I
H. M. MULLER' and D. F. OTHMER Polytechnic Institute, Brooklyn 1,
N. Y.
Hydraulics of the Uniflux Fractionating Tray Valuable hydraulic data on flooding and pressure drop characteristics of the Uniflux tray have been obtained. An operating range has been defined for a nitrogen-absorption oil system
SEVERAL
YEARS AGO, the Socony Mobil Oil Co., Inc., developed the Uniflux fractionating tray which is an improved bubble-cap tray. I t is light in weight, low in cost, and readily installed or removed for inspection and cleaning ( 7 , 2). Over 300 towers with such trays are in operation. In the work reported here, performance and hydraulics of these trays in a !~~,'s-fOOtexperimental column, located a t C. F. Braun and Co., Alhambra, Calif., have been investigated using a nitrogen-oil system.
Equipment The simulator column is equipped with three adjustable trays (Figure 1). The pump capacity for liquid flow is 550 Present address, Socony Mobil Oil Co., Inc., New York, N. Y .
gallons per minute, as recorded by a flowmeter and checked by a manometer. Vapor, a t rates ranging from about 1000 to 15,000 actual cubic feet per minute (ACFM), is recirculated by a large blower, and controlled by a recording flowmeter. The vapor rate is checked by a manometer. Plywood segments, caulked with putty, sealed the downcomers for the dry tray pressure drop runs. Air was used as the circulating gas (Table I and Figure 9). For the wet tray studies a t 24-inch spacing, capacity, activity, pressure drop, and hydraulic gradient data and entrainment measurements were obtained using nitrogen and absorption oil, a t four different liquid rates. Vapor rates ianged from about 1400 actual cubic fevt per minute up to a velocity giving excessive entrainment (Table I1 and Fiqures 7 to 13). For the entrainment studies. the liquid
was fed directly into the downcomer of the top tray. T h e top tray and downcomer were carefully sealed against leakage, and during the entrainment runs, the liquid from the middle tray was piped off the top tray (entrainment tray) to calibrated entrainment drums. Because of the inadequacy of the drawoff system, a certain liquid head was required for liquid flow through this system and consequently, liquid accumulated to a certain height on the top tray before steady state conditions were established. When the liquid reached a certain level, some of the total entrained liquid from the middle tray passed off in the overhead gas stream where it was picked u p in a knockout drum and measured. Care was taken to wait for steady state conditions across the entrainment tray before measurements. T h e level in the downpipe was recorded as inches of
GLASS WINDOW
2 ' - 0 " DIAMETER-
'
'-6"
DIAMETER
+-
\
2- I / 2 "
44-
TYPICAL TRAY SECTION
-1
SECT I ON
LUCITE WINDOW I'-O"
x
2'-0"
MEASUR I NG RULE .GLASS WINDOW 6" DIAMETER
It'
TO I - I /2"
__
_____
.
NO. OF TRAYS NO. OF "S" SECTIONS TRAY DIMENSIONS, INCHES A B C D E
F I N L E T WEIR OUTLET WEIR TOTAL TRAY AREAS, SQ F T I R I S E R AREA 2 REVERSAL AREA 3 ANNULAR AREA 4 & 5 SLOT & GAP AREA
3 6
IO-1/2 3-1/2 AND 2-1/2 3- I/ 2 3-1/2 AND 7/8 I AND 0 24 AND 30 54 48 4.2 2. I 2.7 3. I
Figure 1. Tray action can be seen through two large windows ( 1 X 2 feet) (Figure 3) and two 6-inch portholes with internal lighting. Downcomers were of Lucite so that liquid build-up could b e observed VOL. 51, NO. 5
M A Y 1959
625
b Figure 2. Manometers measured hydraulic gradient across tray 2, liquid head in the downcomer, and pressure drop across trays 1 ,2, and 3
clear liquid and inches of aerated liquid. The first was measured directly by means of a manometer in the center of the downcomer seal pan (Figure 1 and 2) and the second was a direct visual measurement of froth in the downcomer of the tray (Figure 1). Vapor densities were corrected for temperature and pressure conditions in the tower. For the wet tray studies at 30-inch tray spacing, similar data were obtained. T o obtain maximum vapor load from the blower a t the larger tray spacing, the bottom tray was removed to minimize the total pressure drop. Increasing the tray spacing increased the capacity of the Uniflux tray (Table I1 and Figures 7 and 8). Plate stability and tray activity were determined a t low liquid and vapor rates (Table I11 and Figure 6). The build-up of liquid froth in the downcomer was studied as the vapor and liquid loads were increased (Table 11).
