course of the study.More research to evaluate the cost-effectiveness and performance of AOPs at field-scale levels is needed.2. In a successful field scale demonstration. hydrogen peroxide was injected at a site in New Jersey.3.1.2. the use of a membrane bioreactor reduced MTBE from high (2000 mg/L) to very low(40 g/L) concentrations during normal operation [108]. For example. Microorganisms can either utilize MTBE as a sole source of carbon and energy.3. The biodegradation of MTBE has been demonstrated using both suspended growth and fixed film bioreactors [103–109]. Several early studies reported that MTBE is biologically recalcitrant under most environmental conditions [93–95]. two pure cultures designates PM1 andENV735are being used commercially in fluidized bed reactors (FBRs). In-Situ Transformation Technologies 13. or can degrade it cometabolically following growth on short-chain alkanes such as propane.2. are relatively difficult to degrade because of the high energy required by microorganisms to cleave the ether bond and the resistance of the branched carbon structure to microbial attack [92]. and Fenton’s reagent. The potential for success of ex-situ bioreactors has increased over the last several years with the 14 Methyl Tert-Butyl Ether isolation of pure cultures capable of degrading MTBE and TBA at relatively rapid rates [96]. Membrane bioreactors sustain high densities of microorganisms and are therefore suited for treating high strength waste streams as well as waste streams containing slowly biodegraded compounds such as MTBE. 13. E 4 H03 (1944). 58. 3. cometabolism. F. Mol. Clays Clay Miner. J.[111]. Catal. Catal. technologies involving ozone sparging are currently being tested. In addition to the injection of ozone as an oxidant. 5.. C.. Injection of Fenton’s reagent into groundwater at a site in Texas reduced MTBE concentrations by 83.org) have been contributing to the Chapters 11. Barnett et al. (1904) 1065. In-Situ Bioremediation (see also →Bioremediation) In-situ bioremediation involves the use of indigenous or exogenous microorganisms to either destroy or immobilize contaminants. In addition. Adams et al.3. 14. 6. M. 34 (1986) no. Catal. Ozone contained within the bubble and thin film around the bubble is expected to react rapidly and destroy MTBE in the gas phase. 17. Ancilotti et al.. Advis. a recent study has demonstrated the success of propane injection and bioaugmentation at an operating service station in New Jersey where a 93% reduction in MTBE concentration was achieved within two months of system operation [108]. Chem. 2. [113]. 191 – 195. Gicquel. Houston. J. 8. A. Comm. References 1. 10. March 28 – 30. Ancilotti et al. Zentralbl. J. K. 8 (1988) no. Rock. 4 (1978) 37 – 48. 4. two member companies of EFOA (The European Fuel Oxygenates Association – www. J. Wartime Report ACR No.. D.efoa. Obenaus: “Wege zur besseren Nutzung der . Prog.2. Fine bubbles with a high surface-to-volume ratio are injected into the saturated zone to extract dissolved MTBE from contaminated groundwater. 46 (1977) 49 – 57.. C. Torck. Fortum Oil and Gas Oy and Oxeno Olefinchemie GmbH. C. F. Dewitt 1989 Petrochemical Review. In-situ bioremediation strategies could involve direct metabolism. Natl.4% [112]. Ullmann. O. Technology limitations include effective oxidant delivery and the potential of byproduct formation. R. 12. Henry. 5. One of the advantages of ISCO is the potential to destroy MTBE in a relatively short time frame. and 13. 31 (1983) 27 – 31. 4. 4th ed. Energ. 7. Results from pilot studies showed that bioremediation approaches involving oxygen injection and/or bioaugmentation have a strong potential for success at MTBE-impacted sites [110]. 13. H.. 1989. Dunn: “Economics of Producing Butene Intermediate Feedstocks from Butanes”. J. 9. 597 – 609. Aeronaut. B.2. F. bioaugmentation or some combination thereof. Miller: “Arco’s Processes for Oxygenated Fuels – MTBE/TAME and ETBE”. February 8. Houston. March 19 – 21. Wegner). 1989 NPRA Annual Meeting.C4-Kohlenwasserstoffe”. 13. 12. H. DEA (formerly Deutsche Texaco). P. H. O. 14. Miller: Arco MTBE – The Maximum in Flexibility. Humberg. San Francisco. Petrochemical Review. Beck. DE-OS 29 11 077. D. 1982. 1987 (H.. 1989. J. A. 11. 1979 . Shell NL. DGMK-Meeting Karlsruhe. March 23 – 25. Catalytic Destillation Technology and MTBE-Production. 1988. Dewitt 1988. DE 27 0646 5.. Dixon et al. TX. Th.