Tray towers

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... 

TABLE 14-9 Relative Fabricated Cost for Metals Used in Tray-tower Construction ... 

FIG. 15-37 Portion of a perforated-tray tower, arranged for light liquid dispersed. 

TABLE 15-8 Mass-Transfer Data for Perforated-Tray Towers... 

Plate Towers Plate (tray) towers are countercurrent gas-atomized spray scrubbers using one or more plates for gas-liquid contacting. They are essentially the same as, if not identical to, the devices used for gas absorption and are frequently employed in apphcations in which gases are to be absorbed simultaneously with the removal of dust. Except possibly in cases in which condensation effects are involved, countercurrent operation is not significantly beneficial in dust collection. 

FIG. 23-25 Typ es of industrial gas/Hqiiid reactors, (a) Tray tower, (h) Packed, counter current, (c) Packed, parallel current, (d) Falling liquid film, (e) Spray tower, if) Bubble tower, (g) Venturi mixer, h) Static in line mixer, ( ) Tubular flow, (j) Stirred tank, (A,) Centrifugal pump, (/) Two-phase flow in horizontal tubes. 

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. 

FIG. 23"37 Equipment for liquid/liquid reactions, a) Batch stirred sulfonator. (h) Raining bucket (RTL S A, London), (c) Spray tower with Loth phases dispersed. (d) Two-section packed tower with light phase dispersed, (e) Sieve tray tower with light phase dispersed, (f ) Rotating disk contactor (RDC) (Escher B V, Holland). (g) Oldshue-Rushton extractor (Mixing Equipment Co. ). 

Packed columns are gaining ground on trayed columns. Lieberman states that based on his design and operating experience, a properly designed packed tower can have 20-40% more capacity than a trayed tower with an equal number of fractionation stages. 

Eduljee [84] suggests an equation to replace the Gilliland plot as easier to use. The data input must be the same. For tray towers ... 

Constants a and b were determined from a linear regression for x /Xq = 4.75% and x>j and Xq = 0.05% for the packed and tray towers. The optimum stripping factor decreases as the Henry s Law constant decreases. Due to the complex relationship between cost and performance, the authors [143] recommend caution in attempting to extrapolate from the water flotvrate ranges shown. 

Henry s Law Constants and Optimum Stripping Factors for Selected Organic Compounds for Use th Tray Towers... 

Absorption— Determine Component Absorption in fixed Tray Tower (Adapted in part from Ref. 18). 

Example 8-35 Determine Component Absorption in Fixed-Tray Tower... 

An existing 40-tray tower is to be examined to determine the absorption of a rich gas of the following analysis ... 

Downcomers are designed for the same conditions as bubble tray towers. 

A 2-ft tower would be expected to perform satisfactorily with properly designed trays. However, a 2.5-ft tower is the minimum diameter suitable for internal inspection and maintenance. The cost of a tray tower of 2.5-ft has been found to be no more, and from some bids 5 percent less, than the smaller 2-ft. tower. A 2-ft. tower would either be used with packing or with trays inserted from the top on rods with spacers. This would allow removal of the trays for inspection and maintenance. 

To allow for the vertical height required for packed tower distributors and redistributors—and in tray towers the vertical height used by additional trays—typically using 10%-20% of the vertical packed height (10% for 2-in. random packing and 20% for structured packing) [136] the analysis indicated ... 

Kunesh [126] presents tm overview of the basis for selecting rsuidom packing for a column application. In first deciding between a trayed tower or a packed one, a comparative performance design and its mechanical interpretation should be completed, considering pressure drop, capacity limitations, performance efficiencies (HETP), material/heat balances for each alternate. For one example relating to differences in liquid distribution performance, see Reference 126. 

A commercial 4-ft diameter refinery depropanizer unit s performance after replacement of the lower half of a trayed tower with Intalox-2T (R) structured packing is described in Reference 118. 

Packed towers are used in some distillation operations in preference to plate towers. Usually the selection requires an understanding of the fouling characteristics of fluids of the system. These towers have been used even in polymer forming operations. However, other contacting devices can be cleaned easier. For some processes the packed tower is much more effective as well as cheaper than a tray tower. 

The height of the transfer unit has not been satisfactorily correlated for application to a wide variety of systems. If pilot plant or other acceptable data are available to represent the system, then the height of packing can be safely scaled-up to commercial units. If such data are not available, rough approximations may be made by determining Hg and Hl as for absorption and combining to obtain an Hqg (Ref. 74, pg. 330). This is only very approximate. In fact it is because of the lack of any volume of data on commercial units that many potential applications of packed towers are designed as tray towers. 

Kremser-Brown-Sherwood Method — No Heat of Absorption, 108 Absorption — Determine Component Absorption in Fixed Tray Tower, 108 Absorption — Determine Number of Trays for Specified Product Absorption, 109 Stripping — Determine Theoretical Trays and Stripping or Gas Rate for a Component Recovery, 110 Stripping — Determine Stripping-Medium Rate for Fixed Recovery, 111 Absorption — Edmlster Method, 112 Example 8-33 Absorption of Hydrocarbons with Lean Oil, 114 Inter-cooling for Absorbers, 116 Absorption and Stripping Efficiency, 118 Example 8-34 Determine Number of Trays for Specified Product Absorption, 118 Example 8-35 Determine Component Absorption in Fixed-Tray Tower, 119 Nomenclature for Part 2, 121... 

The notch or weir, in which the fluid flows over the weir so that its kinetic energy is measured by determining the head of the fluid flowing above the weir. This instrument is used in open-channel flow and extensively in tray towers 3 where the height of the weir is adjusted to provide the necessary liquid depth for a given flow. 

Packings of random and structured character are suited especially to towers under 3 ft dia and where low pressure drop is desirable. With proper initiai distribution and periodic redistribution, voiumetric efficiencies can be made greater than those of tray towers. Packed internals are used as replacements for achieving greater throughput or separation in existing tower shells. 

Sieve tray towers have holes of only 3-8 mm dia. Velocities through the holes are kept below 0.8 ft/sec to avoid formation of small drops. Redispersion of either phase at each tray can be designed for. Tray spacings are 6-24 in. Tray efficiencies are in the range of 20-30%. 

Pulsed packed and sieve tray towers may operate at frequencies of 90 cycles/min and amplitudes of 6-25 mm. In large diameter towers, NETS of about 1 m has been observed. Surface tensions as high as 30-40 dyn/cm have no adverse effect. 

Reciprocating tray towers can have holes 9/16 in. dia, 50-60% open area, stroke length 0.75 in., 100-150 strokes/min, plate spacing normally 2 in. but in the range 1-6 in. In a 30 in. dia tower, NETS is 20-25 in. and throughput is 2000 gal/(hr)(sqft). Power requirements are much less than of pulsed towers. 

Possible restriction of tray towers to nonfouling materials that are free of suspended solids. 

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