Packed, Tray, and Spray Towers

Packed, Tray, and Spray Towers Packed and tray towers have been discussed in the subsection Mass Transfer in Sec. 5. Typically, the gas and liquid are countercurrent to each other, with the liquid flowing downward. Each phase may be modeled using a PFR or dispersion (series of stirred tanks) model. The model is solved numerically... 

Packed, sieve-tray and spray towers are used without agitation [39-41],... 

Types of air strippers include packed towers, tray towers, and spray towers. Packed towers are packed or filled with small forms made of polyethylene [9002-88-4] stainless steel, poly(vinyl chloride) (PVC) [9002-86-2] or ceramic that provide large surface area to volume ratios which increase transfer rates into the air stream. Packed towers operate in countercurrent mode, that is, the aqueous stream enters at the top of the tower while air is blown in from the bottom. An example of this type of unit is shown in Figure 1. Channeling or short circuiting of the aqueous stream is minimized by... 

The two phases are in direct contact with each other, such as in a packed, tray, or spray-type tower, and the interfacial area between the phases is usually not well defined. In two-phase mass transfer, a concentration gradient will exist in each phase, causing mass transfer to occur. At the interface between the two fluid phases, equilibrium exists in most cases. 

By far the greatest number of instaUations is for the removal or recoveiy of mostly small concentrations of acidic and other components from air, hydrocarbons, and hydrogen. Hundreds of such plants are in operation, many of them of great size. They mostly employ either packed or tray towers. Power requirements for such equipment are small. When the presence of solid impurities could clog the equipment or when the pressure drop must be low, spray towers are used in spite of their much larger size for a given capacity and scrubbing efficiency. 

There are many methods to introduce fresh air for air stripping, including diffused aeration, tray aerators, spray basins, and packed-towers methods. 

There are many commercially available air stripper technologies. There are several different types of air strippers, including packed towers, tray-type, spray aerators, mist aerators, diffused aerators, low-profile packed towers, and centrifugal air strippers. According to the U.S. Environmental Protection Agency (EPA), an estimated 1000 air stripping units were operational at sites throughout the United States in 1991 (see Table 1). 

Transfer of heat by direct contact is accomplished in spray towers, in towers with a multiplicity of segmented baffles or plates (called shower decks), and in a variety of packed towers. In some processes heat and mass transfer occur simultaneously between phases for example, in water cooling towers, in gas quenching with water, and in spray or rotary dryers. Quenching of pyrolysis gases in transfer lines or towers and contacting on some trays in fractionators may involve primarily heat transfer. One or the other, heat or mass transfer, may be the dominant process in particular cases. 

Figure 14.12. Tower extractors without agitation, (a) Spray tower with both phases dispersed, (b) Two-section packed tower with light phase dispersed, (c) Sieve tray tower with light phase dispersed, (d) Sieve tray construction for light phase dispersed (left) and heavy phase dispersed (right), (e) Redistributor for packed tower with light phase dispersed (Treybal, 1963).

The interfacial area is known accurately only in some systems used in laboratory studies falling laminar films, laminar cylindrical jets, undisturbed gas-liquid and liquid-liquid interfaces, and solid castings of known dimensions immersed in liquids. In all reactor systems used industrially such as packed towers, spray towers, and bubble trays, the interfacial area is relatively difficult to determine. Photographic, gamma-ray, light scattering and chemical methods have been used to determine a in bubble dispersions (5, 6, 7, 8, iO, 42). For an average bubble diameter dn, a superficial gas velocity Usa and a bubble rise velocity Un,... 

The equipment typically used in absorption are spray towers, tray towers, packed towers, and Venturi scrubbers. The proper equipment is selected on the basis of the acid gases to be removed, initial and final concentration, and... 

For spray tower and baffle tower, see Section 5.2. For tray and packing see Section 4.2. 

At near atmospheric pressures, oxidation and absorption rates are slow, and earlier atmospheric and low-pressure plants used between five and ten very large towers. Maximum strengths attainable were in the range of 42% to 52% HNO3. To reduce capital investment and to obtain increased operating efficiencies, absorption under pressure was developed, using various types of equipment, e.g., cascade coolers, packed columns, spray towers, and columns incorporatir bubble plates, sieves, and such special devices as the Kuhlman tray. 

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