HETP estimation

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. 

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). 

The Sherwood number, Sh, is estimated from Table 16-9, and the dispersion parameters Yi and Jo ffom Table 16-10 for well-packed columns. Typical values are a 1-4 and b 0.5-1. Since HETP -2HTU, Fig. 16-13 can also be used for approximate calculations. 

This subsection will help you select the type of packing to use for your studies and gives typical HETP and HTU values for ballpark estimates when time is short. 

FIGURE 2.13 From measurements of the retention volume, Vr, and the peak width at half peak height, Wr, of a gaussian peak, an estimate of column efficiency N and relative efficiency, HETP, may be calculated. The last figure is for very well packed columns close to 2 X dp. [Reproduced from Sofer and Hagel (1997), with permission.]... 

The Van Deemter curve (HETP vs fluid velocity) is estimated from the two analytical chromatograms. At a flow rate of 1 ml min the liquid velocity is 0.001 m s the number of theoretical plates associated to the second peak is about 250. The... 

Strigle [94] proposed this term to better describe the performance of a packed column at or near the previously described loading point. Kister [93] evaluated the limited published data and proposed using the MOC at 95% of the flood point. The flood point can be estimated by Equation 9-20 or from the plots in References 90 and 93. The data are reported to be within 15-20% of the prediction [93]. See Figure 9-22 for the identification of MOC on the HETP vs. Cg chart For more accurate information... 

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. 

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... 

One last contribution to HETP in this chapter is to establish tray efficiency by simply running one of the tray programs and inputting T for the tray method selection. The program will give you the well-proven tray efficiency two-film method [1]. Then refer to Table 3.10 and estimate your HETP minimum required. I always recommend adding at least 6 in to the HETP for a reasonable, safe design. 

The Table 11.1 data suggest that the top sections operate at HETPs of the order of 35 to 38 in with 1.5-in Pall rings. The bottom sections have an HETP of about 29 in with 1.5-in Pall rings, and 40 in with 2-in Pall rings. The difference (11 in) is similar to the difference predicted from Eq. (9.34), and therefore, makes sense. It follows that for design purposes, an HETP value of 38 in is a good estimate for the top section of the depropanizer in this example, while an HETP of 40 in is a good estimate for the bottom section. Note that these values are not conservative they match the available data. 

Compare the HETP for the packing under consideration to the rule of thumb in Eq. (9.34). If different, estimate the magnitude of this packing effect from the data, and allow for it in the design. 

On the basis of analysis of published data on distillation operations with packed towers, Murch has presented the following empirical equation as a correlation to be used for estimating HETP f... 

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 ... 

Estimate of the HETP intradiffusion coefficients, when based on HETP dependence on concentration, has been shown to be close in value to the quantity calculated with the theoretical formula obtained by using the model of centrosymmetrical diffusion in an ion-exchanger bead [89,90]. The values in this formula of r /D, where r is the effective particle radius, were determined in thin-layer kinetic experiments with the same ion-exchanger fraction. 

Thus, for multicomponent mixtures, the required number of theoretical stages N, is calculated, and the HETP for the key components is estimated from the approximate relationship. 

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