FLOW THROUGH POROUS MEDIA
Effect of Packing Supports on Mechanical Operation o Packed Towers M A X LEVA,
1030 DALLETT RD., P I T T S B U R G H 27, PA.
J O S E P H M. LUCAS
AND
H E R M A N H. F R A H M E
T H E U N I T E D S T A T E S S T O N E W A R E CO., A K R O N 9. O H I O
e x p e r i m e n t a l data presented in t h i s paper demonstrate t h e m a r k e d effect t h a t tower packing supports m a y exert on t h e flooding l i m i t s , pressure drop, a n d general flow capacities of irrigated packings. A 16-inch-diameter experimental tower was provided w i t h L u c i t e slots t o p e r m i t observation over i t s e n t i r e height. l n t a l o x saddles were used as packing. Ceramic a n d m e t a l p l a i n plates, one plate made of Raschig rings sintered together a t t h e p o i n t s o f contact, a n d t w o weir-type plates of special design were investigated. Free space in t h e plates ranged f r o m 23 t o 92%. T h e range of variables investigated are representative of commercial scale unit operation. T h e tests disclosed t h a t c o l u m n pressure drops m a y increase by m o r e t h a n 50% if a n unsuitable support is chosen. T h i s is especially t r u e if t h e i r r i g a t i o n rates of t h e packing are high. Provision of a large fraction o f free space in a support may, however, n o t guarantee good tower operation, as packing characteristics in relation t o support openings a n d l a y o u t m u s t be considered. T h e superiority of weir-type supports over wire mesh suppnrts is a t t r i b u t e d t o t h e f a c t t h a t gas and l i q u i d passage occur t h r o u g h separate a n d specially provided ports a t a l l times.
0
PERA%TORS of packed distillation and absorption towers have learned, by experience, t,hat packing supports frequently influence tower functioning t o a marked degree. The problem is not always appreciakd by the design engineer. A comprehensive consideration of the mechanics of a packed tower should not only involve examination of the flow capacity of t’he selected packing, but should also give attention to the choice of the packing support plat,e. Generally, it will not suffice to provide plain supports that merely offer the “same percentage of free space” as the packed bed. This is so because, under ordinary packing conditions, packing elements immediately on top of the plates obstruct the openings. As the study shows, support, plates of special construction providing separate flow channels for gas and liquid phases, are superior because t’he effect of obstruction of the openings by adjacent packing elements is greatly lessened. EXPERIMENTAL TOWER
‘
The experiment,al tower consist,ed of two 16-inch-diameter straight flanged sections, each 24 inches high. Each section had a Lucite Observation slot, 3 inches wide and 22 inches long. The column base perinit,ted quick replacement of support plates, and the behavior of six plates was examined. Construction details and physical data are indicated in Figure 1 and Table I. The
JTater distributor was a weir box, carrying 19 ‘/*-inch nipples. Beds of 1- and l’/?-inch Intalox saddles were examined for pressure drop and flooding behavior. The beds were prepared by dry-packing a given weight of saddles to the same height in the tower. Reproducibility for any one support was examined repeatedly, and pressure drops could generally be duplicated \Tithin 10%. -4few tests were also made with supports alone, with and without one layer of packing. Visual observation indicated that mode of liquid build-up and eventual flooding r e r e markedly affected by the support. The flooding phenomena were always normal wit,h the weirtype, fused Raschig ring and wire plates. Normal flooding manifests itself by the gradual and even build-up of the irrigating liquid on the packing surface, until the condition is reached where a major portion of the voids is filled with water. With the plain plates, t’he flooding phenomena were not normal. At a relatively low liquid and gas rate, for which the packing body showed little or no holdup, a hydrost’at’ichead of several inches in height formed in the base of the tower. The height of the head was characteristic of flow rates, and if either the liquid or gas rate was increased slightly, the head rose a significant amount. Evidently, this condition far preceded the flooding limits of the packing, and tower capacity was really determined b y the support.
Table I. Plate Characteristics Plate
Diam., Inches
Thickness, Inches
Plain ceramic 153/a Plain metal I9/l Sintered Raschig ring Ceramic weir type 143/4 (Weir height, 0.5 inch) Metal weir type j53/4 (Weir height, 0.25 inch, rlsers flat) Wire
June 1954
S o of Holes 61 198
131 90
Size of Holes, Inch 7/s 3/4
No. of Risers
Riser Height, Inches
, , . ...
...
‘/z
4
3/4
4
...
Riser Diam., Inches
...
...
Slots/ Riser
... I
.
.
Slot Size, Inches
... ...
No. of Holes in Riser Cap
... ...
(135 1L/a-inch Raschig rings, sintered together) 8 Zn/s X 3/4 56 6 4
3
4
10
Z3/a X 6/s
(Wire mesh screen over steel frame; openings B/s-inch on edge)
INDUSTRIAL AND ENGINEERING CHEMISTRY
37
Size of Holes in Riser Cap, Inch
...
