Data in Chap. 11 (see Figs. 11.2 and 11.4 to 11.6) show that many aqueous systems in structured packings give HETPs roughly twice those of nonaqueous systems. This does not occur in all aqueous systems; for instance, the HETPs for methanol-water in Fig. 11.5, and for H20-D20 in Figs. 11.1 and 11.3, are well in line with those of nonaqueous systems. The exceptions may be explained by arguing that the above methanol-water tests were conducted at the methanol-rich end, and that the H20-D20 system, because of its very low relative volatility, achieves a high efficiency. The poor performance of structured packings with aqueous systems has been attributed to poor wettability (32,33) or underwetting (Sec. 8.2.16). Aqueous systems have high surface tension. The liquid spread on the packing, and therefore the wetting, are diminished at high surface tension. This problem is most serious with stainless steel, and the use of oxidized copper has been cited (33,34) to enhance efficiency in aqueous systems.
The poor wetting theory leaves several questions unanswered. Chen et al. (35) argued that above the minimum wetting rate, wettability and surface tension should only have a minor effect on packing efficiency. Also, high relative volatility and high liquid viscosity appear to be detrimental to efficiency, especially in aqueous systems, and this cannot be explained by poor wettability. Finally, there is uncertainty regarding the concentration at which the transition from an "organic system" HETP to an "aqueous system" HETP takes place.
These uncertainties question the validity of the above theory. Due to the poor understanding of this phenomenon, it is best to exercise caution with HETP predictions for all of the following types of systems on structured packings: aqueous, high surface tension, high liquid viscosity, and high relative volatility.
Liquid inventory. With unstable chemicals, minimizing liquid inventories at hot temperatures minimizes product loss due to degradation and decomposition reactions. In batch distillation, excessive liquid inventory lowers product recovery. With hazardous chemicals, minimizing liquid inventories lowers the hazard.
Figure 8.12ci shows that liquid holdup (percent of packing volume; depends primarily on liquid flow rate and on packing size. Generally, liquid holdup increases as the size of packing particle (random packings) diminishes, or as the flow channel (structured packings) narrows. Figure 8.12d also show;, that liquid holdup is comparable for random and structured packings of similar capacities.
Liquid inventory is the product of liquid holdup (percent of packing volume) and the packing volume. Since structured packings are more efficient than random packings of approximately the same capacity 'Fig. 8.12a), the structured packing volume needed for a given separation is lower. The total liquid inventory (cubic feet of liquid) is in turn lower for structured packings.
Wetting and minimum liquid rates. The capillary action of structured packings promotes self-wetting (Fig. 8.13). Self-wetting is strong in wire-mesh packings and weaker in corrugated-sheet packings. This self-wetting permits efficient operation at liquid rates down to 0.05 gpm/ft2 of tower cross section with wire-mesh packings (20,37-39) and down to 0.1 gpm/ft2 with corrugated-sheet packings (37,38). Some manufacturers (35,37) claim efficient operation even at lower liquid flow rates. These rates are 5 to 10 times lower than the minimum wetting rates of random packings, giving structured packings a major low liquid rate and turndown advantage.
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