HETP

In the case of a plate column the performance of a real plate is related to the performance of a theoretical one by the plate efficiency. In the case of a packed column the height equivalent to a theoretical plate HETP) gives a measure of the contacting efficiency of the packing. 

Sometimes the height equivalent to a theoretical plate (HETP) is employed rather than and to characterize the performance of packed towers. The number of heights equivalent to one theoretical plate required for a specified absorption job is equal to the number of theoretical plates,... 

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. 16. Plots showing (a) variation of (c F/2)J. / ) with 1 for O2 (left plot, X, 0.84- 0.72 mm = 20-25 mesh Q 0.42-0.29 mm = 40-50 mesh) and N2 (right plot, on 3.2-mm pellets) in Bergbau-Forschung carbon molecular sieve and (b) variation of HETP with Hquid velocity (interstitial) for fmctose (soHd symbols), and glucose (open symbols) in a column packed with KX 2eoHte crystals. From refs. 22 and 23.

An alternative to determining packed height is through the use of an empirical term, height equivalent to a theoretical plate (HETP). This term can be measured in a fashion similar to that used for the overall plate efficiency of a column (eq. 44) ... 

Typical experimental values of HETP for a random packing such as 50-mm PaH rings, and a stmctured packing, such as Intalox 2T of Norton Co., under the same system conditions, are shown in Figure 25. Many designers of packed columns prefer the use of HETP instead of but the latter is more fundamental and discrirninates between Hquid- and vapor-phase resistances. It should be noted that terms such as H and N are based on... 

Design data for separation of the particular or similar mixture in a packea column are not available. Design procedures are better estabhshed for tray-type columns than for packed columns. This is particularly so with respect to separation efficiency since tray efficiency can be estimated more accurately than packed height equivalent to a theoretical stage (HETP). 

Height equivalent to a theoretical plate (HETP) plate tower data ... 

Use of HETP Data for Absorber Design Distillation design methods (see Sec. 13) normally involve determination of the number of theoretical equihbrium stages or plates N. Thus, when packed towers are employed in distillation appRcations, it is common practice to rate the efficiency of tower packings in terms of the height of packing equivalent to one theoretical plate (HETP). 

The HETP of a packed-tower section, valid for either distillation or dilute-gas absorption and stripping svstems in which constant molal overflow can be assumed and in which no chemical reactions occur, is related to the height of one overall gas-phase mass-transfer unit Hqc by the equation... 

In design practice, a less rigorous parameter, HETP, is used as an index of packing efficiency. Tne HETP is the height of packed bed required to achieve a theoretical stage. The terms Hoe and HETP may be related under certain conditions ... 

Equations (14-168) and (14-170) have been developed for binaiw mixture separations and hold for cases where the operating hne and equilibrium line are straight. Thus, when there is curvature, the equations should be used for sections of the column where hnearity can be assumed. When the eqiiihbriiim line and operating line have the same slope, HETP = Hog and Nog = (theoretical stages). 

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.)... 

HETP height equivalent to theoretical plate, m HTU height equivalent to a transfer unit, m... 

A high number of plates and a low HETP indicate a high column efficiency. 

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