Macropore pellet size dependence

Fig. 20 Pellet size dependence of macropore diffusion time constant for isobutane diffusion in 13X (cf. [65]) pellets ( ) and CO2 diffusion in 5A (cf. [65]) pellets (o) at 423 K obtained using a model discussed in Sect. 3.2.5...

Most commercial adsorbents consist of small microporous or nonporous crystals formed into macroporous pellets or particles. The solutes carried along the col-lunn by the fluid mobile phase must first be transported from the bulk fluid phase to the external surface of the adsorbent and then they must diffuse inside the particles. Within a particle there are two distinct kinds of diffusion phenomena that contribute to the resistances to mass transfer, the macropore (or inter-crystaUine) diffusion through the pellet and the micropore (or intra-crystaUine) diffusion resistance. The relative importance of macropore and micropore diffusion resistances depends on the pore size distribution within an adsorbent particle. Micropores have diameters smaller than 2 nm, macropores diameters greater than 50 nm while mesopores are in the range of 2 to 50 nm. 

In bidisperse porous adsorbents such as zeolite pellets there are two diffusion mechanisms the macropore diffusion with time constant Rp /Dp and the micropore diffusion with time constant rc /Dc. Bidisperse porous models for ZLC desorption curves have been recently developed by Brandani [28] and Silva and Rodrigues [29]. In bidisperse porous adsorbents, it is important to carry out experiments in pellets with different sizes but with the same crystal size (different Rp, same rc) or pellets with the same size but with different crystals (same Rp, different rc). If macropore diffusion is controlling, time constants for diffusion should depend directly on pellet size and should be insensitive to crystal size changes. If micropore diffusion controls the reverse is true. The influence of temperature is also important when macropore diffusion is dominant the apparent time constant of diffusion defined by Rp2(H-K)/Dp is temperature dependent in the same order of K (directly related to the heat of adsorption) which is determined independently from the isotherm. The type of purge gas is... 

The observed rate will depend on the molecular weight of the inert gas if it is influenced by the first step. External transport can also influence or control the rate of sorption/desorption if the sorbent consists of agglomerates of zeolite crystals such as pellets or layers. The rate of sorption or desorption will then depend on the size or shape of the agglomerates if it is influenced by the transport in the macropores between the crystals. 

Data on pore size distribution can be presented as a plot of cumulative pore volume against pore radius, as shown in Figure 4.5 for pellets made from porous alumina particles. The particles have micropores of 20- to 100-A radius that contribute about 0.4 cm /g to the total pore volume. The remainder of the pore volume is in macropores, the spaces between the small particles, and the macropore volume depends on the pelletizing pressure. Figure 4.5b is an alternate plot of the data, which shows more clearly the pronounced bimodal pore size distribution obtained at low pelletizing pressure. Increasing the pressure reduces the macropore volume, but it does not affect the micropores. 

The major pore sizes in pellet 2 (Fig. 3.4-la) are between 20 to 200 A, although depending on the specific manufacturing details, many other distribution curves are possible. One important special case is where the pellet is made by compressing smaller particles together, for whidi the second peadc in Fig. 3.4-16 (pellet 1") represents the so-called macropores betweoi the particles while the usual peak represents the micrres. Based on the above arguments, most of the catalytic surface is contained in the micix res, but all of the pores can contribute to diffusion resistances. Both pellets were made from 90 grains of alumina, but... 

A composite pellet offers two distinct diffusional resistances to mass transfer the micropore diffusional resistance of the individual zeolite crystals and the macropore diffusional resistance of the extracrystalline pores. A low resistance to mass transfer is normally desirable and this requires a small crystal size to minimize intracrystalline diffusional resistance. However, the diameter of the intercrystalline macropores is also determined by the crystal size and if the crystals are too small the macropore diffusivity may be reduced to an unacceptable level. The macropore resistance may of course be reduced by reducing the gross particle size but the extent to which this is possible is limited by pressure drop considerations. The optimal choice of crystal size and particle size thus depends on the ratio of inter- and intracrystalline diffusivities which varies widely from system to system. 

---