The condensation of polyaromatic hydrocarbons (PAH) often is cited as a mechanism for coke- nduced catalyst deactivation in fixed-bed hydroprocessing units. Saturation of PAH inhibits condensation reactions. One purpose of this paper is to reiterate the fact that PAH chemistry in hydroprocessing units involves more than just a single entity called “naphthenes” and four entities called 1-, 2-, and 3+ “aromatics.” Another is to show that understanding this chemistry can have practical value. The simplest system involving PAH saturation comprises naphthalene, tetralin, decalin, and hydrogen. We modeled this system thermochemically, adding as a proxy for naphthalene condensation a reaction between naphthalene and o-xylene to form chrysene. The calculations show the change in equilibrium concentrations as conditions change from 100 to 3000 psig and 400 to 1500 deg F. As expected, decalin is favored by high pressure and low temperature, and naphthalene is favored by high temperature and low pressure. But thermochemistry also predicts small but significant amounts of chrysene, even at moderate pressures and temperatures. Examples are presented showing how an understanding of this kind of information can be used to explain, and in some cases improve, the performance of hydroprocessing units.
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