Page 133 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Part 3: Mechanical Designs for Tray PerFormance

Determining the number of theoretical and actual trays Entrainment: about three times that of perforated type
in a distillation column is only part of the design necessary plate or sieve tray. Jet-action accompanies bubbling.
to ensure system performance. The interpretation of dis- NmibiZiQ: most flexible of tray designs for high and low
tillation, absorption, or stripping requirements into a vapor and liquid rates. Allows positive drain of liquid from
mechanica1 vessel with internal components (trays or tray. Liquid heads maintained by weirs.
packing, see Chapter 9) to carry out the function requires Application: all services except extremely coking, poly-
use of theoretical and empirical data. The costs of this mer formation or other high fouling conditions. Use for
equipment are markedly influenced by the column diam- extremely low flow conditions where tray must remain wet
eter and the intricacies of the trays, such as caps, risers, and maintain a vapor seal.
weirs, downcomers, perforations, etc. Calculated tray effi- Tray Spacing 18-in. average, 24 to 36in. for vacuum
ciencies for determination of actual trays can be lost by conditions.
any unbalanced and improperly designed tray.
She Truy or Pqfb-uted TYQ~ With Downcmners
Contacting Trays
Vapor rises through small holes (W to 1-in.) in tray floor,
The particular tray selection and its design can materi- bubbles through liquid in fairly uniform manner. Liquid
ally affect the performance of a given distillation, absorp flows across tray floor over weir (if used), through down-
tion, or stripping system. Each tray should be designed so comer to tray below. Figures 8-67A-C, and 868B.
as to give as efficient a contact between the vapor and liq-
uid as possible, within reasonable economic limits. It is not Ccspaeityy: As high or higher than bubble cap at design or
practical in most cases to change the design of each tray to down to 60% of design rates with good efficiency. At lower
fit calculated conditions. Therefore, the same tray design thruputs performance drops as efficiency falls off rapidly.
is usually used throughout the column, or the top section Efficiacy: As high as bubble caps in region of design, but
may be of one design (or type) while the lower section is falls to unacceptable values when capacity reduces below
of another design. The more individual tray designs 60% (approximately)
included in a column, the greater the cost. Entrainment: Only about onethird that of bubble cap
This concept has not gained commercial popularity due trays.
to the proprietary nature of the Fractionation Research, Hexibility: Not generally suitable for columns operating
Inc. (FRI) data being limited to member organizations, under variable load, falling below 60% of design. Tray
and the public literature does not contain much indepen- weeps liquid at low vapor rates.
dent research and application data. General industrial Application: Systems where high capacity near-design
and commercial proprietary designs available are listed in rates to be maintained in continuous service. Handles sus-
Table 8-12, but may not be all-inclusive: pended solid particles flushing them down from tray to
tray. Holes become plugged in salting-out systems where
Tray ’Ippes and Distinguishiug Application Features trays run hot and dry (as underside of bottom tray).
Tray Spacing: Can be closer than bubble cap due to
Bubble Cap improved entrainment. Fifteen inches is average, 9-in., 10-
in. and 12-in. are acceptable, with 20- to 30-in. for vacuum.
Vapor rises up through “risers” or “up-takes” into bub-
ble cap, out through slots as bubbles into surrounding liq- Pe$muted Plate Without Downcome7s: (Dud-, from l?RI)
uid on tray. Bubbling action effects contact. Liquid flows
over caps, outlet weir and downcomer to tray below, Fig- Vapor rises through holes (%e to 1-in.) in tray floor and
ures 8-63-67, 79, and 81. bubbles through liquid. At the same time liquid head
forces liquid countercurrent through these holes and
CU#Q~~~J: moderately high, maintains efficiency. onto tray below. Liquid flow forms random patterns in
Efficiency: most data are for this type, as high as other draining and does not form continuous streamlets from
tray designs. each hole. See Figures 8-67D and &68A

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