w and Jenkins's (127), Frank's (128), Harrison and France's (52), Chen's (98), and Walas's (129) general rules of thumb are practically the same. Strigle's (15) and Rose's (130) general rules of thumb are more optimistic. The author prefers the Porter and Jenkins rule of thumb because it agrees with most other sources, it was successfully tested against an extensive data bank, and it is slightly conservative, and therefore suitable for design.
For small-diameter columns, the rules of thumb presented by Frank (128), Ludwig (63), and Vital et al. (120) are identical. The author believes that for small columns, the more conservative value predicted from either the Porter and Jenkins or the Frank-Ludwig-Vital rule should be selected for design. Summarizing
HETP = 1.5 dp for Pall® rings or similar high-
efficiency packing ^4)
In high-vacuum columns (<2 psia), and where underwetting is a problem, these rules may be optimistic. Further discussion is in Sec. 9.1.3.
Harrison and France (52) presented the only published rule of thumb for structured packings efficiency. This rule states that HETP is 9 in for V-t-inch crimp height, 18 in for 1/2-inch crimp, and 33 in for 1-in crimp. The author found this rule to do well when the crimp angle is 45°, but to be less satisfactory for other angles. Based on data in Chap. 11, the author proposes an alternative rule of thumb.
Crimp heights and specific surface area are listed in Table 8.1. The 4 in added in Eq. (9.35) gives this rule a conservative bias. Eliminating the 4 from Eq. (9.35) will improve the fit of this rule to data, and for a crimp angle of 45° will make it practically identical to Harrison and France's. However, the author feels that just like in the Porter and Jenkins rule (above), the conservative bias is needed to render the rule suitable for design.
Service-oriented rules of thumb. Strigle (15) proposed a multitude of rules of thumb as a function of the service, column pressure, and physical properties. The author extracted these rules of thumb from Strigle's book and listed them in Table 9.3. These rules are based on the extensive experience of Strigle and the Norton Company.
1. Atmospheric distillation (300 mmHg-80 psia)
In HETP - «h - 0.187 In <r + 0.213 In m-/, where nH is given by the tabulation below
Pall* ring (Metal) 1.1308 1.3582 1.6584 IMTP* (Metal) 1.1308 1.3185 1.5686
Intalox* (Ceramic) 1.1308 1.3902 1.7233
Basis: Strigle's regression of Norton's, Billet'«, and published FRI data. Restrictions:
3. High-performance distributors
4. 0.6 < X. < 1.8. Outside this range, HETP is likely to be higher Design: Apply with a safety factor of
20% for easy separations (< 15 theoretical stages) 15% for separations requiring 15-25 theoretical stages Use precise HETP values for more difficult separations 2. Atmospheric distillation (300 mmHg-80 psia)
IMTP packing size_ HETP, ft
Basis: These are typical efficiencies per Strigle's experience. Restrictions:
3. Applies to paraffins, naphthenes, aromatics, alcohols, and ketones with MW <100
4. Do not apply to systems with chemical reactions, chemical association, or high-level ionization in the liquid phase -
5. 0.6 < \ <1.8. Outside this range, HETP is likely to be higher 3. Vacuum distillation (<300 mmHg)
Rule 2 (above) gives typical HETPs for IMTP* packings in vacuum distillation. However, the liquid phase offers more resistance to mass transfer. The HETP increase is correlated as a liquid viscosity correction, as follows:
Liquid viscosity, cP_Relative HETP, %
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