Page 215 - Applied Process Design For Chemical And Petrochemical Plants Volume II

204 Applied Process Design for Chemical and Petrochemical Plants

Percent open tray areas of 20-30% appear to be opti- Sutherland [69] reports for air-water entrainment of
mum for hydrocarbon systems [47]. 0.0001 to 0.1 lb liquid/lb vapor, averaging 0.01 for 15-in.
The larger holes are recommended for high surface tray spacing at hole velocity F, values of 3 to 15. Fh = vo
tension liquids. p;/*. These values are 1-10% of bubble cap plates.
Holes are usually spaced a minimum of 2 do, with 3 do Simkin et al., [64] reports a comparison with the Turbo-
to 4 do being preferable. The distance between holes grid tray giving only 3-60% of the entrainment of bubble
should never exceed 3 in. Thin plates appear to be prefer- caps over a wide range of operation.
able to thick. Sutherland’s [69] relation for air-water on 15-in. tray
spacing correlating %-in. holes on 40% and 23% open
Tray spacing area, and %tiin. holes with 23% open area is:

The height of the liquid-froth mixture on the tray is e, = 6.31 (Fs)4.57 (8-309)
important in determining tray spacing, as tray flooding
moves up the column as the liquid mixture of one tray where e, = entrainment, lb liquid/lb vapor
approaches the underside of the tray above. Tray spacing
is recommended as twice the maximum design height of The correlation of %in. holes in 40% open trays is
liquid-froth mixture on the tray hd. Spacing of 9, 12, 15,
18 and 24 in. have been used with good success. The clos- e, = 2.37 (F,)1.i3 (8-310)
er the spacing, the less the tray flexibility. The 15-in. spac-
Why this deviates from the previous correlation is not
ing is usually a good design value. understood.
The height of aerated liquid-froth mixture on the tray,
h,l, (in.) was determined to agree with the following rela-
tion [69] for air-water for 23% and 40% open trays. Dump Point, Plate Activation Point, or Load Point
These trays will dump liquid excessively through the
hd = 1.23 F, + 0.0005 L + (0.34/p) - 2.45 (8-307)
perforations giving exceeding low efficiencies [47] unless
a minimum vapor rate is maintained for a given liquid
F, = V, p2I2 (8-308)
capacity. The smaller the holes the lower the dump point
L = Liquid rate, lb/ (hr) (ft2 of active plate) (vapor velocity).
Figure 8-147 indicates minimum values of Fh to initiate
This relation does not hold for plates having 10% open acceptable bubbling tray action. Efficiency at this activa-
hole area, as the heights are several times the corresponding tion or load point might be expected to be low; however
heights for 23% and 40% trays at the same vapor rate, F,. Myers results indicate good values at this rate.
For water, the total height of aerated mixture relative to It is recommended that trays be designed for a mini-
the height of clear liquid on the tray, hd/h,l, had values of mum of 10% above the lower plate activation values.
10 to 3. The higher values being obtained from the %win. Below these values the tray will dump liquid and become
(smaller) holes. Liquid flow rate does not appear to influ- inoperable.
ence these values to any extent.
Higher open tray areas tend to produce a spray rather Efficiency
than a froth. High vapor rates produce a spray, while the
higher liquid rates produce a froth [69]. Tray efficiency is as high as for bubble caps and almost
If these trays are used in systems with exceedingly high as high as sieve trays. It is higher than bubble caps in some
foaming tendencies, tray action may be impaired to the systems. Performance indicates a close similarity to sieve
extent of improper performance. In such cases, the foam- trays, since the mechanism of bubble formation is almost
ing tendency should be examined experimentally. identical. The real point of concern is that the efficiency
Antifoam agents have proven quite helpful in some prob- falls off quickly as the flow rate of vapor through the holes
lem cases using these trays. is reduced close to the minimum values represented by
the dump point, or point of plate initial activation. Effi-
Eutrainment ciency increases as the tray spacing increases for a given
throughput.
Data are not available to distinguish between the Myers found only a slight decrease in efficiency with an
entrainment of sieve and perforated trays without down- increase in hole size. Industrial experience indicates that
comers. The relation of Hunt et al. [33] given for sieve large holes of %in. and %-in. can be designed to operate as
trays is recommended, and should apply quite well. efficiently as a small hole, say %in.