Page 232 - Separation process principles 2

6.1 Equipment 197

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Plate
Vapor flow Vapor flow Vapor flow ,
(a) (b) (c)

Figure 6.5 Three types of tray
openings for passage of vapor up
into liquid: (a) perforation;
(b) valve cap; (c) bubble cap;
(d) tray with valve caps.

As shown in Figure 6.5, openings in the tray for the pas- a froth. An 11-ft-diameter column might have trays with
sage of vapor are most commonly perforations, valves, 50,000 A-in.-diameter perforations, or 1,000 2-in.-diameter
and/or bubble caps. The simplest is perforations, usually to valve caps, or 500 4-in.-diameter bubble caps.
in. in diameter, used in a so-called sieve tray (also called a As listed in Table 6.2, tray types are compared on the
perforated tray). A valve tray has much larger openings, basis of cost, pressure drop, mass-transfer efficiency, vapor
commonly from 1 to 2 in. in diameter. Each hole is fitted capacity, and flexibility in terms of turndown ratio (ratio of
with a valve that consists of a cap, which overlaps the hole,
with legs or a cage to limit the vertical rise while maintain-
Table 6.2 Comparison of Types of Trays
ing the horizontal location of the valve. With no vapor flow,
each valve sits on the tray, over a hole. As the vapor rate is Sieve Valve Bubble-Cap
mcreased, the valve rises, providing a larger and larger Trays Trays Trays
Peripheral opening for vapor to flow into the liquid to create
Relative cost 1 .O 1.2 2.0
a froth. A bubble-cap tray has bubble caps that consist of a
Pressure drop Lowest Intermediate Highest
fixed cap, 3 to 6 in. in diameter, mounted over and above a
Efficiency Lowest Highest Highest
concentric riser of 2 to 3 in. in diameter. The cap has rectan-
Vapor capacity Highest Highest Lowest
gular or triangular slots cut around its side. The vapor flows
Typical turndown 2 4 5
UP through the tray opening into the riser, turns around, and
ratio
Passes out through the slots of the cap, into the liquid to form