The advent of oxygenated gasoline in the U.S., and ultimately worldwide, may present refiners with a new operating hazard reactor overheating in naphtha hydrotreaters.
According to an unpublished report by C.S. McCoy, McCoy Consultants Inc., Orinda, Calif., oxygenates such as methyl tertiary butyl ether (MTBE) react in hydrotreaters to form saturate hydrocarbons and water, just as sulfur and nitrogen compounds form saturates and, respectively, hydrogen sulfide and ammonia.
Oxygenates consume more hydrogen and release more heat than do sulfur and nitrogen compounds, says McCoy, but not dramatically so. But he has derived a new measure called "specific heat release," defined as the heat generated from 1 wt % of a compound in naphtha (Table 1).
When expressed this way, oxygenate heat release is substantially greater. But the big difference is seen when the likely concentration each contaminant is taken into account.
Most straight run naphthas contain at most 1,000 ppm sulfur. And some contain as much as 100 ppm organic nitrogen. Because of off spec test results or inventory correction requirements, processing naphtha containing 1 wt % oxygen (56 wt % MTBE) or more, in the form of a fuel oxygenate, soon may be common, says McCoy.
Table 2 shows the heat release values for the three contaminants shown in Table 1, at their likely concentrations. When gasoline containing oxygenates is hydro-treated, says McCoy, heat release, and thus reactor temperature increase, may be greater by a factor of 50 or more, compared to gasoline containing no oxygenate.
McCoy performed this study because of a call he received from a refiner who had a tankful of naphtha containing an unknown amount of MTBE (probably 3 5%) which he wanted to feed to his hydrotreater. On the advice of McCoy Consultants, the refiner blended the material into virgin naphtha very slowly and safely.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
