Note: If reflux is highly subcooled, add one additional theoretical stage to the rectifying section to allow for reflux reheating.

TABLE 9.3 Strtgte'» (15) HETP Rulea of Thumb (Continued)_

4. Pressure distillation (>80 psia)

a. For stripping light hydrocarbons from heavier ones, design HETP values are given by

IMTP Size_C6

Restrictions:

1. 22 < MWt < 72. For aliphatic hydrocarbons with 84 < MWt < 114, HETP will be lower by up to 10% compared to values derived from Eq. (9.37) for MWt = 72

2. For high methane or hydrogen concentrations, HETP can be up to 30* greater than that predicted by Eq. (9.37)

b. Design HETP values for fractionations of C8 and C* hydrocarbons at pressures of 320 and 90 psia, respectively,

IMTP size_HETP, ft

Restrictions:

1. Above refer to separation of similar light components (e.g., Ca splitters, C« splitters). A C2 splitter will have an HETP about 20% higher than a C, splitter.

2. The efficiency in the stripping section may be lower (as much as a 10% higher HETP value) than in the rectifying section with the same packing.

9.1.6 HETP predictions—data Interpolation

Interpolation of experimental HETP data is the most reliable means of obtaining design HETP values. This is hardly surprising, and can even be anticipated in an area where our understanding of the theory is so poor that rules of thumb can do better than theoretical models. The author feels that it is always best to derive HETP from experimental data, and to check it against a rule of thumb.

Eckert (72), Chen (98), and Vital et al. (120) tabulated experimental HETP data for various random packings. The author extended these tabulations and included data published for structured packings. Chapter 11 presents this collection of published HETP data and a detailed procedure for interpolating such HETP data.

A prerequisite to any interpolation of packing data is thorough familiarity with the factors that affect HETP. These factors are listed in Sec. 9.1.3, and elaborated on in various other sections (which are re ferred to in Sec. 9.1.3). Overlooking any of the factors listed can very easily lead both the data interpolation and the resulting design to be grossly incorrect.

9.2 Maldistribution and its Effects of Packing Efficiency

9.2.1 Effects of liquid maldistribution on HETP: an overview

Maldistribution in packed columns may cause a severe reduction in column efficiency. For 1-in packings, HETP may increase by a factor as high as 2 or 3 due to maldistribution (116,131,132).

Early models (133,134) expressed the effect of liquid maldistribution on packing efficiency in terms of a simple channeling model. A portion of the liquid bypasses the bed, undergoing negligible mass transfer, and then rejoins and contaminates the rest of the liquid. Huber et al. (132,135) and Zuiderweg et al. (136) replaced the simple bypassing by variations in the local LfV ratios (Sec. 9.2.2). The overirrigated parts have a high LfV ratio, the underirrigated parts a low L/V ratio. Regions with low L/V ratios experience pinching (Sec. 2.2.5), and therefore, produce poor separation.

Huber, Yuan, et al. (116,132,135,137) added lateral mixing (Sec. 9.2.3.) to the model. Lateral deflection of liquid by the packing particles tends to homogenize the liquid, thus counteracting the channeling and pinching effect.

A third factor is the nonuniformity of the flow profile through the packing (Sec. 9.2.4). This nonuniformity was observed as far back as 1935 (138) and first modeled by Cihla and Schmidt (139). Hoek (140) combined this factor with the previous two for modeling the effect of maldistribution on packing efficiency.

9.2.2 Effect of maldistribution on local L/V ratio

When maldistribution occurs, some areas in the bed receive more liquid and other areas receive less. Thia causes variation in the LfV ratio along the bed cross section. The effect of these variations in LfV ratio on column efficiency are best illustrated by an example using the x-y diagram. A similar example, recently presented by McMullan et al. (140a), proved effective for analyzing a troublesome maldistribution case histoiy.

Example 9.1 A packed column is used to achieve the separation described in Example 2.1, Sec. 2.2.4. Due to fouling, some holes in the reflux distributor are plugged. The blockage pattern is such that one half the column receives 46 percent of the liquid, and the other half receives 55 percent of the liquid (± 5 percent maldistribution). This maldistribution pattern persists throughout the rectifying section. The liquid is redistributed at the feed point, and distribution is perfect below this point.

1. How many stages are required in the rectifying section?

2. To what extent is the rectifying section efficiency reduced due to this mat distribution?

solution

Step 1 Same as step 1 of Example 2.1, Sec. 2.2.4. Step 2 Same as step 2 of Example 2.1, Sec. 2.2.4.

Step 3 Simulate the rectifying section of the column as two columns operating in parallel. Subscripts 1 and 2 describe the column that receives 55 and 45 percent of the liquid, respectively. The following equations apply:

L{ = 0.55L' L{ = 117 lb-mole/h Li = 0.45L' L3 = 96 lb-mole/h Vi = 0.5V Vi = 142 lb-mole/h V2 = 0.5V V3 = 142 lb-mole/h D[ = V1-L\* 142 - 117 = 25 lb-mole/h D'z = V2 - U2 - 142 - 96 = 46 lb-mole/h

Step 4 Apply Eq. (2,9) to derive equations for the component balance lines for each of the parallel columns.

also

Column 1

Vi vx

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