Packed Towers Better Than Trays

The very first continuous distillation column was the Patent Still used to produce Scotch whiskey in the 1830s. It had 12 bubble-cap trays with weirs, downcomers, tray decks, and bubble caps with internal risers. Current trayed towers are quite similar. As most distillation towers have always been trayed rather than packed, one would have to conclude that trayed towers must have some sort of inherent advantage over packed towers. And this is indeed true in a practical sense, even though, in theory, a packed tower has greater capacity and superior separation efficiency than a trayed column. 

The two features that should be maximized in packed beds are  

Structured packing has about 50 percent more open area than Raschig rings and two or three times their wetted surface area. Hence, structured packing has largely replaced packing in the form of rings in many packed towers. 

In any type of packed tower, the liquid or internal reflux drips through the packing and forms a thin film of liquid on the surface of the packing. Vapor percolates up through the packing and exchanges heat and molecules with the thin film of liquid on the surface of the packing. 

In a trayed tower, vapor-liquid contact occurs only 5 or 6 inches above the tray deck, and the majority of the tower s volume is not used to exchange heat or mass between vapor and liquid. In a packed tower, the entire packed volume is used for this vapor-liquid contacting. 

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Packed Towers Better Than Trays 133 11.2.3 Packing Holddowns... 

Chapter 10 Packed Towers Better than Trays 107... 

Example 9.2 asked (Sec. 9.4.2) whether a packed tower would be better than trays for the depropanizer. The design with trays is in Sec. 6.5.11. For the comparison, it is assumed that filter evaluation of the alternate design (with third-generation packing Sec. 9.4,9) it was found to be sound and not too tight. 

For new towers, the designs will usually develop to utilize the entire tower cross-section. However, for existing towers with perforated trays being installed to replace bubble caps or packing, the optimum active tray area may not utilize the entire cross-section. If the number of holes required is small compared to available area, it is better to group the holes on 2.5 dg to 3.5 do than to exceed these limits. Holes separated by more than 3 in. are not considered effective in tray action so necessary for good efficiency. Blanking strips may be used to cover some holes when more than required have been perforated in the tray. 

Turndown. Moving valve and bubble-cap trays normally give better turndown than packings. Unless very expensive distributors are used, packed tower turndown is usually limited by distributor turndown. 

Example S.2 For the depropanizer in Examples 2.4 (Sec. 2.3.1), 3.4 (Sec. 3.2.5). and 6.1 (Sec. 6.5.2), would a packed tower be better than a tray tower Proeeas loads and physical properties are the same as those in Table 6.10. The service ia nonfouling, the streams have a negligible zolid content, the corrosive tendency is low. and preeeure surges are unlikely. 

Steam stripping is used to remove dilute concentrations of ammonia, hydrogen sulfide, and other volatile components from pharmaceutical waste stream. Steam stripping can typically achieve contaminant removal of 99% or better and is effective for the removal of organics having boiling points of less than 150 C. The steam stripping process is carried out in a distillation column, which may be either a packed or tray tower. Steam enters at the bottom ofthe column while waste water is countercurrently supplied fi"om the top of the distillation column. The product stream, rich in volatile components, may further be treated to recover these components. 

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