HETP versus HTU. Packed-height computations can be carried out using either the HTU or the HETP approach. Both approaches should give essentially the same result. The HETP approach is usually preferred because it has the following advantages:

1. The HETP approach is suitable for multicomponent systems, while the HTU approach is difficult to apply for these.

2. The HETP approach can use the stage-by-stage computer programs that are used for multistage calculations.

3. The HTU approach is more complex and more difficult to use, but appears to achieve no improvement in accuracy when compared with the HETP method.

4. The HETP approach enables easier comparison with plate columns.

The main advantage of the HTU method is that it enables easier analysis in terms of mass transfer coefficients, and therefore it is more suitable for fundamental analysis and model development. The HETP can simply be calculated from the HTU using Eqs. (9.10) and (9.12).

9.1.3 Factors affecting HETP

Packing size and type (Sees. 8.1.2, 8.1.4, and 8.1.10). Generally, packing efficiency increases (HETP decreases) when

■ Packing surface area per unit volume increases. Efficiency increases as the particle size decreases (random packing) or as the channel size narrows (structured packing).

a The packing surface is better distributed around a packing element.

Vapor and liquid loads {Fig. 8.16). For constant L/V operation in the preloading regime, generally

■ Liquid and vapor loads have little effect on random packings HETP.

■ HETP increases with loadings in structured packing. The effect is most pronounced in wire-mesh structured packings, and much less pronounced in corrugated-sheet structured packings. With larger-crimp corrugated-sheet structured packings, HETP is usually practically independent of vapor and liquid loads.

Distribution (Sec. 9.2). Both liquid and vapor maldistribution have a major detrimental effect on packing efficiency.

L/V ratio. Most packed-column efficiency testing has been at total reflux. Some tests for both random (3,61,115) and structured packings (3,32,116) suggest that efficiencies at finite reflux are simijar to those at total reflux.

Some tests by Koshy and Rukovena (110,111) with aqueous, high-relative-volatility systems (a >2) gave much lower random packing efficiencies at high and low L/V than close to total reflux. They interpreted the results as an L/V ratio effect; however, underwetting (below) can also explain these data.

Pressure. Generally pressure has little effect on HETP of both random (117,118) and structured (3,32) packing, at least above 1 to 2 psia.

At deep vacuum (< 1 to 2 psia) there are data to suggest that efficiency decreases as pressure is lowered for random packings (117,118), but most of these data can also be explained by poor wetting or maldistribution. For distillation at high pressures (> 200 to 300 psia), there is evidence to suggest that structured-packing efficiency diminishes as pressure is raised (24,3 lo).

Physical properties. Data presented by a number of workers (98,115,119,120) suggest that generally, random packing HETP is relatively insensitive to system properties. A survey of the data in Chap. 11 will lead to a similar conclusion. The data in Chap. 11 also indicate that the insensitivity to system physical properties extends to nonaqueous systems in structured packings. For water-rich systems, structured-packing HETPs tend to be much higher than for nonaqueous systems (Sec. 8.1.10).

Underwetting (Sec. 8.2.16). With aqueous-organic systems, HETP tends to increase at the aqueous end of the column, both with random and structured packings.

Errors in VLE. These affect packing HETP in the same way that they affect tray efficiency (Sec. 7.3.1). The discussions, derivation, and Fig. 7.6 apply equally to tray and packed towers.

Two liquid phases. Harrison (121) presents two case studies: in one, adding water to two water-insoluble organics had no effect on HETP. In another, a key component was soluble in both liquid phases, and HETP was about 50 percent higher than normal. Harrison argues that a second liquid phase leads to lower efficiency only when it impairs diffusion of the key species. On this basis, Harrison expects efficiency loss also when an "inert" liquid or vapor represents a large fraction of the liquid or vapor phase.

Summary. In the preloading regime, packing size, type, and distribution affect HETP. With aqueous-organic systems, HETP may be sensitive to underwetting and composition. HETP of structured packings may also be affected by pressure (at high pressure), and vapor and liquid loads.

9.1.4 HETP predictions—mass transfer models

Three approaches are commonly used for HETP prediction: mass transfer models, rules of thumb, and data interpolation. These ap proaches will be discussed in the next three sections, starting with the mass transfer models.

The Bravo and Fair correlation (122). This correlation is based on the two-film model, that assumes resistance to mass transfer in both the vapor and liquid phases. The correlation treats the mass transfer coefficient independently from the interfacial area, and accounts for effects of partial wetting. The correlation is based on extensive commercial and pilot scale efficiency measurements for first generation packings and Pall® rings, and applies to random packings only.

For calculating mass transfer coefficients, the Bravo and Fair correlation uses the relationship by Onda et al. (123).

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