HETP influence

Due to the unique HETP characteristics, it is important to consult the manufacturer on the specific system involved and operating conditions. Care must be exercised in selecting these or any other type of packing since plugging with suspended solids, polymer formation on surfaces, and similar mechanical problems can influence performance and life of the packing system. 

Various authors [90] and researchers have published factors that influence the magnitude of HETP values. The lower the HETP value (i.e., number of feet (or inches) per theoretical plate/tray/stage) the more efficient is the packing (random or structured) for any particular separation system. Other influences include ... 

The summary of HETP values of Vital [142] for various types and sizes of packings are believed to be referenced to typical industrial distributors for the liquid. This variation can influence the value of HETP in any tabulation the effect of distributor design is discussed in an earlier section of this chapter. Porter and Jenkins [143] developed a model to improve the earlier models of Bolles and Fair from about 25% deviation to about a 95% confidence using a 20% factor of safety [139]. 

N and HETP. Under optimal working conditions [2,10,22,35], of, (proper thermostating), o (proper capillary rinsing) and o (matching of sample and buffer electrolyte conductivity), o (proper capillary selection) are under control. When the influence of oq can also be neglected, then [Pg.595]

Plate theory disregards the kinetics of mass transfer. Thus, it reveals little about the factors influencing HETP values. Plate theory tells us that HETP becomes smaller with decreasing flow-rate however, experimental evidence shows that a plot of HETP versus flowrate always goes through a minimum. 

Van t Hoff plots of In k versus the inverse of temperature (generally 1000/T for convenience) are very often linear, especially with monomeric bonded phases. They can exhibit nonlinear behavior, and the transition temperature is often close to the undefined room temperature. Temperature optimization is one trend in LC. A rising temperature increase reduces viscosity and increases the diffusion rate, thereby enhancing mass transfer, which flattens the HETP curve at high velocities (31). Conversely, Sander and Wise (32) investigated the influence of temperature reduction. 

In summary, bed depth may significantly influence HETP. This adds uncertainty to scale-up. Shallow test beds should be avoided. Most investigators use beds at least 1.5 m tall, and often more than 3 m tall. The FRI sampling technique (below) can detect maldistribution along the bed height. 

Sampling Fractionation Research Inc. (FRI) developed a sam-ling technique that eliminates the influence of "end effects and etects a maldistributed composition profile. This technique [Silvey and Keller, IChemE Symp. Ser. 32, p. 4 18 (1969)] samples the bed at frequent intervals, typically every 0.6 m or so. HETP is determined from a plot of these interbed samples rather than from the top and bottom compositions. 

Closely examine the considerations in Sec. 9.3.3. Use these to scale up the HETP from the above steps to your column. Pay attention to effects of diameter, height, and wetting. Judgment is required. It may pay to look at the original reference from which the data were derived in order to check whether distribution, data scatter, or test procedure have influenced the data. 

As mentioned in Sec. I.B. the efficiency of separation is measured by the HETP, so it is important to study the influence of increasing concentrations of EDTA on the HETP. The effect of this variable on HETP was monitored by using the separation of Tm-Er as a suitably representative system. The results obtained are listed in Table 9. 

Curve 1 illustrates the effect of the solution rate of flow on HETP values in the dense fixed beds of ion exchanger when the solution moves down the column from top to bottom. The data have been obtained in columns with inner diameters of 18, 26, and 79 mm.The fact that the points lie along one curve indicates that column diameter does not influence HETP values. 

Compared with Bo , which is independent of the interstitial velocity (Eq. 7.32), Steffi is inversely proportional to interstitial velocity (Eq. 7.20). This means that the influence of mass transfer resistance will grow and surpass the influence of axial dispersion at high interstitial velocity, which is almost always the case for preparative chromatographic processes. In some extreme cases, where the mass transfer coefficients are small and the chromatographic column is operated at high flow rates, the HETP equation for the calculation of Nt can even be simplified further to ... 

Influence of Different HETP Coefficients for Every Component... 

In the case of a nonlinear isotherm, there is no general solution available that would compare to the equation derived by Lapidus and Amxmdson. Thus, we do not have a tool comparable to the coliunn height equivalent to a theoretical plate (HETP) for the investigation of the influence of the column performance on the band profiles. 

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