Bimodal catalyst

FIGURE 10.1 Diagram of bimodal catalyst pore structure. 

Xu, B. L., Fan, Y. N., Zhang, Y., Tsubaki, N. 2005. Pore diffusion simulation model of bimodal catalyst for Fischer-Tropsch synthesis. AIChE Journal 51 2068-76. 

Bimodal catalysts were designed as a compromise between the important features of both HDS and HDM catalysts. The bimodal catalyst has micropores (<50A) and mesopores in the 50- to 100-A pore diameter range which provide the high surface area and large macropores... 

Fig. 48. Experimental vanadium deposit distributions in a microporous catalyst (100 A micropore diameter/204 m2/g SA) macroporous catalyst (1300 A pore diameter/14.5 m2/g SA) and bimodal catalyst (120 A micropore, 25,000 A macropore diameters/200 m2/g SA) (Plumail e al., 1983).

As discussed in Section IV, Agrawal and Wei (1984) and Ware and Wei (1985b) have successfully modeled experimental deposit profiles by using the theory of coupled, multicomponent first-order reaction and diffusion. Wei and Wei (1982) employed this theory to evaluate the influence of catalyst properties on the shape of the deposit profile. Agrawal (1980) developed a model for the deactivation of unimodal and bimodal catalysts based on the consecutive reaction path. These approaches represent a more realistic consideration of the HDM reaction mechanism than first-order kinetics and will, accordingly, be discussed in more detail. 

Agrawal (1980) adopted the grain model of Sohn and Szekely (1972) to model the deactivation of a bimodal catalyst for the HDM reaction. The schematic in Fig. 59 illustrates the proposed physical structure of the catalyst pellet. The macrospherical pellet of radius Ra is composed of numerous microspheres of radius / , where the number of microspheres per unit volume is given by... 

Using this model, Agrawal (1980) computed the initial demetallation rate as a function of micropore radius and microsphere radius (grain size) for the bimodal catalyst. The results are shown in Fig. 60 for a typical set of... 

Agrawal (1980) also computed the effect of time on stream on the HDM reaction rate for various cases of bimodal and unimodal catalysts. These comparisons are shown in Fig. 61. As is evident, improvements in stability and overall activity rather than initial activity are gained, whether unimodal or bimodal catalysts are used, by increasing the micropore size. The relative capacity of the catalysts can be visualized as the area under the curves in Fig. 61. 

Additional comparisons of fresh vs. used catalyst are shown in Table VII. The coke laydown occurred mainly in the micropore region, causing a substantial loss of surface area. Coke laydown also was observed in the macropore region of the bimodal catalyst shown at the bottom of the table. 

Hetero Transition Metals Assembly and Development of Bimodal Catalyst... 

The S/C and V/C ratios in residual asphaltenes in the products from different nms are plotted in Figs. 3a and 3b. The lowest sulfur and vanadium concentrations in the asphaltenes are noticed for the run conducted with the monomodal macropore catalyst Q. The catalyst R which contains a large proportion of both macro and meso pores ranks next for the removal of sulfur from asphaltenes. Although the conventional bimodal catalyst (S) with a large proportion (> 50%) of micro pores and about 20% macropores shows good activity for removal of sulfur from asphaltenes similar to catalysts R its activity for vanadium removal is poor. 

Zhang Y, Y oneyama Y, T subaki N Simultaneous introduction of chemical and spatial effects via a new bimodal catalyst support preparation method, Chem Commun 11 1216—1217, 2002. 

In order to select optimal support material for a certain application, information regarding the transport characteristics of the micro/meso- and macropore system of the available bimodal catalyst supports is required. The transport parameters of the macropore system of a formed body can be determined using a Wicke-Kallenbach diffusion cell. Except for the well examined molecular sieves, there is only little information regarding the transport properties of the micro- or mesoporous primary pore system of catalyst supports [5]. 

Recent investigations, both experimental measurements [45,46] and simulation studies (42) have indicated that bimodal catalysts have considerable potential. The large pores provide greater rates of diffusion than the smaller pores However the small pores provide greater surface area on which the conversion reactions can occur. Ideally, if both large and small pores are combined, an optimum can be attained. 

Zhang et al. [134] introduced a bimodal catalyst support for CO reforming reactions. This catalyst contains both macropores and mesopores, where macropores positively influence the mass transfer of reactant and products and mesopores improve the dispersion of the metal. Si02 Si02 biomodal support used for this reaction was prepared by impregnation of silica nanoparticles into the macropores [135]. Development of mesopores occurred through... 

Spatial Effects via a New Bimodal Catalyst Support Preparation Method Chem. Commun. 2002,1216-1217. 

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