Structured Packing Fp

Fp. Packing factor, fl

Figure 8.12 Random versus structured packings, (a) Specific surface area versus packing factor.

alone. Other factors (Sec. 8.1.2), such as spread of the surface ares, also affect efficiency. Figure 8.126 suggests that at low specific surfat» areas (<60 ft2/ft3), these factors tend to favor random packings, Le, that a second- or third-generation packing is more efficient than a structured packing of the same specific surface area. The difference ia efficiency appears to widen as the specific surface area decreases. Fifr

+ Pall rings

6 Third generation random pocVmgs

O Corrugoted sheetslructvred pockings

0 Wire- mesh structured packings

Corrugated sheet-0 structiroJ peeking

10 20 30 50 100 200

Specific surface areo, ft2/ft5 (b>

10 20 30 50 100 200

Specific surface areo, ft2/ft5 (b>

Figure 8.12 (CWiniiid) Random versus structured packings. (i>) HETP versus specific surface area, metal packings, chlorobenzene-ethyl-benzene, 50 mmHg, total reflux, data by Billet (3) and Spiegel and Meier (21); (c) pressure drop per theoretical stage, chlorobenzene-ethylbenzene, 50 mmHg, total reflux, data by Billet (3)

Figure 8.12 (CWiniiid) Random versus structured packings. (i>) HETP versus specific surface area, metal packings, chlorobenzene-ethyl-benzene, 50 mmHg, total reflux, data by Billet (3) and Spiegel and Meier (21); (c) pressure drop per theoretical stage, chlorobenzene-ethylbenzene, 50 mmHg, total reflux, data by Billet (3)

Figure 8.12 {Continued) Random versus structured packings, id) Liquid holdup, air-water data by Billet (3), preloading regime.

Figure 8.12 {Continued) Random versus structured packings, id) Liquid holdup, air-water data by Billet (3), preloading regime.

ure 8.126 is based on test data for the chlorobenzene-ethylbenzene system at 50 mmHg (3,21).

Overall, structured packings have an efficiency and/or capacity advantage over random packings when operated at lower liquid loads (< 20 gpm/ft2). This advantage is somewhat less pronounced than one may infer from Fig. 8.12a alone, and is due to the nature of the resistance to vapor flow. In random packings, resistance to vapor flow is mostly due to expansion and contraction. This mechanism gives a high pressure drop. In structured packing, the regular flow channel keeps expansion and contraction to a minimum. The main friction loss mechanism becomes pressure loss through bends, which incurs a far lower resistance to vapor flow. This lower resistance permits incorporating more surface area in a bed of structured packings.

Pressure drop per theoretical stage. This parameter is of major importance at deep vacuum. For instance, a column containing 20 theoretical stages, with a packing that has a pressure drop of 0.2 in of water per theoretical stage, will have a pressure drop of 7.5 mmHg. If the column bottom pressure must not exceed 10 mmHg (e.g., to avoid excessive bottom temperatures and product degradation), the top pressure must be 2.5 mmHg or less. If the packing pressure drop were only 0.1 in of water per theoretical stage, the pressure drop will be 3J mmHg, and the top pressure 6.2 mmHg. This permits roughly a 20 la 30 percent reduction in column diameter and lowers the costs of geo-erating and maintaining the vacuum.

Figure 8.12c shows that structured packings have a much lower pressure drop per theoretical stage than random packings. This is a major advantage in deep vacuum services.

Performance in high-pressure/high-liquid-flow services. The capacity and efficiency advantage of structured packings over random packings rapidly erodes as liquid rate or pressure increases (e.g., Fig. 8.12a). Numerous cases of structured packing failures have been experienced by the industry in high-pressure and/or high-liquid rate services. Successful experiences in such services have also been reported (24, 31a, 31c, 31d, 31e).

The causes of poor structured packing performance at high-pressure and/or high-liquid rate services are not well-understood. Work by Kurtz et al. (31a) and Kister and Gill (316) identifies high-flow parameters Fjv and high-system frothiness as key factors. Flow parameters are high when either the pressure or the liquid rate (or both) are high. Frothiness is promoted by the low surface tensions experienced at high-pressure distillation. The high capacities and efficiencies demonstrated by Kean et al. (31c,31g0 for numerous structured packings in glycol dehydrators support these statements. In these contactors, flow parameters were very low (<0.03) due to the low liquid rates. Frothiness was low due to the high glycol surface tension and the foam-prevention measures taken by Kean et al. The contactors performed well at pressures as high as 600 to 700 psia.

The above can be explained by a "downcomer choking" type of phenomenon. At higher liquid rates and/or higher pressures, disengagement of vapor from the liquid becomes more difficult. High frothiness will further retard vapor disengagement. As structured packings permit far less lateral movement of fluids than random packings, far more vapor will be carried downward, causing recycling and reduction of effective capacity and efficiency. Improved liquid distribution, and an increase in the size and number of perforations in the corrugated sheets will help spread the liquid and counteract any premature choking. Kurtz et al. (31a) report similar ideas and present two high-pressure case studies where better liquid redistribution improved both efficiency and capacity of structured packings.

Due to the poor understanding of the phenomenon, the lack of predictive methods, and the large number of failures experienced in the industry at high-pressure and/or high-liquid-rates, the author recommends extreme caution with structured packings at liquid rates exceeding 10 gpm/ft2 and pressures higher than 100 to 200 psia. In this region, the author would use structured packings only in services where a demonstrated trouble-free track record has been established [e.g., glycol contacting (31c, 31c?)].

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