Page 19 - Gas Purification 5E

Introduction 9

Table 1-4
Relative Costs of Columns

Relative Costs of Tray Columns (for equal diameter and height)

Bubble Cap 1 .oo
Koch Kascade 1.243
Plate Tmy 0.842
Sieve Tray 0.874
Turbogrid 0.855
Valve Tray 0.911
Relative Costs of Column Packings (installed cost for equal volumes
of packing, Wcu. ft in 1973 dollars)
1-in. Dia. 2-in. Dia.
Berl saddles, stoneware 13.10 -
Berl saddles, steel 26.30 -
Berl saddles, stainless steel 32.80 -
Intalox saddles, ceramic 13.30 10.40
Pall rings, polypropylene 36.90 26.30
Pall rings, stainless steel 13.60 9.80
Raschig rings, stoneware 6.30 4.38
Raschig rings, stainless steel 15.70 10.90
Raschig rings, steel 12.60 8.79
Tellerettes, HD polyethylene 26.30 -
Data of Bleckr and Nichols, 1973

upward through openings in the tray. After traversing the tray, the liquid flows into a down-
comer, which conveys it to the tray below. Downcomers typically occupy 5 to 20% of the
column cross-sectional area.
In countercurrent trays, which are also available but less popular than crossflow types, the
liquid flows from one tray to the next lower tray as free falling drops or streams. Examples
of countercurrent trays include perfomted (Dual How), slotted (Turbogrid), and perforated-
cormgated (Ripple). The trays are reasonably efficient, but lack flexibility because tray
holdup and operating characteristics are highly dependent on gas and liquid flow rates.
Baffle or shower deck tray columns also approximate countercurrent contactors. These
trays are nonperforated horizontal or slightly sloped sheets, each of which typically occupies
slightly more than half of the tower cross-sectional area. The liquid flows off the edge of one
tray as a curtain of liquid or series of streams and falls through the gas stream to the tray
below. Typically, the trays are half moon in shape on alternate sides of the column, or disc
and donut designs with centrally located discs that are slightly larger than the openings in
donut-shaped trays located above and below them. Baffle trays are used for extremely dirty
liquids when highly efficient contact is not required and for heat exchanger duty-particular-
ly the quenching of hot, particle-laden gas streams. Photographs of typical commercial trays
are shown in Figure 1-2.