Column packed, HETP value

Distillation efficiency commonly is expressed as height equivalent to a theoretical stage (HETP). In a trayed column, the HETP value is the tray spacing divided by the fractional overall tray efficiency. A theoretical stage is a mass transfer stage from which the liquid and vapor streams leave in equilibrium. In a packed column, the liquid in the packed bed would be in equilibrium with the gas stream located one HETP value above it in the packed bed. Thus, the stepwise procedure McCabe and... 

The situation is very much poorer for stmctured rather than random packings, in that hardly any data on Hq and have been pubHshed. Based on a mechanistic model for mass transfer, a way to estimate HETP values for stmctured packings in distillation columns has been proposed (91), yet there is a clear need for more experimental data in this area. 

FIG. 14-74 HETP values for Max-Pak structured packing,. 35 kPa (5 psia), two column diameters. Cyclohexane/n-heptane system, total reflux. For 0.4.3 m (1.4 ft) column perforated pipe distributor, 400 streams/m2, 3.05 m (10 ft) bed height. For 1.2 m (4.0 ft) column tubed drip pan distributor, 100 streams/m ,. 3.7 m (12 ft) bed height. Smaller column data. University of Texas/Austin Larger column data. Fractionation Research, Inc. To convert (ft/s)(lb/ft ) to (m/s)(kg/m ) , multiply by 1.2199. (Couiiesy Jaeger Troducts, Inc., Housion, Texas.)... 

Next, to determine packed column height use Table 9 for distillation HETP values, leaning tow ards the high side of the range for studies. For use of Kqa values see Section A—Absorbers. Bed height per packed bed runs up to 20-30 ft for metal or ceramic packings, but plastic packing is usually limited to 24 ft. 

Katz et al. [1] also examined the effect of particle diameter on resistance to mass transfer constant (C). They employed columns packed with 3.2 im, 4.4 p,m, 7.8 pm, and 17.5 pm, and obtained HETP curves for the solute benzyl acetate in 4.3%w/w of ethyl acetate in n-heptane on each column. The data were curve fitted to the Van Deemter equation and the values for the A, B and C terms for all four columns extracted. A graph relating the value of the (C) term with the square of the particle diameter is shown in Figure 8. 

Strigle [82, 94] describes the hydraulics and HETP performance of a packed column by referring to Figure 9-22. As noted, the HETP values are essentially constant over a wide range of Cg alues shown as B-C on the figure. Note that Cg can be expressed ... 

To convert an available HETP value for a given system and column diameter to a different packing in the same series (X or S), assume HETP is inversely proportional to the specific surface (consult manufacturer for this detail). 

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)... 

A column having a smaller HETP value is a good column because diffusion inside the column is small, resulting in better separation. The HETP value is given by the Van Deemter equation, which describes the peak broadening of packed columns through which a non-compressible solvent is moving. 

The HETP curve clearly shows, that for a packed column, the particle size has a profound effect on the minimum value of the HETP of a column and thus the maximum efficiency attainable. It would also appear that the highest efficiency column would be obtained from columns packed with the smallest particles. This will in due course be shown to be a fallacy, but what is true, is that the smaller the particle diameter the smaller will be the minimum HETP and thus, the larger the number of plates per unit length obtainable from the column. At this time it will suffice to point out that the total number of theoretical plates that can be obtained will depend on the length of the column which, in turn, must take into account the available inlet pressure... 

The identification of the pertinent HETP equation must, therefore, be arrived at from the results of a sequential series of experiments. Firstly, all the equations must be fitted to a series of (H) and (u) data sets and those equations that give positive and real values for the constants of the equations identified. The explicit form of those equations that satisfy the preliminary data, must then be tested against a series of data sets that have been obtained from different chromatographic systems. Such systems might involve columns packed with different size particles or employ mobile phases or solutes having different but known physical properties. 

Thus it can be seen that the observed HETP is not only a function of column packing but depends upon operating conditions and the properties of the solute. This is why different values of HETP (or different numbers of theoretical plates per unit column length) are obtained for various solutes. 

Increased column diameter may cause a reduction in the homogeneity of the packing, resulting in flowrate distortion of the component zones across the column cross-section and higher h (HETP) values. Diminution of column efficiency also can be related to (1) the nonuniformity of liquid coating which in turn affects mass transfer, (2) the radial temperature gradient that... 

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... 

As long as dumped packing in commercial columns is properly wetted [more than 1000 lb/(h)(ft2) or 5000 kg/(h)(m2)], the following HETP values will result in a workable column ... 

It should be noted that the values of HETP given in Table 1-12 may be taken as only a rough approximation of the value obtained with a column of the given type. With packed columns, the HETP varies with the uniformity of packing and may vary markedly from column to column. Also, the values of HETP are not independent of the mixture used in determining the number of theoretical plates (Table 1-13). It is strongly advised that each new or repacked column be checked for its efficiency before being put into use. 

The smallest size packings in the Bolles-Fair correlation are nominally 13 mm. If the usually required minimum ratio of column diameter to packing diameter of 8 is to be retained (to avoid bypassing at the wall), scaleup studies require a minimum column diameter of about 100 mm. Research in progress may lead to the use of a special, small laboratory packing (e.g., Pro-Pak , from Scientific Development Co.) for columns as small as 25 mm. HETP values of two sizes of Pro-Pak, for cyclohexane/n-heptane at 1.0 atm, are shown in Figure 12.66, from reference 84. 

The smaller the HETP is, the better the column. The older 6 mm diameter packed columns would have values from 1 to 3 mm/plate, whereas values of 0.25 mm/plate are common for capillary columns. One equation to determine the number of theoretical plates and the HETP is the van Deemter equation shown below ... 

Cobalt oxide, as adsorbent additive, 177 Column chromatography, see Elution Column length, effect on N, 115-117 Column packing, see HETP values Column pressure, effect on TV, 115-117 Coluisns, for liquid chromatography, 347-348... 

The number of theoretical plates is a function of numerous apparatus and operating parameters. Thus, it has not been possible yet to precalculate the HETP value for a packed column on the basis of mathematical considerations alone. So far, only empirical relations have been established resulting from a large number of experimental tests (chap. 4.2, 4.8). 

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