Packed towers, separations distillation

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

If a waste contains a mixture of volatile components that have similar vapor pressures, it is more difficult to separate these components and continuous fractional distillation is required. In this type of distillation unit (Fig. 4), a packed tower or tray column is used. Steam is introduced at the bottom of the column while the waste stream is introduced above and flows downward, countercurrent to the steam. As the steam vaporizes the volatile components and rises, it passes through a rectification section above the waste feed. In this section, vapors that have been condensed from the process are refluxed to the column, contacting the rising vapors and enriching them with the more volatile components. The vapors are then collected and condensed. Organics in the condensate may be separated from the aqueous stream after which the aqueous stream can be recycled to the stripper. 

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. 

Distillation stage calculations are usually performed with ideal stages, The number of ideal stages required for the separation is divided by the overall column efficiency (Sec, 7,1,1) to obtain the required number of trays. In packed towers, the number of stages in the column is multiplied by the HETP (Height Equivalent of a Theoretical Plate, see Sec. 9.1,2) to obtain the packed height. 

Adsorbers, distillation colunms, and packed towers are more complicated vessels and as a result, the potential e. ists for more serious liazards. These vessels are subject to tlie same potential hazards discussed previously in relation to leaks, corrosion, and stress. However, tiicse separation columns contain a wide variety of internals or separation devices. Adsorbers or strippers usually contain packing, packing supports, liquid disuibutors, hold-down plates, and weirs. Depending on tlie physical and chemical properties of tlie fluids being passed tluougli tlie tower, potential liazards may result if incompatible materials are used for tlie internals. Reactivity with tlie metals used may cause undesirable reactions, which may lead to elevated temperatures and pressures and, ultimately, to vessel rupture. Distillation colunms may contain internals such as sieve trays, bubble caps, and valve plates, wliich are also in contact with tlie... 

Separations in Packed Towers 398 Mass Transfer Coefficients 399 Distillation 401... 

The subsequent oxidation proceeds with air at 30 to 80°C and pressures up to 5 bar, if necessary after catalyst separation and a precautionary filtration. It can be carried out in CO- or countercurrent mode, in a single step or multistep process. The hydrogen peroxide formed during the oxidation is extracted from the reaction mixture with water e.g. in pulsating packed towers. The extraction yield is ca. 98%. The hydrogen pieroxide solutions obtained are 15 to 35% by weight and must be freed from residual organic compounds before they can be concentrated by distillation. 

Step-Ill The organic layer is transferred to a batch distillation still equipped with a packed tower. The mixture is distilled. As the water-n-butanol system forms a heterogeneous azeotrope, the condensate separates into two layers in the receiver. The organic layer is refluxed, and the water layer is drawn as distillate and discharged as effluent. 

Later brine fi om the deeper and stronger Sylvania Formation (Table 2.4) was utilized, and since it also contained iodine, it and many of its derivatives were also produced (Fig. 2.41). The brine was first acidified and treated with just enough chlorine to react with the iodide ions and covert them into elemental iodine. The iodine was then blown from the brine with air in a packed tower, and re-absorbed for separate processing in a smaller tower. The residual brine was next heated, passed through a second large packed tower, and blown with steam and chlorine. The bromide ions were converted to bromine by the chlorine and carried by the steam from the tower. As much water as possible was separated from the bromine, and the bromine was further dried with sulfuric acid and re-distilled in a separate tower. Magnesium chloride and calcium chloride were then recovered from the brine (Pavlick, 1984). 

Example 12.4-1 Benzene-toluene distillation in a packed tower You want to separate a saturated liquid feed of 3500 mol/hr containing 40% benzene into a distillate containing 98% benzene and a bottoms with 2% benzene. The reflux ratio should be 1.5 times the minimum the packing has an HTU of 0.2 m. 

---