Methanol synthesis, diffusion limitations

Examples of a simultaneous numerical solution of molar and energy balance equations for the gas bulk and catalyst particles are introduced in Figure 5.27, in which the concentration and temperature profiles in a methanol synthesis reactor are analyzed. The methanol synthesis reaction, CO -F 2H2 CH3OH, is a strongly exothermic and diffusion-limited reaction. This implies that concentration gradients emerge in the catalyst particles, whereas the heat conductivity of the particles is so good that the catalyst particles are practically isothermal. 

Graaf et al. [51] used the dusty gas model to investigate the relative importance of Knudsen and bulk diffusion for the intra-particle diffusion limitation problem for the methanol synthesis. In their steady-state model, they neglected the pressure gradient and assumed isothermal conditions throughout the spherical particle. Their simulation results indicated that both bulk and Knudsen diffusion mechanisms should be incorporated in the single pellet model to obtain reliable results. 

Grad H (1949) On the kinetic theory of ratified gases. Comm Pure Appl Math 2 331-407 Graaf GH, Scholtens H, Stamhuis EJ, Beenackers AACM (1990) Intra-particle diffusion limitations in low pressure methanol synthesis. Chem Eng Sci 45 773-783 Greiner W (2003) Classical mechanics systems of particles and hamUtonian dynamics. Springer, New York... 

Another synthesis process proposed to receive benefits from operating with monolith catalysts is the conversion of methanol for gasoline production [16,17J. The catalyst used was the ZSM-5 zeolite. However, rather than binding the catalyst onto the wall by use of a washcoat, it was uniformly crystallized on the cordierite honeycomb (62 cells/cm ) wall surfaces (up to 30% by weight), similar to the method described in the patent assigned to Lachman and Patil [18]. The effects of methanol partial pressure on conversion and temperature on hydrocarbon selectivity were determined. Three regimes of mass transfer resistances are experienced in this reaction reactant transfer to the reactor walls within the monolith channels through the laminar flow, diffusion resistance at the surface between zeolite crystals on the walls, and diffusion into the zeolite molecular-size pores to the active sites within the crystals, where the reaction rate limit is anticipated. 

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