Packed Columns Design and Performance

Table 1 Evolution of packing

Firstgeneration, Secondgeneration, Thirdgeneration, after before 1950 1950-1970 1970

Firstgeneration, Secondgeneration, Thirdgeneration, after before 1950 1950-1970 1970

Random packings

Rashing rings Lessing rings Saddles

Intalox® (Norton) Pall Ringsa

IMTP® (Norton) CMR® (Koch Glitsch) Chempak®"

Fleximax® (Koch Glitsch) Nutter Ring® (Nutter)

Grids

C-Grid (Koch Glitsch)c EF-25 (Koch Glitsch)c

Structured packing

Wire-mesh typed

Sulzer BX and CY Mellapack® (Sulzer) Flexipack® (Koch Glitsch) Gempack® (Koch Glitsch) Intalox® (Norton) Montz packing (Montz)

developed by BASF, still marketed (or variations of it) by most packing manufacturers. ^Developed by Leva, marketed by Nutter.

cVariations of these grids are now offered by most packing manufacturers. ^Developed by Sulzer, they are now offered by other manufacturers.

developed by BASF, still marketed (or variations of it) by most packing manufacturers. ^Developed by Leva, marketed by Nutter.

cVariations of these grids are now offered by most packing manufacturers. ^Developed by Sulzer, they are now offered by other manufacturers.

The main variable influencing the column design requirements is the relative volatility, a. Figure 1 illustrates the effect of a on the column performance:

• As a increases, the number of theoretical stages (NTS) required to achieve a fixed product quality decreases, since NTS is proportional to 1/ln(a). As a decreases and approaches 1, the number of stages required increases approaching infinity. At any given a, the minimum number of stages required to achieve a given separation corresponds to a total reflux operation. At total reflux all overhead vapours are condensed and returned to the column as reflux, so that there is no net product. The minimum reflux sets the limiting slope of the operating line, required to achieve a given separation. At constant a, the NTS increases as the product purity increases. The increase is proportional to the logarithm of the key components purity ratio.

It can be also demonstrated that:

At constant product purity, the minimum reflux decreases as a increases.

At constant product purity, the minimum number of stages decrease as a increases. At constant a, the minimum reflux decreases as the product purity decreases.

Figure 1 Number of stages required vs. relative volatility at several product purities.

• At constant a, the minimum number of stages increases as the product purity increases.

All these statement say that a defines the separation difficulty. For values around 1.1 and lower, separation by distillation becomes very difficult, requiring very large and expensive columns. For a = 1 the mixture is azeotropic and would require the addition of selective entrainers if azeotropic or extractive distillation is to be applied.

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