With the advent of the second stage of the European Auto Oil Program and similar legislation in other parts of the world, the aromatics content of gasoline has to be reduced significantly. It is anticipated, that this will lead to an oversupply of aromatics. Therefore, in this paper we suggest a novel catalytic process for the hydroconversion of pyrolysis gasoline (from naphtha steamcrackers) or other low-value aromatic fractions into an excellent synthetic steamcracker feedstock mainly composed of ethane, propane and nbutane.
It is demonstrated that this can be achieved via a two-stage route comprising ring hydrogenation on conventional metal catalysts and consecutive hydrogenative ring opening of the resulting cycloalkanes on monofunctional, i.e., acidic zeolite catalysts. For the second stage, the key parameters (i.e., the reaction temperature, the hydrogen pressure and the zeolite pore architecture) to yield a high-quality steamcracker feedstock are varied systematically, and the product compositions achievable in the steamcracker are simulated.
Additionally, it is shown that the aromatics can also be hydroconverted directly on bifunctional zeolite catalysts. The compositions of the resulting steamcracker feedstocks and the respective products in the steamcracker are given for various aromatics with 6 to 9 carbon atoms.
Due to their high octane numbers, aromatics have been used till now as octane boosters for gasoline on a very large scale. For example, before the year 2000 in Western Europe the average content of aromatics in gasoline was about 45 vol.-% (including about 2.5 vol.-% of benzene)1.
However, the combustion of aromatics leads to the formation of carcinogenic benzene in the exhaust gases. Thus, according to American and European research programs, reducing the total aromatics content of gasoline from 45 to 20 vol.-% would bring about a reduction in total exhaust air toxics from motor vehicles (mainly benzene) of 28%2. Therefore, the content of aromatics in gasoline is more and more restricted. For example, with the first stage of the Auto Oil Program of the European Union in the year 2000, the aromatics content of gasoline had to be reduced to BLOCK 2 - - FORUM 9 243
below 42 vol.-% (cf. Table 1). With the advent of the second stage in 2005, Western European refiners will be forced to further decrease it to below 35 vol.-%3. For the year 2010, an even lower maximum value of 30