HETP-value

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. 

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. 14-72 HETP values for four sizes of metal pall rings, vacuum operation. Cyclobexane/n-beptane system, total reflux,. 35 kPa (5.0 psia). Column diameter = 1.2 m (4.0 ft). Bed height = 3.7 m (12 ft). Distributor = tubed drip pan, 100 streams/nv. [Adapted from Shafiat and Kunesh, Ind. Eng. Cbem. Res., 34,1273 (1.9.9.5),] Reproduced with permission. Copyright 1995 American Chemical Society. 

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. 

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 ... 

With increasing vapor rate, the contact between liquid and vapor increases to increase the rate of mass transfer and the HETP value will improve in efficiency of contact and drop from point C to E to point D. With increasing vapor rate, liquid entrainment will occur into the vapor phase and lower the efficiency (and raise the HETP) to... 

Thus, for mass transfer performance design a specific design HETP value should be established, which in effect represents the range from point B through E for Cj values above point F, the HETP values will be greater (and thus less efficient contact). 

Size No. Pieces/Ft Ft2/Ft3 Lb/Ft % Void Packing Factor HETP Values ... 

HETP values nearly independent of flow rate. 

Figures 9-21G and 9-211 present the proprietary estimating capacity charts for the various sizes of the Intalox packing for a non-foaming system. The system base HETP of a packing is the flat HETP value produced by uniform distribution, see Figure 9-22.

For commercial towers with good liquid/vapor distribution Norton [96] recommends standard designs use HETP ralues 13% above the system base HETP. If the system under consideration does not meet the physical properties limit, either use a conservative estimate or use actual plant or published data for the system. For comparison of HETP values for selected packings see Strigle and Rukovena [94], Figure 9-28. 

Tests by FRI and Nutter [132] emphasize that distribution of liquid must be uniform and at minimum values to achieve good HETP values over a range of system pressures for hydrocarbons distillation. 

Experimental HETP Values from Various Sources for Packings Equivalent to Stainless Steel X-200... 

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). 

In general, for the same liquid and vapor rates the HETP and HTU values for fieri saddles and others with a/e lower than Raschig rings should be lower. Correlating data is not available, except tests of Teller [71] which indicate 1-in. fieri saddles have HETP values 0.75 as compared to 0.85 for 1-in. Raschig rings. 

The accuracy of establishing HETP -values for new conditions in the same packing referenced to known values as suggested by Planovski [57] has not been tested by other literature references. 

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]. 

For easy separations (less than 10 theoretical stages) a 20% design safety factor can be applied to a typical HETP value. 

HETP values for random dumped packing have been found to be 25% greater at a greater ascosity than a lower viscosity, i.e., viscosity change from 0.15 cps to 0.44 cps. 

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)... 

In SEC the HETP value is due almost entirely to the contribution from stagnant mobile phase dispersion and... 

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. 

HETP is another quantity that is used to express the efficiency of a device for carrying out a separation, particularly in which mass is transferred by a stage-wise action rather than a differential contact. For example, in a tray column, the HETP value is the tray spacing divided by the fractional overall tray efficiency. 

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. 

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... 

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... 

Data Interpolation Interpolation of experimental HETP data is the most reliable means of obtaining design HETP values. This is hardly surprising in an area where our understanding of the theory is so poor that rules of thumb can do better than theoretical models. The author believes that it is best to derive HETP from experimental data, and to check it against a rule of thumb. 

In particular, a set of four HETP values obtained for cyclohexane for a 4 xl sample injected into a 40% (3, P -oxydipropionitrile column were ... 

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