Basic HETP

Structured packings are produced by a number of manufacturers. The basic construction and performance of the various proprietary types available are similar. They are available in metal, plastics and stoneware. The advantage of structured packings over random packing is their low HETP (typically less than 0.5 m) and low pressure drop (around 100 Pa/m). They are being increasingly used in the following applications ... 

Equation (10) is the basic form of the Van Deemter equation and will be expanded and discussed with other HETP equations in the next chapter. 

The model postulates th t in the absence of maldistribution, there is a basic (or true or inherent ) HETP which is a function of the packing and the system only. This HETP can be inferred from data for small towers, in which lateral mixing is strong enough to compensate for any pinching (Sec. 9.2.3). 

HETP and plate number The determination of these parameters, which characterize the efficiency, is more complex. The basic equations are explained in Section 2.4.2. The problem is to determine the peak width or variance from a measured chromatogram. Especially, if the second central moment has to be calculated the result depends very much on the extension of the base line, which has to be chosen for the integration interval. Figure 2.22 gives an example for a slightly asymmetric peak. 

As pointed out in Tab. 6.5, the HETP plot is needed for tracers and solutes. With tracers the theoretical HETP plot would be a straight horizontal line (B = 0, feeff = oo) that determines A and, thereby, Dx/umt. To fit within the framework presented here, for every solute only the slope B is determined and kefr is subsequently evaluated. Basically, A can also be derived from these plots, but in practice different axis intercepts A might be observed for solutes and tracers. Because of the limited precision of the second moment and the linearized HETP plot, the accuracy of the derived parameters is limited and the values for A and B (D-c/ujM and keff) should be taken as initial guesses" only (Chapter 2.7, Section 6.5.3.3). Thus, the differences in the observed axis intercepts can be safely ignored and are often negligible. For further discussions of HETP plots see, for example, Van Deemter et al. (1956), Grushka et al. (1975), Weber and Carr (1989) or recent publications on NMR measurements by, for example, Tallarek et al. (1998). 

Thus, if theoretical stages are computed by one of the methods described earlier, it is only necessary to multiply by some characteristic value of the packing, the HETP, to arrive at the required height. This approach is often used in practice, with the packing vendor supplying values of HETP. The problem is that the vendor may not take into account the basic requirements of determining efficiency, as stated earlier for trays ... 

The quantity Af is a measure of the quality of a separation that is based on a single peak. Since the HETP is a function of many different parameters (see Section 2.2), the plate count is not a basic property of a column and therefore not a measure of column quality. Unfortunately, it is frequently used this way, especially in the marketing literature of column manufacturers. 

The van Deemter equation provides a simple and useful description of the basic phenomena in a packed bed. However, it has two shortcomings. The first is the fact that the A term, describing the nonuniformity of the packed bed, has been introduced in a rather ad hoc manner and does not withstand careful theoretical examination. The second problem arises from the fact that in many practical cases, especially in gas chromatography, a downward curvature is observed on the right-hand branch of the HETP-linear velocity plot, which cannot be accounted for by the van Deemter theory. Something is wrong. 

Giddings (9,10) argued that the origin of the problem rests with a basic conjecture of the van Deemter concept the assumption that the different contributions to the HETP are independent of each other and that therefore the variances of these different contributions can be summed up ... 

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