-3
-
Table 1. Run
No. 1 2 3 4 5 6 7 8
Results
Tray Operation and Maximum Tray Capacity. Tray capacity was evaluated by rate of entrainment a t specific liquid and vapor rates, and relative pressure drops across trays 1, 2, and 3. The liquid flow during these runs. (Table 11) was set at a fixed r a t e - e g ,
Air Rate Cu. Ft./ Min. Lb./Min. 1,398 102.9 1,790 127.4 3,380 237.6 5,070 355.4 6,720 470.4 8,500 597.6 9,820 699.2 11,800 844.9
626
Cu. Ft.
Tray Temp,., O F.
0.0736 0.0712 0.0703 0.0701 0.0700 0.0703 0.0712 0.0716
91 103 111 116 123 127 129 137
Pv,
Lb./
Abs. Press. below
Tray Pressure Drop,
Tray 1, in H20
Tray 1
409.8 409.5 410.8 413.9 418.1 422.8 429.0 437.8
0.1 0.2 0.85 2.20 4.05 6.10 8.6 12.3
In. of Oil Tray 2 0.15 0.25 0.95 2.20 3.95 5.95 8.3 11.9
Tray 3 0.15 0.25 0.90 2.15 3.80 5.80 8.15 11.6
7
A
Figure 3. The side window between trays 2 and 3 (without liquid)
Dry-Tray Pressure-Drop Studies
U
Figure 4. A. Through the side window, droplets were visible above the layers of dense froth. Liquid i s flowing from right to left at the relative high rate of 400 gallons per minute. Vapor rate of 7500 actual cubic feet per minute i s at mi!d entrainment. 6. When the liquid rate i s 400 gallons per minute, but vapor rate is increased to about 8500 actual cubic feet per minute, heavy entrainment or flooding occups, and froth height increases
INDUSTRIAL AND ENGINEERING CHEMISTRY
7
sr:
02 ?
& ?
I-
N.? h b
?
-N
N
2
B
E
6;-
n
.-a
s
ji
d
3
N
*
d Im
ERE
404 gallons per minute (10,000 gallons per hour per square foot of downcomer area). As the vapor rate was gradually increased from a low value, the tray pressure drop along with the froth height also increased. Froth appeared less dense at higher vapor loadings (Figures 4,5; and 13). At a certain value of the vapor rate, mild entrainment began to occur and the froth height was roughly two thirds of the tray spacing. The froth with liquid droplets bouncing above its surface was in a state of turbulence and proceeded across the tray (from right to left in Figure 4) in a rotating motion at a fairly constant level. Most of the froth moved across the tray toward the downcomer. However, at the top of the layer, some recycled back across the tray so that it appeared to be moving clockwise (Figure 4). Some of the liquid droplets dancing above the froth were swept in with the vapor through the risers of the tray. The liquid carryover during this mild entrainment was from 1 to 10 pounds per 100 pounds of vapor as measured from the top tray. At this stage, the pressure drop across the middle tray was a little higher than that across the bottom tray. As the vapor rate was slowly increased above the point of mild entrainment, the rate of entrainment increased fairly rapidly and the tray capacity was approached. At this stage an appreciable difference between pressure drops across the bottom tray and across the middle tray indicated a high degree of entrainment. The pressure drop across the top entrainment tray was even higher than that across the middle tray because of the accumulation of the entrained liquid on the top tray. The entrainment rate was not measured until the pressure drop across the top tray became constant, The froth height on the middle tray in addition to being higher was also in an unsteady state across the tray. Instead of a void space with liquid droplets dancing above the froth, a pulsation occurred and geysers of liquid were blown against the tray above. Each picture shown in Figure 5 illustrating pulsation was snapped at about 3-second intervals with the liquid and vapor rate being held constant at 400 gallons per minute and 8000 actual cubic feet per minute. A large part of this liquid was carried through the riser to the tray above. The entrainment rate a t these conditions, being greater than 10 pounds of liquid per 100 pounds of vapor, was considered heavy. Some froth characteristics reported by other workers (5-8) were observed. For example, the density of the froth on the tray appeared to decrease with increased vapor rate and constant liquid rate. In addition, at high vapor rates (about 7000 actual cubic feet per VOL. 51, NO. 5
M A Y 1959
627
Figure 5. Pulsation on the tray was photographed at about 3-second intervals, using a constant liquid and vapor rate of 400 gallons and 8000 actual cubic feet per minute
