Channel shift model

Figure 12. Illustration of Channel Shift Model. Alkali migration behaviour before and...

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

The 7-shifting method depends on our ability to identify a unique bottleneck geometry and is particularly well suited to reactions that have a barrier in the entrance channel. For cases where there is no barrier to reaction in the potential energy surface, a capture model [149,150,152] approach has been developed. In this approach the energy of the centrifugal barrier in an effective onedimensional potential is used to define the energy shift needed in Eq. (4.41). For the case of Ai = 0, we define the one-dimensional effective potential as (see Ref. 150 for the case of AT > 0)... 

Some research groups working on the modeling of MCFC include the reforming reactions in their process models in different ways. He and Chen [1] and Yoshiba et al. [2] only consider the water-gas shift reaction in a spatially distributed anode channel. Due to its high rate, they assume the shift reaction to be in chemical equilibrium. Lukas and Lee [3] and Park et al. [4] also describe the water-gas shift reaction in equilibrium, but in addition they include the steam reforming reaction of methane as an irreversible reaction with a finite reaction rate. In particular, Park... 

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