Higher Ethers as Replacement Oxygenates for Methyl Tertiary Butyl Ether in Gasoline: Synthetic and Environmental Aspects
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The use of oxygenates, particularly methyl tertiary butyl ether (MtBE), in reformulated gasoline has reduced the levels of carbon monoxide and unburned hydrocarbons in ambient air. However, the widespread contamination associated with MtBE use has prompted a search for replacement oxygenates. Among the alternatives are higher carbon ethers. Two ethers are of particular importance because they can be prepared from readily available petroleum refinery feedstocks. These two ethers are methyl tertiary hexyl ether (MtHxE) and methyl tertiary octyl ether (MtOcE). In order for the higher ethers to compete in the fuel oxygenates market, an economically feasible process for their production must be developed. It would be desirable to produce these ethers via the etherification of olefins with methanol derived from synthesis gas, with the synthesis gas in turn being derived from coal or biomass. An economic advantage would be provided if the olefins could be etherified directly with synthesis gas, without the isolation of the intermediate methanol. Chapter III reports a parametric study of the preparation of MtHxE and MtOcE from olefins and methanol, and identifies the optimum initial conditions for the development of a continuous process to produce these ethers from olefins and synthesis gas in a single-step etherification reactor. The introduction of these ethers into fuel supplies guarantees their introduction into the environment as well. There are two main sources of these contaminants; direct emissions into the atmosphere from automobiles and releases from leaking underground fuel tanks. Therefore, two different models were employed to assess the associated risks. Chapter IV presents the results of atmospheric contaminant transport modeling studies, which indicate that these ethers are likely to contaminate air at about the same concentration as MtBE. Multimedia fate modeling studies suggest that atmospheric dispersion and deposition of these ethers is not likely to be ecotoxicologically relevant. On the other hand, these ethers are likely to be carcinogens, and humans could be exposed to unacceptable concentrations of these ethers in urban air. Groundwater contaminant transport modeling studies indicate that these ethers may contaminate community water supply wells at concentrations similar to those that are known to cause widespread public health concern for MtBE. The screening-level risk assessment presented in Chapter V suggests the need for a more rigorous risk assessment before these compounds are widely used to replace MtBE in gasoline.