minute), the average froth density appeared to increase with increasing liquid rate, whereas a t the lower vapor rate (about 2000 actual cubic feet per minute), it appeared to decrease with increasing liquid rate (Figure 13). Average froth density on the tray was calculated as the average static head of liquid on the tray in inches of clear liquid, measured by manometer, divided by the average froth height on the tray, measured visually : Froth densitytray= head in inches clear liquid head in inches of froth T h e effect of both liquid and vapor rate on the tower capacity and minimum allowable operating rates can be seen from the capacity curve (Figure 7 , A ) . The maximum capacity of the Uniflux tray modification tested in this size column for the nitrogen-absorption oil
system is shown by the constant entrainment parameter of 20 pounds per 100 pounds of vapor. Vapor flow is expressed as actual cubic feet per minute and superficial F factor (pL.p.l'*). Liquid flow is expressed as gallons per minute. This type of curve illustrates tray capacity as a function of liquid and vapor rate. It has been observed from several sources of unpublished data that towers with a fixed type of fractionating device and same diameter tend to flood at lower F factors with increasing pressure and vapor density a t comparable liquid rates. I t is believed that if the proper units can be developed, a curve of this nature should facilitate expressing the capacity characteristics of all types of systems for a particular fractionating tray design. Figure 7 , A also shows the minimum vapor loading for tray stability for two contraction areas. The maximum tray capacity was
reached when excessive entrainment occurred, as evidenced by high froth height and pulsation (or geysering). At this condition the total tray pressure drop rose sharply and exceeded the liquid head in the downcomer. Liquid buildup in the downcomer, per se, had no bearing on the flooding characteristics of this tray. Ai a constant liquid rate of 400 gallons per minute, the vapor rate was increased from about 7400 actual cubic feet per minute at 24-inch tray spacing, to about 8800 actual cubic feet per minute a t 30inch tray spacing for similar entrainment conditions (10 pounds of liquid per 100 pounds of vapor or 10 weight 70). 4 t a constant vapor to liquid ratio. this increase in capacity is about 167, (Figure 7,B) for the nitrogen-absorption oil system. Tray Pressure Drop. From the data in Tables I and I1 and Figures 8 and 9, pressure drop is broken down to three elements : APtotai
=
APdry
+
APllsutd
+
APinotlon
(1)
For this series of tests, added friction loss at high vapor loadings was only appreciable through the entrainment range. The following equations for calculating the three increments of total tray pressure drop have been developed. From Figure 9, the equation for pressure drop through a dry tray is
where sg, is specific gravity of liquid at tray temperature. The additional pressure drop caused by the liquid on the tray at moderate vapor loads before the entrainment range is reached has been evaluated by subtracting the dry tray pressure drop obtained from Equation 2 from the total pressure drop. This element of pressure drop is equal to the slot submergence (0.75 inch) plus the height of the crest over the outlet weir, h o w , as calculated by the modified Francis weir formula, c r
When the liquid-vapor loading reaches the entrainment range, the additional pressure drop caused by the liquid on the tray is greater than Equation 3. This additional pressure drop through the entrainment range, APfrlction, has been numerically evaluated from these data by subtracting values of A p e y -k A P l i q u i d (at a liquid rate equivalent to a head over the outlet weir of 2 inches and a t a tray spacing of 24 inches) from A P t o t a l and plotting APfriction us. u,pV1/' as shown on Figure 10, or APtotal
Figure 6. Tray action i s about 100 gallons and 1400 acrual cubic feet per minute. All rows were active and the tray was stable and well sealed. No liquid backflow was evident 628
INDUSTRIAL AND ENGINEERING CHEMISTRY
-
APdry
- APliquid
=
0.013
APfrietion
(~,~p,)*.s
(4)
Since APfriotion appears to be a function of both vapor and liquid loading, a
FRACTIONATING TRAY F
ACFM
ACFM
=
3
C
LL
C
I50 -
60CO
2
,
-
'
3 IO0
-
050
-
.' /
MIN STAB WITH LARGE CONT AREA,
,,
';
D =3-1/2,': ! "B"=3-1/2 1 I MIN STAB WITH SMALLER CONT AREA
!
