Monolith particle-filled

The ratio between the through-pore size (ca. 8 pm) and the skeleton size (ca. 2.2 pm) shown in Fig. 5.2a is much greater than in a packed bed of a particle-filled column. Figure 5.3 shows the plots of skeleton size against the through-pore size in a column for a silica monolith prepared in a capillary or in a mould, as well as in a particle-packed column. The through-pore size/skeleton size ratios observed with the... 

The sol-gel method has been extensively used for the preparation of n-metal oxides and organic compounds. The important examples are n-NiO, n-Mn02, n-W03 and n-Fe203 etc. which have homogeneous particles, pore sizes and densities. This method affords easy control over the stoichiometry and homogeneity which is not possible with conventional methods. Further, the materials with special shapes monoliths, fibers, films and powders of uniform and very small particle sizes can also be prepared. The most important attribute of NMs prepared by this method is that they also contain pores of similar dimensions. These pores may be filled with another phase to form a nanocomposite which has proved to be of significant use to the HEMs community [98]. 

In order to obtain a reliable mathematical apparatus there has to be a reasonably simple, yet accurate, model of the structure of a syntactic foam. Attempts to extend the known models of monolithic plastics to syntactic structures filled with solid spherical particles have not proved successfull 8,76). Models which rely on close microsphere packing 157,158> have not been very accurate either. For example, Krzhechkovsky et al.,59) showed that syntactic foams can be treated as homogeneous uniform materials with small Ks (up to 20%) only if there is no stress field in the polymer matrix around the microspheres (see Sect. 3.5). 

Magnetic resonance imaging permitted direct observation of the liquid hold-up in monolith channels in a noninvasive manner. As shown in Fig. 8.14, the film thickness - and therefore the wetting of the channel wall and the liquid hold-up -increase nonlinearly with the flow rate. This is in agreement with a hydrodynamic model, based on the Navier-Stokes equations for laminar flow and full-slip assumption at the gas-liquid interface. Even at superficial velocities of 4 cm s-1, the liquid occupies not more than 15 % of the free channel cross-sectional area. This relates to about 10 % of the total reactor volume. Van Baten, Ellenberger and Krishna [21] measured the liquid hold-up of katapak-S . Due to the capillary forces, the liquid almost completely fills the volume between the catalyst particles in the tea bags (about 20 % of the total reactor volume) even at liquid flow rates of 0.2 cm s-1 (Fig. 8.15). The formation of films and rivulets in the open channels of the structure cause the further slight increase of the hold-up. 

The potential of structured packings as catalyst carriers for reactive stripping, film-flow-monoliths, Sulzer DX -packings, both coated with zeolite BEA, and katapak-S , filled with BEA-particles, was explored in cold-flow experiments and under reactive stripping conditions in a pilot-scale plant. 

Molecularly imprinted polymers with a variety of shapes have also been prepared by polymerizing monoliths in molds. This in situ preparation of MIPs was utilized for filling of capillaries [20], columns [21], and membranes [22, 23]. Each specific particle geometry however needs optimization of the respective polymerization conditions while maintaining the correct conditions for successful imprinting. It would be advantageous to separate these two processes, e.g., to prepare a molecularly imprinted material in one step, which then can be processed in a mold process in a separate step to result the desired shape. 

It is suggested that four mechanisms are basically involved in the process of compression of particles deformation, densification, fragmentation, and attrition. The process of compression is briefly described as follows small solid particles are filled in a die cavity and a compression force is applied to it by means of punches and then the formed monolithic dosage form is ejected. The shape of the tablet is dictated by the shape of the die while the distance between the punch tips at the point of maximum compression governs the tablet thickness, and the punch tip configuration determines the profile of the tablet. The compression cycle in a conventional rotary tablet press will be described in detail in this chapter and is illustrated in Figure 1. 

Channels filled with a single-particle string have much better solid flow characteristics than a packed bed, and so application of monoliths as moving-bed reactor internals is appealing. This structured moving-bed reactor design opens a wide range of applications. 

Reactors used in the lab-scale facility shown in Figure 1 were glass made and had several sizes to place the different monoliths. Commercial 15x15 x 30 cm monoliths were cut to smaller ones, of around 2.0 x 2.0 x 20 cm. Number of cells used in these "small monoliths were 3 x 3, 4 x 4 or 5 x 5, depending on cell density. To avoid bypass in the catalytic reactor, the free space between the monolith and the reactor wall was always carefully filled with carburundum (SiC). For some tests, monoliths were crushed, sieved to several sizes below 1 mm, and then used as particles. Temperature was measured with two thermocouples located just at the inlet and exit of the monolith. Tenqjcrature differences between these two points were < 2 C. Gas san ting and analysis methods in this small facility were already reported [6,7]. 

The method requires a high-purity fused capillary filled with standard HPLC packing materials constituted of particles that can be of very small diameters (1—3 gum) or of continuous bed (monoliths) since there is no back pressure. 

Figure 7.2 SEM images of microfluidic chips. (A) Cross-section through a microfluidic LC channel packed with 5 pm particles. (Reprinted with permission from ref. 35). (B) Pumping/valving channels ( 2 pm deep, placed 25 pm apart). (Reprinted with permission from ref. 35). (C) Cross-section through a microfluidic LC channel filled with methacrylate-based monolithic packing material. (Reprinted with permission from ref. 36).

---