Height equivalent to a theoretical plate values

In equation (6) is the width at the base of the peak, measiued in the same units as tj.. The theoretical plate model assumes the column to be made of a series of plates. The distribution of the analyte between the mobile and stationary phase occurs at each plate. Therefore, the higher the number of plates, the better the separation since more sorption-desorption cycles occur. Coliunn efficiency can also be expressed in terms of Height Equivalent to a Theoretical Plate value (HETP or H-value) ... 

Equilibrium-stage methods are usually adequate for nearly ideal distillation systems when coupled with calculations of plate efficiency to estimate actual trays or, in the case of packed towers, when HETS (height equivalent of a theoretical stage) or HETP (height equivalent to a theoretical plate) values are known from experience or from experiment to enable the estimation of packed height. For absorbers, strippers, and nonideal distillation systems, mass-transfer models are preferred, but their use requires a value for the tower diameter and a tray layout or type and size of packing. Even when mass-transfer models are preferred, initial calculations are usually made with equilibrium-stage models. Also, note that data for reliable mass-transfer coefficients is often difficult to obtain. 

Source From Column efficiency in liquid-solid adsorption chromatography. H.E.T.P. [height equivalent to a theoretical plate] values as a function of separation condition, in Anal. Chem. ... 

The height equivalent to a theoretical plate, H, is that length of column that represents one theoretical plate, or one equilibration step. Obviously, the smaller the value of this parameter, the more efficient the column. The more theoretical plates packed into a length of column, the better the resolution. It is calculated by dividing the column length by N ... 

The efficiency of a column can be assessed in a similar manner to that described for HPLC and values for the resolution index of two solutes, the number of theoretical plates and the height equivalent to a theoretical plate may also be calculated. Although it is easier to measure gas pressure, it is the actual gas flow, which is affected by the particle size and compression of the packing, that should be used in column assessment investigations. 

In summary, the efficiency TV of a TLC plate is variable. The height equivalent to a theoretical plate has a minimum value, as in HPLC. However, it is not possible, unlike in HPLC, to vary the flow rate of the mobile phase in order to increase separation efficiency. 

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. 

Dingenen [9], who studied the effect of the mobile phase velocity on the height equivalent to a theoretical plate (HETP) at different temperatures for benzotriazole derivatives, obtained the results shown in Figure 22, which represents the HETP values found for methanol and the hexane-ethanol mixture. Both curves of Figure 22 clearly demonstrate that the kinetic circumstances are less favorable at low temperatures. A slow mass transfer between the two phases clearly determines the band-broadening process at temperatures below 20°C. This... 

The amount of resin to pack in a column, column geometry, flow rates, pressure, column hardware, and wetted materials of construction should all be evaluated in development. Chromatography columns must be properly packed prior to validating the purification process. From a business perspective there should be some criteria other than purification of the product by which the quality of the packed column can be assessed prior to applying the feedstream, which by this time in the process is quite expensive. Height equivalent to a theoretical plate (HETP) and asymmetry determinations can be used to evaluate the quality of column packing, but may have limited value for some types of packed columns... 

A gas chromatographic study (Carrott and Sing, 1990) of the adsorption of a series of hydrocarbons (C2-C6) by the AMOCO AX21 carbon has revealed some unusual features. Thus, values of the height equivalent to a theoretical plate (HETP) over the range 90-l20°C were found to be independent of both the adsorptive and the temperature. Furthermore, over the range of gas flow rate studied, only the initial section of the Van Deemter plot could be observed (in contrast to the behaviour of other microporous carbons). At present, the explanation for this behaviour is not entirely... 

Height Equivalent to a Theoretical Plate. Provided both the equilibrium and operating lines are straight, HETP values may be estimated by combining the HG and HL values predicted by the above correlations and by translating the resulting HQG into HETP by combining equations 47, 51, and 56 with equation 85, which is discussed under bubble tray absorption columns ... 

The value of a is related to another parameter used to characterize zone spreading, namely the height equivalent to a theoretical plate H which is defined as... 

Assumption 1 The column is assumed to be radially homogeneous. Experiments show that it is possible to pack wide columns (up to at least 80 cm in diameter) and achieve packed beds that are nearly as homogeneous as those in analytical columns 1/4 inch in diameter. This homogeneity is witnessed by the values of the reduced height equivalent to a theoretical plate achieved, sometimes less than 3 [10]. Such a result is possible only if the input profile is radially homogeneous. Accordingly, appropriate flow distributors should be used [11,12]. This also requires that the column be operated isothermally or adiabatically. For the influence of a radial gradient of the column temperature, see [13]. 

The reduced height equivalent to a theoretical plate, h, is the ratio of the height equivalent to a theoretical plate, H, to the particle diameter, dp. Values of h below 3 are considered as characterizing good columns. 

With increasing solute amount values of the height equivalent to a theoretical plate increase and k values decrease if other parameters are fixed. Estimate of the maximum sample amount that can be charged to a column so as to avoid loss of column performance occurring due to overloading, can be made from the value of the adsorbent linear capacity, 0°. On this basis, it is found that the maximum sample size for porous silica is about 2 x 10-4 g of sample per gram of silica. 

Another useful parameter for column efficiency is the height equivalent to a theoretical plate, HETP (the H value). The following simple relationship shows that H has the units of length. 

If the dimensions of stationary phase coating thickness and diffusion distance to the film from the gas phase have been optimized, for further improvement of GC resolution, it becomes necessary to increase the length of the column. This is seen from the simple relation of Eq. (11.7), which indicates that for an optimal minimized value of H (the height equivalent to a theoretical plate) the number of plates, N, is proportional to the length of the column. From Eqs. (11.8) and (11.9) we note that the resolution is proportional to the square root of N. For columns packed with particles of optimal size, and operated at the optimal linear flow rate at the minimum of the Van Deempter curve (Fig. 11.3), the typical maximum pressure of 100 psi achievable from a regulated gas cylinder requires that most packed columns be less than 4-7 m long. More typically they are only 1 -2 m in length. These considerations hmit the resolution achievable in packed column GC. 

As the value of N is proportional to column length, L, the other parameter H or HETP, the so-called height equivalent to a theoretical plate, is Introduced as... 

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