Intensification of Catalytic Processes through the Pellet Structuring: Steady-State Properties of a Bifunctional Catalyst Pellet Applied to Generic Chemical Reactions and the Direct Synthesis of DME

Abstract

Structuring of different types of catalytic active centers at a single-pellet level appears to be a promising and powerful tool for integration and intensification of multistep solid-catalyzed chemical reactions. However, the enhancement in the product yield and selectivity strongly depends on the proper choice of the distribution of different catalysts within the pellet. To demonstrate potential benefits from properly designed catalyst pellet, numerical studies were conducted with the aid of the mathematical model of a single spherical bifunctional catalyst pellet. The analysis was performed both for a system of two generic chemical reactions and for a real process, i.e., direct synthesis of dimethyl ether (DME) from synthesis gas via methanol. Evaluation of the pellet performance was done for three arrangements of the catalytic active sites within the pellet, i.e., a uniform distribution of two types of catalytic active centers in the entire volume of the pellet, and two core–shell structures. It was demonstrated that, especially for the larger pellets typical for fixed-bed applications, the product yield might be significantly improved by selecting proper catalyst arrangements within the pellet.

Model Analysis of the Role of Kinetics, Adsorption Capacity, and Heat and Mass Transfer Effects in Sorption Enhanced Dimethyl Ether Synthesis

“…Moreover, it is reported that in hybrid pellets, deactivation may also occur because of the close interaction between the two active phases, possibly leading to detrimental cross migration of elements at the boundaries . The core@shell catalysts are proposed as alternative to the mechanical mixture and the hybrid configurations, as the diffusion lengths are significantly reduced ,, while allowing for a better temperature control . Besides, the core@shell design significantly reduces the contact area between the active phases, partially preventing the aforementioned deactivation .…”

“… 56 The core@shell configurations instead, consist in the layering of the methanol synthesis and dehydration active phases in a single pellet. 51 , 54 , 55 , 57 − 66 In the case of MeOH@DME configuration, the CZA catalyst core (brown) is surrounded by a shell of γ-Al 2 O 3 , (gray), whereas in the DME@MeOH, the core and the shell formulations are inverted.…”

“…The hybrid pellet is a configuration with the catalytic materials interdispersed inside the same particle, creating a homogeneous active phase distribution . The core@shell configurations instead, consist in the layering of the methanol synthesis and dehydration active phases in a single pellet. ,,,− In the case of MeOH@DME configuration, the CZA catalyst core (brown) is surrounded by a shell of γ-Al 2 O 3 , (gray), whereas in the DME@MeOH, the core and the shell formulations are inverted.…”

“…Once the operating conditions as temperature and pressure are fixed, the adsorption capacity and reaction kinetics can be managed by acting on the regeneration methods, on the time-design of the regeneration cycle and on the adsorbent/catalyst ratio. , The effect of this latter parameter is addressed in the present analysis, considering its kinetic and thermodynamic consequences and their impact on the thermal behavior of the reactor. In addition, the effect of the active phase distribution at the pellet scale is investigated to address the impact of the intraparticle diffusion limitations in catalyst pellets, which were shown to markedly affect kinetics, and consequently the reactor performances in direct DME synthesis. − …”

Recent Progress in Direct DME Synthesis and Potential of Bifunctional Catalysts

“…The simplest system would be mixing the particles or pellets physically on the reactor level (hybrid catalyst bed) [63,133,175]. A physical mixture on the particle level increases the proximity of the active sites for the two reactions and, thus, decreases the probability for the intermediate (MeOH) to leave the reactor without getting dehydrated to DME [176][177][178][179][180][181][182]. For the aforementioned scheme, one could utilize mixing the dried STM-and MTD catalysts [177,180,182].…”