Particle-packed columns, limitations

There is much interest in high-efficiency- and high-speed separation media for liquid chromatography. The plate numbers available in practice have been in the range of 10,000-30,000 in HPLC for 20 years or so, but these are low compared to well over 100,000 theoretical plates in capillary gas chromatography or in capillary electrophoresis. This is caused by the limitation in the use of small-sized particles for HPLC, where a particle-packed column is commonly used under a pressure-drop of up to 40 MPa. An increase in column efficiency by using small particles, which is the approach taken in the past, is accompanied by an increase in the pressure-drop, as expected from Eqns. 5.2 and 5.3, below. Eqns. 5.1-3 describe the efficiency (plate height) and flow resistance of a column packed with particles [1-3], where N stands for the... 

Monolithic columns do have disadvantages. Although very high flow rates are used to speed up separations, this generates a considerable amount of solvent waste for >4.6-mm-bore columns. The number of phases and column sizes is very limited at present, as is the number of commercial manufacturers. Also, the technology of particle-packed columns is not static, but continues to improve as well. Monolithic columns have not yet demonstrated the performance capabilities exhibited by sub-2-pm particles and UHPLC. However, advances in monolithic column technology in the years to come promise to bridge that gap. 

The macroporous monolith approach, introduced by Frechet and Svec, seemed to address many of the problems associated with open-tubular and particle packed columns. First, the adsorptive capacity of capillary monoliths has been found to be 3-5 orders of magnitude larger than that of both open channel and bead-packed columns [32]. Next, since the polymerization takes place within the fused-silica capillary, the tedious process of packing the capillary columns may be avoided. Furthermore, the limitations in chromatographic efficiency caused by irregularities in particle packing and by the nonuniformity of particle sizes are eliminated. 

In a packed column, however, the situation is quite different and more complicated. Only point contact is made between particles and, consequently, the film of stationary phase is largely discontinuous. It follows that, as solute transfer between particles can only take place at the points of contact, diffusion will be severely impeded. In practice the throttling effect of the limited contact area between particles renders the dispersion due to diffusion in the stationary phase insignificant. This is true even in packed LC columns where the solute diffusivity in both phases are of the same order of magnitude. The negligible effect of dispersion due to diffusion in the stationary phase is also supported by experimental evidence which will be included later in the chapter. 

This paper will be limited to a discussion of our packed column studies in which we have addressed attention to questions regarding, (a) the role of ionic strength and surfactant effects on both HDC and porous packed column behavior, (b) the effects of pore size and pore size distribution on resolution, and (c) the effects of the light scattering characteristics of polystyrene on signal resolution and particle size distribution determination. 

Column reactors are the second most popular reactors in the fine chemistry sector. They are mainly dedicated reactors adjusted for a particular process although in many cases column reactors can easily be adapted for another process. Cocurrently operated bubble (possibly packed) columns with upflow of both phases and trickle-bed reactors with downflow are widely used. The diameter of column reactors varies from tens of centimetres to metres, while their height ranges from two metres up to twenty metres. Larger column reactors also have been designed and operated in bulk chemicals plants. The typical catalyst particle size ranges from 1.5 mm (in trickle-bed reactors) to 10 mm (in countercurrent columns) depending on the particular application. The temperature and pressure are limited only by the material of construction and corrosivity of the reaction mixture. 

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