Effect of transport limitations on C2+ selectivity in the oxidative methane coupling reaction using a NaOH/CaO catalyst

Advanced reactor concepts for oxidative coupling of methane

“…Minor optimizations can be achieved by controlling mass transfer limitations that can be present in all the three systems (gas-particle mass transfer in packed beds, gas-wall mass transfer in monoliths or tubular reactors with wall coated catalyst, bubble-to-emulsion phase mass transfer in fluidized beds, and bubble-to-liquid mass transfer in molten salts systems). Mass transfer resistances can either have a beneficial or detrimental effect on the C 2 selectivity: on the one hand, it can decrease the local oxygen concentration at the catalyst surface, which is known to be favorable for the selectivity of the OCM reaction (this was also proven by experimental studies with different catalyst particle sizes (22), and is confirmed by the main kinetic models for OCM proposed in the literature); on the other hand, it can increase the concentration of C 2 close to the catalyst, which increases the rate of unselective consecutive reactions of combustion and reforming of the formed C 2 's. For this reason, controversial effects of mass transfer resistance have been reported in the literature even for the same reactor concept, depending on the choice of the catalyst, as reported by Mleczko and Baerns (20).…”

“…The particle size can have, on the other hand, an influence on the internal heat and mass transfer limitations in the porous pellets. While it is clear that larger particles can decrease the gas pressure drop and increase the internal mass transfer resistances, the effect of these resistances on the C 2 yield for the OCM reaction system is controversial (20,21), as it may be beneficial for some type of catalysts (22). When using catalytic wall reactors, on the other hand, the gas flow rate must be tuned to avoid bypass, especially if the tubes are large.…”

Irreducible Mass-Transport Limitations during a Heterogeneously Catalyzed Gas-Phase Chain Reaction: Oxidative Coupling of Methane

“…The effect of pellet diameter on selectivity was also investigated by Follmer et al (1989), Schiebisch et al (1991), and Reyes et al (1993b). Follmer et al (1989) experimentally found an increase in the selectivity with increasing pellet diameter and attributed this to the lower intraparticle concentration of oxygen. A low oxygen concentration can improve the C 2 selectivity.…”

Enhanced catalytic methane coupling using novel ceramic foams with bimodal porosity

“…Comparing foam and powder, very interesting conclusions can be drawn. As far as the C 2 yields (in %) are concerned, the foam outperforms the powder bed by a factor of up to 2; the space time yield (STY) per mass is increased by a factor of 3-4, while the STY referenced to the specific surface area is even more increased (by a factor of [20][21][22][23][24][25][26][27][28][29][30]. (Yet, as pointed out in the introduction, the surface area is not a good point of reference since increasing surface area not necessarily means improved C 2 selectivity.)…”

“…Using CO oxidation as a test reaction, Kraushaar-Czarnetzki et al investigated the influence of the different gas flows in packed beds of beads, honeycomb structures and foams and found for the latter an intermediate situation with respect to the mass transfer between the gas phase and the catalyst surface. 7,24,25 In this context, it is important to know that a certain degree of mass transport limitation was shown to improve the C 2 selectivity 26 (because of a lower oxygen partial pressure at the surface 27 ). Therefore, the smaller mass transfer is likely to be the reason for the better performance of the foam as compared to the particle bed, whereas the honeycomb structure is inferior because of the laminar flow pattern.…”