-
2000
4000
---,
i
l
2 000
/
,"
1
/'
0
200
100
300
400
500
LIQUID FLOW GPM
1
/
1
--
; I + 4 0 2GPMT '- A
B
~
-.-P L--
345GPM
-
-
-68-
C A P A C I T Y I N C R E A S E FROM 24"-j TO 30" TRAY SPACING A T CONSTANT VAPOR TO L l O U l D R A T I O = ( 4 0 2 - 3 4 5 ) / 3 4 5 = 57/345 = I6 5 %
' I i
I
r - r -
I
-4
I
600 b
At 24-inch tray spacing, the probable maximum operating conditions are attained a t a constant entrainment Figure 7. A. rate of 2 0 pounds of liquid per 100 pounds of vapor. Minimum operating conditions for a stable tray vary with the contraction area under the downcomer apron B. A capacity increase of about 16% is obtained when the tray spacing is increased from 24 to 3 0 inches
correction factor has been empirically incorporated into Equation 4 to allow for a variance of liquid loads between the range of 100 to 550 gallons per minute (hou = 0.5 to 2 . 5 ) at 24-inch tray spacing, or
(4a)
T h e fluid mechanics causing this incremental increase in pressure drop at high vapor loading are not yet fully understood. At the high vapor loading conditions through the moderate and heavy entrainment range, it has been determined experimentally in subsequent tests that the liquid holdup on the tray was suddenly increased by about 10 to 2OYc. However, the incremental in-
crease in tray pressure drop did not appear to be a function of the increased hydrostatic head of liquid on the tray. It is probable that some of this energy loss is caused by vapors passing through the froth which is in a high state of turbulence. I t is also true that during these series of tests, liquid entrainment was evident a t the high vapor loading conditions. Consequently. it is probable the entrained liquid flowing through the fractionating device-Le., such as riser and reversal area-with the vapor contributed to the tray pressure drop because of the higher mass velocity of the vapor-liquid mixture. Equations 1, 2, 3 and 4 appear to check reasonably close for towers of 5'12 feet in diameter. For smaller towers. especially for high pressure systems.
12 8
400
xDRY
12
I
10
8
6 4
37 2000
ACTUAL
4000
2
'
6 0 0 0 ' 8000
I
l0,OOT
CUBIC F E E T PER MINUTE (ACFM)
0
I
2000
4000
6000
I
80'00 10,000
A C T U A L CUBIC F E E T P E R MINUTE (ACFM)
Figure 8. Dry and total tray pressure drops measured across tray 2 vs. vapor and liquid rate A. Constant entrainment lines a r e shown for 24-inch tray spacing. 24- and 30-inch tray spacing
B.
Total tray pressure drops a t
Equations 2 and 3 appear valid a t low to moderate loading conditions, but as the flooding point is approached, they do not agree with actual data; Equation 4 or 4a is not a measure of the additional pressure drop due to friction or entrainment. This could be caused by the effect of the circular tower wall on the flow of froth across the tray which causes back-splashing or recycling to build up a higher froth height than would be evident at equivalent loadings in larger towers. Entrainment. Entrainment is defined simply as the liquid carried n i t h the vapor from one tray to the tray above. From the considerable data published, only two general qualitative conclusions can be drawn: Entrainment increases with vapor and/or liquid rate, and tray efficiency is deleteriously affected by entrainment. The lack of any generally accepted mathematical correlation relating entrainment to tray capacity and efficiency can be attributed to the fact that most investigations reported have been with small scale equipment operated a t low liquid and vapor velocities. Also, the limited data reported for commercial equipment encompass so many different fractionating tray designs under so many different loading conditions that a quantitative analysis is impossible. In Figure 11, the liquid rate to the top tray is not a true value for the internal liquid loading, because it does not include the quantity of entrained liquid being recycled internally within the tower, However, for this system (vapor density of nitrogen, about 0.073 pound per cubic foot), the quantity of entrained liquid being recycled at 10 weight 70 entrainment is less than 570 of the total liquid load, and is not believed to be VOL. 51, NO. 5
M A Y 1959
629
90 80 70 60
80 60
50 0
%
40
g
30 25
0 LL
10 9 8
From tray 2 operated at 60" F., an equation was derived for dry tray pressure drop
high enough to affect the hydraulic characteristics of the tray. It is significant that such is not the case for systems of higher vapor density and this feature must be recognized when considering the effects of entrainment on tray capacity and tray efficiency for systems of high vapor densities. The total rate of entrainment measured and shown on Figure 11 is believed to include the entrained liquid that is recycled internally within the tower on each tray. Tray efficiency is not materially reduced until 10% entrainment (10 pounds of liquid per 100 pounds of vapor) is reached (9). Figure 11 shows that entrainment increases gradually from 1 to 10% with increasing vapor rate while the entrainment, when above 1070, increases more sharply with small increases in liquid or vapor rate until the maximum capacity of the tray is approached. From these data it appears that for the Uniflux tray, tray efficiency is not appreciably affected by entrainment until the tray capacity is approached. Figure 11 also shows how the rate of entrainment is related to the tray capacity. T h e flooding point at a constant liquid rate can be determined by the rapid rise in entrainment with a small increase in vapor rate. When the entrainment rate reaches 40 pounds of liquid per 100 pound of vapor, the capacity of the tray is certainly approached.
