Petroleum Resid Conversion: Opportunities and Limitations

In the conversion of vacuum resids one has the option to form coke as a byproduct or to devise conversion processes that prevent coke formation while forming a heavy liquid byproduct. The theme of this presentation is that both options can be greatly improved over current commercial practice but there are limitations imposed by petroleum macromolecules and economics. Coking processes and indeed, the Conradson carbon residue test itself, actually form coke precursors that are not native to the vacuum resid. By inhibiting the formation of new coke precursors, one may reduce the coke yield to only that formed by the native coke precursors. As a result, the yield of coke may be less than Conradson carbon residue, rather than 1.2 to 1.6 times Conradson carbon residue, without lowering the quality of the liquid product.

The secret of converting vacuum resids without forming coke is to keep the converted asphaltenes soluble while maintaining a moderate concentration of hydrogen donors. Although hydroconversion processes clearly maintain the hydrogen donor concentration, many are limited by the solubility of converted asphaltenes. By using hydrogen gas to strip out the asphaltene nonsolvents, the volatile liquid product, hydroconversions of greater than 90% may be achieved. However, conversions in this range usually run into an economic limit caused by very slow kinetics and accelerating hydrogen consumption. Meanwhile, there are processes, such as the Eureka and the HSC processes that very effectively use steam to strip out the volatile liquids during thermal conversion, relying on only the natural hydrogen donors. As a result, conversions are in the range of coking and ebullating bed hydroconversion processes without forming coke or consuming hydrogen.

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