Small column diameter

For packed columns, band broadening is minimized by small particle diameters. For capillary columns, small column diameters reduce band broadening. 

These calculations ignore the fact that for small column diameters the resultant significant pressure gradients require a more fundamental form of the Golay equation (8,9). Because the optimum linear velocity becomes increasingly more independent of the diameter as the number of plates increases for diameters of this size... 

LC uses mostly packed columns, as the use of open tubular columns in this method is not practical because of the extremely small column diameters required for good separation. In gas chromatography, both packed and open tubular columns can be used, but the latter are far more popular because of their vastly superior properties. The mobile phase is usually forced through the stationary phase at elevated pressure, although other approaches are also possible (e.g., electrically driven flow in electrochromatography (EC), gravity driven flow in classical LC or flow driven by capillary forces in TLC). 

The section of column wall directly opposite a bottom feed (or reboiler return) nozzle is often prone to corrosion and erosion attacks. High inlet velocities, small column diameters, and corrosive chemicals are conducive to this problem. The author has experienced a case where metal was thinned and the colximn wall was eventually punctured opposite the bottom inlet of a 2.5-ft-diameter column. Installing an impingement plate on the wall will shield it from such corrosion and erosion attacks. 

High Speed Capillary SFC. Until recently the practice of SFC was restricted to packed columns (17,20-26.43). Capillary colusins are clearly superior for most applications in gas chrootatography, while packed columns in liquid chromatography have tremendous advantages compared to open tubular capillary columns due to the small column diameter (< 10 ym) required to produce equivalent separations (44). Since supercritical fluids have properties intermediate between those of a gas and those of a liquid, one might expect that both packed and capillary SFC methods would be competitive and the method of choice depend on the particular separation desired. 

The inoease in HETP with column diameter appears to be most pronoimced for small column diameters (< 1 ft Refs. 56,164) and with smaller packings (<1 in Refs. 66,136). It has therefore been recommended tiiat for scaleup purposes, a column at least 1 ft in diameter, and prefmably larger, ould he used (164). 

In upflow bubble operation the consumption of the gas phase by reaction must also be considered in the model if the reactor operates under lower pressure (<20 bar) and if the reactor length is of technical dimensions (L>2 m) additionally gas phase dispersion (radial and axial) may have an influence on conversion [65]. As this reactor type is also used in waste water treatment as well as in fermentation processes, the possible non-Newtonian behavior of the liquid phase as well as the coalescence behavior of the system must be taken into account. Finally, it should be remembered that - comparable to fluidized bed reactors - results from laboratory reactors with small column diameter and/or particle sizes smaller than 0.2 cm usually cannot be regarded as representative for technical upflow units, because capillary force as well as lare scale circulation in the liquid phase may be significantly different. 

We should emphasize that all the data which are presented here concern a small column diameter with a good initial distribution. They should be extrapolated with great care especially in the trickling flow regime for which in large size column it is often a challenge to have a uniform distribution of the liquid all over the reactor. 

The greater dp, the greater the sample capacity—an important factor in SFC because of the demands on detector sensitivity imposed by small column diameters. On-column detection by UV absorption, fluorescence, etc., is feasible even for 50-jum columns column lengths of a few cm may be used [34]. 

Even on the small diameter columns used (i.e., 50 pm) diffusion coefficients dictated an optimum linear velocity (popt) in the order of 0.1 cm s Since columns tended to be 10 m long, the optimum column hold-up time was in the order of 16.7 min. Since this is unrealistically long for routine analysis, most workers operated up to 10 times Popo making the column hold-up time less than 2 min with typical run times of 30 min to 2h. Efficiency dropped from 200000 plates near optimum to roughly 20000 plates at the higher velocity. Very small column diameters made it difficult to make reproducible injections. 

At constant k a increases with increase in T. The relationship can be approximately described by k a qc r°. This is in contrast to the small column diameter data, which show a negative dependence on T. 

Therefore, using ALOT columns with small column diameter for high-speed analysis is very promising. ALOT columns can eilso allow faster chromatographic separations in routine analysis. For example, Fig. 2-5 shows the separation of 16 hydrocarbons C1-C4 within 300 seconds using ALOT column with alumina [29]. As can be seen from... 

With modem equipment, most of the zone broadening occurs within the column. These effects may be grouped into two major categories. The first is axial diffusion. As the sample ions are resolved into discrete zones, the zones tend to expand because of diffusion of the ions. Since diffusion across the column is limited by the small column diameter, the net zone diffusion takes place along the column and hence is called axial diffusion . 

List pros and cons of large/small column diameters and packing size. For more problems on this topic, see Chapter 8. 

Small column diameter, in order to keep gas-phase diffusion effects to minimum. 

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