Analysis of the drying stage in the technology of supported catalysts

Theory of preparation of supported catalysts

“…If the sorption effects during impregnation are negligible (P « 1), the solution is uniformly distributed in the pellet volume. However, the component's distribution may change essentially as a result of drying (1,10,(17)(18)(19).…”

“…It is possible to propose three main mechanisms of liquid transfer in a porous particle upon drying: volume transfer of the capillary condensate, adsorption film flow, and recondensation through the gas phase (27¿28). Of these three mechanisms, the former one which causes the component's redistribution (10,(17)(18)(19) is the most important and probably needs some explanation. The reaction proceeding via this mechanism is determined by the Pap dependence upon the meniscus radius r (eq 6).…”

“…Certainly, this analysis is rather approximate since it does not take into account a series of factors, namely the correlation between the pore radii and the existence of the adsorption film. Nevertheless it depicts correctly a qualitative picture of liquid distribution and is consistent with the experimental data, e.g., of ref 33 and 34. Further analysis (17,31) leads to the establishment of the value of critical moisture content, U", for a given porous medium. At the moisture content lower than this value, the liquid is in the form of individual clusters.…”

“…Distribution of Liquid in the Porous Medium during Drying. Proceeding from the above ideas on the mechanism of moisture transfer in porous media as well as the character of liquid distribution inside the porous medium with a uniform moisture concentration it is possible to give a scheme of liquid distribution during drying of the porous particle (17)(18)(19). In the general case four characteristic zones are formed inside this particle (Figure 4): (i) external dry zone with a zero moisture concentration; (ii) external two-phase zone.…”

“…The distribution of an active component on the surface of the support pores is primarily determined at the steps of impregnation and drying (10)(11)(12)(13)(14)(15)(16)(17)(18)(19). The principles of the distribution depend on the type of binding between a precipitated component and a support surface and also on the mass-transfer conditions in the pores.…”