Prediction of the Thermal Runaway Limit and Optimal Operation of Heat Transfer-Limited, Fixed-Bed Reactor Systems

We derive a new prediction for thermal runaway starting from the alpha model for fixed-bed reactor systems. This method accounts for thermal resistance internal to the reactor tube and the radial temperature gradients that result. To showcase our method, we compare its predictions to other common criteria for thermal runaway using o-xylene oxidation as the example chemistry. Even in systems where internal heat transfer is negligible, the empirical practical design criterion for thermal runaway is inaccurate. For cases where internal heat transfer is relevant, our runaway limit is more stringent than limits derived from simpler 1-D models. To augment our work, we optimize the product yield with the thermal runaway constraint using orthogonal collocation. Using the alpha model, the results illustrate that the thermal runaway limit can be accurately determined using either numerical or analytical methods

Kinetics and Mechanism of Ethanol Dehydration on γ‑Al₂O₃: The Critical Role of Dimer Inhibition

Steady state, isotopic, and chemical transient studies of ethanol dehydration on γ-alumina show unimolecular and bimolecular dehydration reactions of ethanol are reversibly inhibited by the formation of ethanol–water dimers at 488 K. Measured rates of ethylene synthesis are independent of ethanol pressure (1.9–7.0 kPa) but decrease with increasing water pressure (0.4–2.2 kPa), reflecting the competitive adsorption of ethanol–water dimers with ethanol monomers; while diethyl ether formation rates have a positive, less than first order dependence on ethanol pressure (0.9–4.7 kPa) and also decrease with water pressure (0.6–2.2 kPa), signifying a competition for active sites between ethanol–water dimers and ethanol dimers. Pyridine inhibits the rate of ethylene and diethyl ether formation to different extents verifying the existence of acidic and nonequivalent active sites for the dehydration reactions. A primary kinetic isotope effect does not occur for diethyl ether synthesis from deuterated ethanol and only occurs for ethylene synthesis when the β-proton is deuterated; demonstrating olefin synthesis is kinetically limited by either the cleavage of a C_β-H bond or the desorption of water on the γ-alumina surface and ether synthesis is limited by the cleavage of either the C–O bond of the alcohol molecule or the Al–O bond of a surface bound ethoxide species. These observations are consistent with a mechanism inhibited by the formation of dimer species. The proposed model rigorously describes the observed kinetics at this temperature and highlights the fundamental differences between the Lewis acidic γ-alumina and Brønsted acidic zeolite catalysts

Kinetics of Direct Olefin Synthesis from Syngas over Mixed Beds of Zn–Zr Oxides and SAPO-34

A packed bed containing a physical mixture of both Zn–Zr mixed oxide catalyst and SAPO-34 converts syngas directly into a mixture of C2–C5 olefins and paraffins. Specifically, the mixed oxide catalyst is responsible for intermediate oxygenate synthesis from syngas while the molecular sieve catalyzes olefin synthesis from the oxygenate intermediates. Kinetic measurements with cofed propylene over each catalyst independently confirm olefin hydrogenation activity over both components of the composite bed. The addition of either water or CO to the feed drops the activity of propylene hydrogenation over the Zn–Zr oxide. In sum, under reaction conditions of syngas feed and produced water, olefin hydrogenation predominantly occurs over the SAPO-34 catalyst, rather than over the catalyst responsible for hydrogenating CO into oxygenate intermediates. A developed kinetic model consistent with this conclusion describes measurements at differing feed compositions, temperatures, space velocities, and bed catalyst mixing ratios. Technoeconomic analysis of the process indicates that the olefin-to-paraffin ratio is a key performance metric for commercial scale syngas conversion and highlights the importance of considering olefin hydrogenation rates over the molecular sieve component

Effects of Composition and Structure of Mg/Al Oxides on Their Activity and Selectivity for the Condensation of Methyl Ketones

The effects of chemical composition and pretreatment on Mg–Al hydrotalcites and alumina-supported MgO were evaluated for the gas-phase, self-condensation reaction of C₃–C₅ biomass-derived methyl ketones. We show that the selectivity toward the acyclic dimer enone and the cyclic enone trimer can be tuned by controlling the temperature of hydrotalcite calcination. Methyl ketone cyclization is promoted by Lewis acidic sites present on the hydrotalcite catalysts. XRD and thermal decomposition analysis reveal that the formation of periclase MgO starts above 623 K accompanied by complete disappearance of the hydrotalcite structure and is accompanied by an increase in hydroxyl condensation as the formation of well-crystallized periclase. ²⁷Al MQMAS and ²⁵Mg MAS NMR show that at progressively higher temperatures, Al³⁺ cations diffuses out of the octahedral brucite layers and incorporate into the tetrahedral and octahedral sites of the MgO matrix thereby creating defects to compensate the excess positive charge generated. The oxygen anions adjacent to the Mg²⁺/Al³⁺ defects become coordinatively unsaturated, leading to the formation of new basic sites. A kinetic isotope effect, <i>k</i>_H/<i>k</i>_D = 0.96, is observed at 473 K for the reaction of (CH₃)₂CO versus (CD₃)₂CO, which suggests that carbon–carbon bond formation leading to the dimer aldol product is the rate-determining step in the condensation reaction of methyl ketones. We also show that acid–base catalysts having similar reactivity and higher hydrothermal stability to that of calcined hydrotalcites can be achieved by creating defects in MgO crystallites supported alumina as a consequence of the diffusion of Al³⁺ cations into MgO. The physical properties of these materials are shown to be very similar to those of hydrotalcite calcined at 823 K