Page 285 - Applied Process Design For Chemical And Petrochemical Plants Volume II

274 Applied Process Design for Chemical and Petrochemical Plants

Figure 9-18 plots HETP versus FP for optimized trays at At FPs of 0.3-0.3:
24in. spacing, and No. 2 and No. 2.5 Nutter Rings and
Intalox@ 2T structured packing. Efficiency and capacity for the trays, the random pack-
In summary, Kister, et al. [136] are quoted in their con- ing, and the structured packing decline with a rise in
clusions: flow parameter.
“Comparing trays at 24in. tray spacing with a state-of- The capacity and efficiency decline is steepest in the
the-art 2-2.5-in. (nominal) random packing, and with a structured packing, shallowest in the random packing.
state-of-the-art structured packing of 67 ft2/ft3 specific sur- .At an FP of 0.5 and 400 psia, the random packing
face area, all optimally designed, we found that: appears to have the highest capacity and efficiency,
and the structured packing the least.
At FPs of - 0.02 - 0.1:
The above results are based on data obtained for opti-
.The trays and the random packing have much the mized designs and under ideal test conditions. To trans-
same efficiency and capacity. late our findings to the real world, one must factor in liq-
The structured packing efficiency is about 50% high- uid and vapor maldistribution, which is far more
er than either the trays or the random packing. detrimental to the efficiency of packings than trays. In
As FP increases from 0.02 to 0.1, the capacity advan- addition one also. must account for poor optimization or
tage of the structured packing (over the trays or over restrictive internals, which are far more detrimental to the
the random packing) declines to 0 from 3040%. capacity of trays than packings.”
Chen [ 1331 highlights the long-term growth of the tech-
At FPs of 0.1-0.3:
nically popular use of bubble cap trays, valve and sieve trays,
The trays and the random packing have much the followed by the increased popularity of packed columns
same efficiency and capacity. accompanied by the development of random and struc-
The structured packing has much the same capacity as tured packings. There are some applications in chemical/
the trays and the random packing. petrochemical/petroleum/gas treating processes where
As FP increases from 0.1 to 0.3, the efficiency advan- one type of contacting device performs better and is more
tage of the structured packing over the random pack- economical than others. Chen [ 1331 points out:
ing and over the trays declines to about 20% from
about 50%. 1. A typical tray has opening area ranging 8% to 15% of
the tower cross-section area.
2. A typical packed tower design has more than 50% of
open tower cross-section, with the void fraction of a
packed tower being higher at around 90% of tower
volume, resulting in the following:
(a) Pressure drop per theoretical stagepacked tow-
ers usually result in lower pressure drop per theo-
retical stage than trays. Trays often are 3 to 8 mm
Hg per theoretical stage, with packing having
about 1 to 2 mm Hg for random packing and 0.01
to 0.8 mm Hg for structured packing. For high
pressure systems, the difference may not be signif-
icant, while for atmospheric and below atmos-
pheric pressures, the difference can be quite sig-
nificant.
(b) Liquid hold-up-Trays usually hold-up 8 to 12%
of tower volume, compared to 1 to 6% for
3 0.05 0.1 0.2 0.3 0.5 packed towers. This can be significant for sys-
FP, flow parameter
‘Adjusted for vertical height consumed by dlstribitor, rediitributor and end bay; see equations 1 to 3 tems involving thermal degradation and requir-
ing very short residence times, which also aids in
Figure 9-18. Overall comparison of efficiency for “state-of-the-art” sharp separations.
random and structured packing with trays at 24-in. spacing. Repro- (c) Liquid/vapor ratios-Trays are designed for low
duced with permission of the American Institute of Chemical Engi-
neers; Kister, H. Z., Larson, K. F., Yanagi, T. Chemical Engineerins liquid/vapor ratios, while packed towers are oper-
Pmgress, V. 90, No. 2 (1 994) p. 23; all rights resewed. ated from low to high liquid/vapor ratios (often