HETP measurement

Equation (15) gives the variance per unit length of a GC column in terms of the outlet pressure (atmospheric) the outlet velocity and physical and physicochemical properties of the column, packing, and phases and is independent of the inlet pressure. However, equation (13) is the recommended form for HETP measurements as the inlet pressure of a column is usually known, and is the less complex form of the HETP expression. 

Model cycling of the first column in the purification stream (this usually is subjected to the harshest cleaning conditions and sees the dirtiest feedstocks), process monitoring of production columns (yield and purity of product, HETP measurements, pressure-vs.-flow tests)... 

Figure 9.7 shows HETPs measured in teBts that simulate various types of maldistribution in a 4-ft column containing a 12-ft bed of 1-in Pall rings (181,160). The y-axis is the ratio of measured HETP in the maldistribution tests to the HETP obtained with an excellent distributor. 

Safety factor. Even if the above techniques are followed, some uncertainty still remains. To allow for this uncertainty, it has been recommended (98,120) to add 6 to 12 in to the HETP measured in small-scale columns, and possibly more in vacuum columns operating at low liquid rates. This recommendation has been criticized (168) for being too conservative. An alternative recommendation (167) is to add a 10 to 15 percent safely factor. 

Equation [31 -8] is based on theory. A more practical method is needed for day to day use and equation [31 -9] based on common HETP measurements for chromatography can be applied. 

It is evident that the HETP measures only the overall resistance to mass transfer and cannot provide evidence concerning the nature of this resistance. The chromatographic response is indeed remarkably insensitive to differences in... 

Two methods are available for expressing the efficiency of a column in terms of HETP measurement of the peak width (Figure 2.18) at (1) the baseline (Equation 2.9) and (2) half-height (Equation 2.11). In determining N, we assume that the detector signal changes linearly with concentration. If it does not, N... 

In the case of a plate column the performance of a real plate is related to the performance of a theoretical one by the plate efficiency. In the case of a packed column the height equivalent to a theoretical plate HETP) gives a measure of the contacting efficiency of the packing. 

HETP values obtained in this way have been compared to measured values in data banks (69) and statistical analysis reveals that the agreement is better when equations 79 and 80 are used to predict and than with the other models tested. Even so, a design at 95% confidence level would require a safety factor of 1.7 to account for scatter. 

An alternative to determining packed height is through the use of an empirical term, height equivalent to a theoretical plate (HETP). This term can be measured in a fashion similar to that used for the overall plate efficiency of a column (eq. 44) ... 

The solute diffusivity will also depend on the nature of the mobile phase beitmay a gas or liquid. Very little work has been carried out on the effect of different carrier gases on column efficiency. Scott and Hazeldene [9] measured some HETP curves... 

Katz and Scott measured (k ) and (k"), the diffusivities, and the total HETP curves (identifying the magnitude of 2Xdp)for 69 different solutes. This data were inserted in... 

Thus, a practical procedure would be as follows. Initially the HETP of a series of peptides of known molecular weight must be measured at a high mobile phase velocity to ensure a strong dependence of peak dispersion on solute diffusivity. 

FIGURE 2.13 From measurements of the retention volume, Vr, and the peak width at half peak height, Wr, of a gaussian peak, an estimate of column efficiency N and relative efficiency, HETP, may be calculated. The last figure is for very well packed columns close to 2 X dp. [Reproduced from Sofer and Hagel (1997), with permission.]... 

Height equivalent to a theoretical plate (HETP) A measure of the efficiency of a column usually expressed in millimeters. HETP — LIN, where L is the length of a column and N is the number of theoretical plates. The reciprocal of HETP is also used to describe efficiency and is often expressed by the terms plates per meter. 

The efficiency of an open tubular column can be measured in several ways the most widely used methods are the number of theoretical plates (n), the nu3aber of effective theoretical plates (N), the height equivalent to a theoretical plate (HETP) or effective plate, the coating efficiency and the separation number (SM). No single method is ideal, standardization is lacking and... 

The conceptual idea of a theoretical plate can be used in SEC to measure column efficiency and to compare the performance of packed coluians. For column comparisons it is usually measured with small molecules, such as toluene, acetone or benzyl alcohol, which can explore all of the pores of the packing (K jc - 1). Plate counts measured in this way produce HETP values lower than the actual values measured with monodisperse polymers and proteins. The plate count in this case can be expressed by equation (4.40)... 

Before releasing a process column for chromatography, it is advisable to perform some test to measure efficiency, such as calculating height equivalent theoretical plates (HETP), both to forestall any problems in the column bed and to provide a benchmark by which to measure column reproducibility and predict degradation of the bed or material. Examples of compounds that are relatively innocuous for use in pharmaceutical applications are 1% NaCl (for gel filtration), concentrated buffer solutions (for ion exchange), and benzyl alcohol and parabens for reverse phase LC.10... 

In liquid chromatography, the diffusion rates are slower than that in gas chromatography, and the values of DM and D are very small therefore, the minimum H value is obtained at a low flow rate, as shown by curve E in Figure 5.6. The value of H increases slowly at higher flow rates in liquid chromatography. An experimental result is shown in Figure 5.7. The HETP was minimal at a certain flow rate, and the measured optimum value was less than 10 pm for this column. The optimum flow rate was about 0.9 ml min - corresponding to a linear flow velocity of about 55 mm min -. ... 

Figure 6.1 Calculation of HETP and symmetry. Wh equals width at half of the peak height. A and B are measured at 10% of peak height.

Another example is the confusion between the terms efficiency, efficacy, and selectivity of a separation. It is not always mentioned how these responses are obtained. Usually, the efficiency is either expressed as the number of theoretical plates or plate number, N, or as the height equivalent to a theoretical plate or plate height, HETP or With efficacy, usually efficiency, thus plate count, is meant, although it is not always clearly specified. The selectivity, in general, is found by calculating the selectivity or separation factor, a, which is a measure for the... 

The magniiude of relative band spreading in the column is often measured by the height equivalent to a theoretical plate (HETP) which is ilriuilcil l>) H rtiul calculated rioiii (he o plcssiun tU)J... 

The Van Deemter equation remained the established equation for describing the peak dispersion that took place in a packed column until about 1961. However, when experimental data that was measured at high linear mobile phase velocities was fitted to the Van Deemter equation it was found that there was often very poor agreement. In retrospect, this poor agreement between theory and experiment was probably due more to the presence of experimental artifacts, such as those caused by extra column dispersion, large detector sensor and detector electronic time constants etc. than the inadequacies of th Van Deemter equation. Nevertheless, it was this poor agreement between theory and experiment, that provoked a number of workers in the field to develop alternative HETP equations in the hope that a more exact relationship between HETP and linear mobile phase velocity could be obtained that would be compatible with experimental data. 

Efficiency of the distillation column is measured by the height equivalent to a theoretical plate, abbreviated HETP or simply h. The length of the column is L, thus h=L/n. The h value is independent of L, whereas the n value is dependent on L. 

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