Catalyst bidispersed

The final size and shape of the catalyst particles are determined in the shape formation process, which may also affect the pore size and pore size distribution. Larger pores can be introduced into a catalyst by incorporating in the mixture 5 to 15 % wood, flour, cellulose, starch, or other materials that can subsequently be burned out. As a result, bidisperse catalyst particles are obtained. 

Figure 82 Schematic of (a) monodis-perse and (b) bidisperse catalyst.

In the above considerations the total pressure should also be taken into account. With increasing pressure, the efficiency of a bidisperse catalyst decreases because diffusion in micropores turns from Knudsen to ordinary and the difference between Df n and Dejt disappears. At a pressure of 1-10 MPa, a uniform porous structure with a pore size close to the mean free path is the most favorable [6]. 

Thus diffusion limitations decrease the yield twofold. These results may be generalized to include interface and intraparticle diffusion for bidisperse catalysts [5]. The effect of diffusion limitation on the concentration distribution over the reactor length can be calculated from Equations 8.24, 8.29 and 8.33. An example of such calculations is shown in Figure 8.4 for the case CM = 0, DtA = D and kjkx = 0.1. Although the rate of... 

Macro- and micropore volumes and porosities for bidisperse catalyst pellets are calculated by the same methods as used for monodisperse pore systems. Example 8-5 illustrates the procedure. 

U. Maheshwari, S. Nigam, and D. Kunzuru, "Diffusional Influences on Deactivation Rates of Bidispersed Catalysts", AIChE Journal. 1985, 31-8. 1393-1396. 

Wakao and Smith [20] originally developed the random pore model to account for the behaviour of bidisperse systems which contain both micro- and macro-pores. Many industrial catalysts, for example, when prepared in pellet form, contain not only the smaller intraparticle pores, but also larger pores consisting of the voids between compressed particles. Transport within the pellet is assumed to occur through void regions... 

The hydrodenitrogenation activity for both, mono- as well as bidispersed, decreased significantly within 100-150 hr of oil-catalyst contact time. Carbonaceous depositions seem to be the primary cause for catalyst activity decay. 

Two Co-Mo-alumina catalysts obtained from a commercial vendor as either marketed or special research samples were used in this study. The surface area and pore-size distributions (using the mercury penetration technique) were determined by an independent commercial laboratory. The catalyst properties are given in Table II. Note that the monodispersed (MD) and bidispersed (BD) catalysts have the same metallic composition and are chemically similar. 

Figure 3.1 Pore-size distribution in catalysts (r in nm) (a) monodisperse (b) bidisperse.

The described treatment of mass transport presumes a simple, relatively uniform (monomodal) pore size distribution. As previously mentioned, many catalyst particles are formed by tableting or extruding finely powdered microporous materials and have a bidisperse porous structure. Mass transport in such catalysts is usually described in terms of two coefficients, a effective macropore diffusivity and an effective micropore diffusivity. 

A significant increase in catalytic activity as compared to the limiting values, shown in Figure 8.1, can be achieved by the use of bidisperse porous structures. Such catalyst pellets are formed by compressing, extruding or in some other manner compacting finely powdered mkroporous material into a pellet. Ideally the micropores are due to the porosity in the individual microparticles of catalyst. The macropores result from voids between the microparticles, after pelletization or extrusion. In such catalysts, most of the catalytic surface is contained in the micropores, since S llre. The bidisperse structure is illustrated in Figure 8.2 compared to monodisperse particle. 

A single effective diffusion coefficient cannot adequately characterize the mass transfer within a bidisperse-structured catalyst when the influence of the two individual systems is equally important. In a realistic model the separate identity of the macropore and micropore structures must be maintained, and the diffusion must be described in... 

A bidisperse structure can be advantageous because the effectiveness factor in the microparticles is often close to unity (their size being three to four orders of magnitude smaller than the usual size of the industrial catalysts). It is of interest to estimate the ratio of the conversion rates with mono- and bidisperse structures, having the same size, when the porous structure of the microparticles is identical to that of a monodisperse pellet. This ratio can easily be found when the micropore effectiveness factor is close to unity, as in the case for many industrial systems. Since the external surface area of the... 

Cylindrical pellets of four industrial and laboratory prepared catalysts with mono- and bidisperse pore structure were tested. Selected pellets have different pore-size distribution with most frequent pore radii (rmax) in the range 8 - 2500 nm. Their textural properties were determined by mercury porosimetry and helium pycnometry (AutoPore III, AccuPyc 1330, Micromeritics, USA). Description, textural properties of catalysts pellets, diameters of (equivalent) spheres, 2R, (with the same volume to geometric surface ratio) and column void fractions, a, (calculated from the column volume and volume of packed pellets) are summarized in Table 1. Cylindrical brass pellets with the same height and diameter as porous catalysts were used as nonporous packing. 

Both catalysts were mono- or bidispersed with mean pore radii about 70 and 2000 run diffusion and permeation measurements were performed with four inert gases (H2, He, N2 and Ar). 

Two porous catalysts in the form of cylindrical pellets were used industrial hydrogenation catalyst Cherox 42-00 with monodisperse pore structure (Chemopetrol Litvinov, Czech Rep. height x diameter = 4.9 x 5.0 mm) and laboratory prepared a-alumina, A5 (based on boehmite from Rural SB, Condea Chemie, Germany) with bidisperse pore structure (height x diameter = 3.45 x 3.45 mm). 

A final word should be said about the variety of porous materials. Porous catalysts cover a rather narrow range of possibilities. Perhaps the largest variation is between monodisperse and bidisperse pellets, but even these differences are small in comparison with materials such as freeze-dried beef, which is like an assembly of solid fibers, and freeze-dried fruit, which appears to have a structure like an assembly of ping-pong balls with holes in the surface to permit a continuous void phase. ... 

In the past, similar bidispersed systems have been investigated and modeled in the catalyst deactivation area (5-7). However, modeling of immobilized affinity adsorbent beads is more complex due to the non-linearity introduced by the rapid ligand binding reaction which is dependent on the product concentration. 

The porous structure of most catalysts is polydisperse. Therefore, capillary condensate fills only part of the pore-space — mostly small pores. The bidisperse globular structure (Figure 23.1) is convenient to consider as a model for rough estimations of the influence of external mass transfer and intraparticle diffusion on the total reaction rate. Such an analysis was made by Ostrovskii and Bukhavtsova [8]. According to this model, the only pores inside globules (micropores) will fill with liquid, and space between globules (macropores) fill with gas. Then the total porosity can be written as... 

The pores of the solid may be interconnected (accessible to fluid from both ends of the pores), dead-end (connected to the outside of the solid only from one end), or isolated (inaccessible to external fluid). The pores in most solids are neither straight nor of constant diameter. Catalyst particles manufactured by pressing powders containing micropores into pellets, with macropores surrounding the powder particles of a different order of magnitude in size, are said to be bidisperse [19]. 

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