Free Space, % 20 45
58 ‘/a
60
‘/a
60
92
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
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GAS- INJECTION
WIRE M E S H 7
The specially constructed plate (Figure 2 ) consists of a circular perforated base that carries a definite number of cylindrical slotted risers. The risers are capped by hollow perforated hemispheres. The side of the risers carry elongated ports, the bases of which are slightly above the base plate. I n an installation, the packing elements are always chosen small enough so that they can descend between the risers. A major portion of the liquid ordinarily leaves the tower through the openings in the ONE INCH O f I-SIZE INTALOX SADDLES base plate. Should the free space in the base plate be insufficient for liquid passage, a small head of perhaps 1 G-2120 inch will form, and overflow through the slots in the N3 PACKING O N TOP periphery of the risers will yield additional flow passages. T h e gas 500 1000 enters the toFer at GAS MASS VELOCITY - L E . /FZ' HR. all times through Figure 5. Flow Data through the upper portion Fused Raschig Ring Plate of the slots and the Gas rate parameter perforated h e m i spherical caps. The risers are laid out on a triangular G ,960 pitch pattern, ensuring uniform gas distribution i n t o the bottom of the tower. The plate is m e c h a n i c a l l y very strong and may be employed to support ceramic p a c k i n g s of t h e usual heights of 10 to 15 feet. Weirtype plates are also Figure 6. Flow Data through Plain Ceramic Plate suitable as redistributors. Instead Gas rate parameter
0 0 0 0 0 0 0 0 0 0 0
PLAIN PLATE
WIRE PLATE
GAS INJECTDN-WEIR.TYPE PLATE
FUSED RASCHIG RNG
PLATE
Figure 1.
W El R - T Y PE PLATE
Test Plates
u I
t
P L A N VIEW -GAS
GAS RISER?
FLOW
_ _ _ _LIQUID _ - FLOW
,
SECTION A - A
Figure 2.
Weir-Type Plate Details
-
I/
s -
\-
L:10300
8' ]PLATE
/
L
o
o'
COVERED WITH GNEMCH OF I'SIZE INTA-
m w
=; 10E
-
50-
9I n -y
g05-
Q
10-
w
-
w
a
GAS M A S S VELOCITY -
L *Xi70
-
LEI. /FT*.HR.
*IO
P
S
-
I
a
r-WEIR-TYPE PLATE.CGVERE0 WITH ONE INCH OF I'SIZE INTALOX SADDLES
-
-
-
405-
2 1
I I I t 1 1 1
I
I
I 1 1 1 1 1
I
I I I l l
I
I
I
l 1 1 1 i
G A S MASS V~LGCITY- 1 6 . /FT:
R.
Figure 7. Data of Lerner and Grove (2) 1.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Liquid mass velocity, Ib. (sq, ft.) (hr.)
TOP.46,No. 6
FLOW THROUGH POROUS MEDIA
m
II
I
of requiring a separate packing support and a follow-up redistributor, the weir-type plate combines both features, leading to a shorter tower height and simpler construction. HYDRAULIC DATA
Some typical gas-liquid flow data are shown in Figures 3 to 6 . The plain plates yielded pressure drops sufficiently high to be recorded. W t h the other plates, the per cent free space was so large that pressure drops for the plates alone were generally too small to be recorded reliably. The readings were magnified by covering the plates with one layer of I-inch Intalox saddles. This merely raised the numerical level of the data without obscuring the hydraulic characteristics of the plate. For the data with liquid rate as parameter (Figures 3 and 4), the line for L = 0 has a slope of 2.0 (logarithmic coordinates), indicating turbulent flow. This is supported by the order of mag-
I GAS FLOW RATE-
Figure 8.
Operating Stages of Support Plates
nitude of the gas Reynolds number
(?
= 25000).
As liquid
is admitted to the top of the plate, the pressure drop data level rises. The ring plate data show four separate branches, Points A and B have been termed the “build-up” and “flooding” points, respectively. The section below point A proceeds essentially parallel to the nonirrigated pressure drop line. The slope is smaller along A B . Bejrond R, a sudden and almost vertical increase occurs, followed by a gradual flattening out. This type of curve seems typical, as has been demonstrated b y Lerner and Grove (g), Figure 7. In an attempt to explain the reason for the various branches, the operating stages of support plates have been indicated schematically in Figure 8. They are t o be examined with sectional sketches 1to 4 of Figure 9, which represent essentially the conditions that prevail for a constant liquid rate a t one singular and typical average plate openings. The liquid head conditions a t Gg are still those of the preceding course, indicating that there is no increase in gas flow passage as the Interpretation of Flow Data Leading t o Hydrostatic Head gas rate is increased to Gz. At GP, however, a limitFormation ing condition has been reached beyond which the gas velocity through the openings becomes sufficient to enlarge the flow path. This enlargement in flow path must yield to a relatively lower pressure drop. At the same time, the liquid, now descending in an annular ring (in the ideal non-
P
1.T-
Figure 9.
s W VI
2
