Height equivalent of theoretical plate

Murch, D. R, Height of Equivalent Theoretical Plate in Packed Fractional Columns. Ind. Eng. Chem., 45, 2616 (1953). 

MURCH, D. P. Ind. Eng. Chem. 45 (1953) 2616. Height of equivalent theoretical plate in packed fractionation columns. 

Figure 42-1. The dependence of the height of equivalent theoretical plate (HETP) on the flow velocity of the mobile phase for LiChrospher RP phases with different particle diameters. The test solute u.sed was benzo(a)anthracene and the mobile phase was acetonitrile/water 75/25 (v/v) (Source Merck).

K is a thermodynamic member and corresponds to the median distribution coefficient of two neighbouring coefficients). Assuming almost equal selectivities in the gas and liquid phases and a uniform R value of 0.5 with GC and LC which is common in practice, LC approaches the efficiency of GC, as far as comparable N values or heights of equivalent theoretical plates (HETP) which are closely related to this. 

Height of a theoretical plate (HETP) Height of packing in a distillation column that gives a separation equivalent to one theoretical stage. 

The number of equivalent theoretical plates is intermediate between NTUql and NTUqg- Similarly, the height equivalent to a theoretical plate, HETP, is intermediate between H rUoi, and HTUoa- Transfer units and theoretical plates are equal in number and height if and only if S is =1 (Eq. 10). 

The efficiency of a plate can be predicted with more certainty than the equivalent term for packing (height of a theoretical plate [HETP] or heights of transfer units [HTU]). 

Now, the variance per unit length of a column is numerically equivalent to ratio of the column length to the column efficiency [2] [i.e., the height of the theoretical plate (//)] thus. 

The equivalent height of a theoretical plate H, as already defined (expression 1.5), is calculated for reference compounds to permit a comparison of columns of different lengths. H does not behave as a constant, its value depends upon the compound chosen and upon the experimental conditions. 

Thus, the plate theory supplies an approximate description of the spreading of a component band. If the chromatographic peak width is known, one can calculate the number of theoretical plates, which characterize the column efficiency and hence the equivalent height of a theoretical plate ... 

The plate theory does not consider the effects of the carrier gas flow rate and the rate of mass transfer on the peak width, so that it says nothing about the factors which influence the equivalent height of a theoretical plate. According to plate theory, H decreases continuously with increase of flow rate, as distinct from experimental results which reveal a minimum at a certain flow rate. 

Many operating variables, such as sample volume, flow rate, column length, and temperature, must be considered when performing any separation. The relative importance of these variables for Toyopearl HW-55F resin columns has been specifically evaluated. For example. Fig. 4.47 shows the relationship between column efficiency, or height equivalent of a theoretical plate (HETP),... 

FP = flow parameter, dimensionless HETP = height equivalent of a theoretical plate, in. 

Commercial preparations of these supports are available in narrow mesh-range fractions to obtain particles of uniform size the material should be sieved to the desired particle size range and repeatedly water floated to remove fine particles which contribute to excessive pressure drop in the final column. To a good approximation the height equivalent of a theoretical plate is proportional to the average particle diameter so that theoretically the smallest possible particles should be preferred in terms of column efficiency. Decreasing particle size will, however, rapidly increase the gas pressure necessary to achieve flow through the column and in practice the best choice is 80/100 mesh for a... 

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... 

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... 

The performance of real stages is related to an equilibrium stage by the concept of plate efficiencies for plate contactors, and height of an equivalent theoretical plate for packed columns. 

The methods for estimating the heights of transfer units, HTU, given in Section 11.14.3 can be used for distillation. The relationship between transfer units and the height of an equivalent theoretical plate, HETP is given by ... 

As with distillation, the correlation for overall tray efficiency for absorbers, given in Equation 10.7, should only be used to derive a first estimate of the actual number of trays. More elaborate and reliable methods are available, but these require much more information on tray type and geometry and physical properties. If the column is to be packed, then the height of the packing is determined from Equation 9.64. As with distillation, the height equivalent of a theoretical plate (HETP) can vary... 

The efficiency of a chromatographic separation can be described by the height equivalent of a theoretical plate (H), where lower values of H correspond to more efficient separations. The Van Deemter equation describes the relationship between H and mobile phase flow velocity (u) as the sum of three major terms, A, B, and C, each of which represents a different contribution to band broadening in a chromatographic column. 

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