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Generalized pressure-drop correlation (GPDC) charts have been the standard for predicting packing flood points and pressure drops. This article guides engineers on the correct and incorrect uses of the GPDC interpolation procedure, and updates the method for today's new state-of-the-art packings.
For several decades, the Sherwood-Eckert generalized pressure-drop correlation (GPDC) chart has been the standard for predicting flood points and pressure drops in packed columns. This chart [initially developed by Sherwood, et al. (2) and later modified by Lobo, et al. (3)] contained only a single curve that predicted packing flood points. Leva added a new family of curves to the chart's flood-point curve to predict packing pressure drop (4). Eckert proposed further modifications (5-7), the most recent of which omitted the flood curve, retained only the pressure drop curves, and incorporated several other minor modifications. Strigle changed the scales of this latest Eckert version from log-log to semi-log to make interpolation between adjacent pressuredrop curves easier (8). Known as Strigle's chart (Figure 1), this is the "best and latest," and preferred, version of the GPDC, as discussed in current distillation texts (8-10).
Those versions of the GPDC are based on random packing only. For structured packings, Kister and Gill (11) developed a modified chart [GPDC(SP), where SP stands for structured packings], Figure 2, that empirically was a better fit to a large database of published structured-packing data.
The GPDC chart ordinate is the capacity parameter (CP), given by:
CP = C^sub S^F^sub P^^sup 0.5^V^sup 0.05^ (1)
In Eq. 1, ν is the kinematic viscosity of the liquid. Note that the kinematic viscosity (in centistokes) is obtained by dividing the dynamic viscosity (in centipoises) by the liquid density in g/cm^sup 3^ - not by the liquid density in English units.
F^sup P^ is the packing factor, which is an empirical factor characteristic of the packing size and shape. Packing factors for common packings are listed in most distillation texts (8-10).
C^sub S^ is known as the C-factor, which is the superficial gas velocity (U^sub S^) corrected for vapor and liquid densities (ρ^sub G^ and ρ^sub L^). It is given by:
C^sub S^ = U^sub S^[ρ^sub G^/ρ^sub L^ - ρ^sub G^)] 0.5 (2)
The C-factor describes the balance between the vapor momentum force, which acts...