Higher hydrocarbons to chemicals in millisecond reactors.

摘要

Millisecond chemical reactors could replace many conventional industrial processes to convert hydrocarbons into value-added chemicals. In this thesis, the potential of millisecond reactors for the partial oxidation of C₆–C₈ hydrocarbon feeds is investigated. Most commercial methods are liquid-phase processes plagued by poor conversions, high recycle costs, long residence times (minutes to hours), and expensive catalysts. High-temperature vapor-phase oxidation followed by fast cooling is a radically different technology. The catalyst contact time is up to six orders of magnitude lower, allowing higher throughputs and thus lower capital costs.; The feeds studied include cyclohexane, n-hexane, methylcyclohexane, n-heptane, toluene, p-xylene, isooctane, and gasoline. Depending on the catalyst and operating conditions, product distributions can be tuned to favor oxygenates, olefins, or synthesis gas (syngas, H ₂ and CO). With a cyclohexane-oxygen feed at C₆H₁₂/O ₂ = 2, a Pt–10%Rh single-gauze catalyst can give total selectivities exceeding 80% to oxygenates and olefins (mostly 5-hexenal and cyclohexene) at 25% cyclohexane conversion and complete oxygen conversion. Many catalysts are explored for cyclohexane oxidation, including Pt, Rh, Pt–Rh, Pt–Sn, Co, Mo, and Ag.; The oxidation of liquid fuels to produce syngas is motivated by current interest in improved engine efficiency and fuel cells. A wide variety of fuels are reacted with air over Rh-coated monoliths at short contact times. Very high yields (typically >90%) of syngas are produced from n-hexane, cyclohexane, isooctane, and toluene, representing model linear, cyclic, branched, and aromatic hydrocarbon fuels, respectively.; Density-functional theory is used to predict rate-constant parameters for the partial oxidation of cyclohexane. The model includes 46 elementary reactions and 31 species. According to two-dimensional computational fluid-dynamics simulations, the primary role of the catalyst is to oxidize some cyclohexane on the surface, generating heat to initiate gas-phase reactions. A narrow reaction zone near the catalyst produces species which do not decompose due to fast mixing and cooling in the wake of the wires. Thermal gradients are on the order of 106 K/s near the catalyst. Numerical reactor simulations suggest ways to adjust operation for desired product distributions, by allowing the investigation of costly or potentially dangerous experiments, such as high pressure.