Packed particle size/permeability

Unfortunately the particle size affects quadratically the column permeability. When the packing particle size is divided by two, the column driving pressure must be four times higher in order to obtain the same flow-rate. Often, very small particle size is associated with short columns in order to reduce the working pressure. 

All packing materials produced at PSS are tested for all relevant properties. This includes physical tests (e.g., pressure stability, temperature stability, permeability, particle size distribution, porosity) as well as chromatographic tests using packed columns (plate count, resolution, peak symmetry, calibration curves). PSS uses inverse SEC methodology (26,27) to determine chromatographic-active sorbent properties such as surface area, pore volume, average pore size, and pore size distribution. Table 9.10 shows details on inverse SEC tests on PSS SDV sorbent as an example. Pig. 9.10 shows the dependence... 

Permeability is another method for obtaining information about pcirticle diameters. If one packs a tube with a weight of powder exactly equal to its density, and applies a calibrated gas pressure through the tube, the pressure drop can be equated to an average particle size. The instrument based on this principle is called the "Fisher Sub-Sieve Sizer ". Only one value can be obtained but the method is fast and reproducible. The instrument itself is not expensive and the method can be applied to quality control problems of powders. Permeametry is usefiil in the particle range of 0.5 to 50 n. 

As e is increased, flow through the bed becomes easier and so the permeability coefficient B increases a relation between B, e, and S is developed in a later section of this chapter. If the particles are randomly packed, then e should be approximately constant throughout the bed and the resistance to flow the same in all directions. Often near containing walls, e is higher, and corrections for this should be made if the particle size is a significant fraction of the size of the containing vessel. This correction is discussed in more detail later. 

Commercially available monolithic columns are based either on silica or organic polymer and are generally characterized as a polymeric skeleton with macropores, with a diameter of approximately 2 pm, and mesopores, with a diameter of approximately 13 nm. The role of the macropores (through-pores) is to provide channels with high compounds permeability, which permits the use of higher flow rates with respect to columns based on conventional particle size, and an extended surface area, which is comparable to conventional columns packed with 3 pm particles. 

The particle size of the packing material determines the number of theoretical plates per unit length that can be generated (see Section 1.3.4). Small particle sizes result in high efficiencies however, increased backpressure can occur as a result of the decreased column permeability. If the small particles are packed into shorter columns the speed of analysis will increase at the same efficiency. 

Due to small particle size, the columns packed with micropellicular stationary phases have low permeability (27) and therefore, can not be operated at very high flow rates due to pressure limitations of commercial HPLC instruments. In comparison to porous particles, the surface area of stationary phases per unit column volume is low, and hence, their loading capacity is correspondingly smaller. This is particularly evident in the isocratic analysis of small molecules where the column can be easily overloaded. Therefore, micropellicular sorbents do not appear to offer advantages in the HPLC of small molecules. 

Particle behavior is a function of particle size, density, surface area, and shape. These interact in a complex manner to give the total particle behavior pattern [28], The shape of a particle is probably the most difficult characteristic to be determined because there is such diversity in relation to particle shape. However, particle shape is a fundamental factor in powder characterization that will influence important properties such as bulk density, permeability, flowability, coatablility, particle packing arrangements, attrition, and cohesion [33-36], Consequently it is pertinent to the successful manipulation of pharmaceutical powders that an accurate definition of particle shape is obtained prior to powder processing. 

Besides the packing density, the pore structures belong to the important properties of particulate structures. They constitute an useful aspect of packed particles, since they control properties such as filtration, permeability, fluid trapping, etc.. Packing structures of powders with continuous size distribution are very complex. The pores in ordered packings of monosized spheres would seem to be the simplest case to study. 

Packed capillary columns have also been described which have an internal diameter of < 0.5 mm with a solid support particle size of between 100 and 200 p. The characteristics of these columns are more akin to those of simple capillary columns than to the characteristics of packed columns. Their permeability is higher than that of the standard packed column and, therefore, they give fast analysis with a high separation power. The sample loading is considerably higher than what can be introduced without overloading onto a capillary column and, as wider choice of stationary phases is available, packed capillary columns have a wide range of application. 

In a further study, Chellam and Wiesner (1997) showed that the specific resistance of the deposit increased with shear rate and decreased with initial flux. This implied that the deposit structure is also important. Additionally, Veerapaneni and Wiesner (1994) simulated the deposition of particles on permeable surfaces. Small particles ( 1 pm) and low fluid velocities favoured the formation of loose deposits on the surface, while particles > 1 pm formed dense deposits. These results show the impact of colloid size on particle packing and thus the permeability of the deposit. Particle-particle interactions, however, were neglected. 

Kinetic factors will lead to dispersion of the fronts being much more important for favorable isotherms. Intraparticle mass-transfer resistance can be eliminated or decreased by using peUicular packings, reducing particle size or increasing particle permeability as shown in Fig. 3.4-4. 

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