Maximum Loading at Approach to Flooding (24-in tray spacing; active vapor area = 19 sq. ft.; p v = 0.073) External Maxmium Vapor Rate Liquid Near Flooding Rate, G.P.M. Acfm F, = U,P,'/~
630
550
7600
400 200 100
8200 8900 9300
m m 1
1.8 1.93 2.1 2.2
40
30
7
i-'
6
W
5
4
z a + z W
3 25
0
10 8
0
6
a I 20
[r
s v
9.
100
a 80 0 3 60 0 1
15
Figure
20
0
w
s
30
1
20
F =,UP5
50 40
3
1
2 15
'
15
20 25 3035 Fs
=4.FV5
Figure 10. When vaporliquid loading reaches the entrainment range, additional pressure drop caused by liquid on the tray has been numerically evaluated
-
Entrainment is related to the froth height on the tray and to the degree of pulsation. I t is hypothesized that these two factors are in turn related to the mechanics of fluid flow-Le., to the angle of resultant force and to the transmission of a surge wave. With liquid flowing horizontally across the tray and with vapor flowing vertically upward, a resultant force of motion a t some angle to the horizontal in the direction of liquid flow is evident. T h e angle a t which this resultant force acts depends on the shape and type of fractionating device used and on kinetic energy of the vapor and liquid rate. Depending on this angle, the liquid is aided more or less as the froth flows across the tray, and it is believed that the froth height is a function of this angle and of the vapor distribution across the fractionating device itself. The froth in flowing across the tray must overcome some resistance to flow partially caused by the frictional drag of the fractionating device and the tower walls. The circular cross section of the tower is also believed to have some effect on the flow pattern. T h e velocity, height, and density of the froth varies somewhat as it proceeds across the circular tray. I t is believed that the froth in flowing across the tray can be compared to a liquid flowing across an open channel where the vapor pressure at the surface of the liquid is constant across the channel and a surge wave is transmitted
INDUSTRIAL AND ENGINEERING CHEMISTRY
.
nated by F on- the curves
to the froth ( 4 ) . The magnitude of the surge wave is believed to be related to the resistance to the flow of froth. T h e frequency of pulsation is believed to be related to the velocity of the surge wave in relation to the velocity of the froth, and to the manner in which the vapor is distributed across the tray which in turn is related to the dry tray pressure drop or the resistance to the vapors flowing through the tray. An exact physical analysis is complicated; to develop such an analysis, more controlled tests must be made on systems of different liquid and vapor densities using several Uniflux tray dimensions. Minium Tray Capacity or Tray Stability at Low Liquid and Vapor Rates. The dimensions D and E shown in Figure 1 had a marked effect on tray stability at low vapor rates. When D is 31/* inches and an inlet weir or E is 1 inch, the minimum tray stability occurred at vapor loads down to about 2100 actual cubic feet per minute -about 30y0 of normal design rate for a usual fractionating tower of this diameter. With these dimensions at low vapor rates, the majority of incoming liquid hurdled the center of the inlet weir because of the half-moon shape of the downcomer formed by the circular tower wall. T h e high quantity of liquid ccming over the center of the first row of slots resulted in a small amount of dumping or backflow through this row at low vapor rates. When the inlet weir was removed and the contraction area decreased (D decreased to '/8 inch), a marked improvement in liquid distribution and in tray activity occurred, and the vapor rate was re-
FRACTIONATING TRAY duced to about 1000 actual cubic feet per minute, about 1570 of normal design rate, before any appreciable backflow was detected. This decrease in contraction area had no effect on the maximum capacity of the tray. This indicates the Uniflux tray, properly sealed, has a n operating range of about 8 to 1 (8500 to 1000 actual cubic feet per minute). Figure 6 shows tray action a t low loading conditions. At vapor rates between 10 and 1570, the tray was semistable. Approximately 10% was equivalent to a superficial vapor velocity of 0.9 foot per second (vapor volume divided by the total cross-sectional area minus the area occupied by two downcomers) or a superficial F factor, u, p y l / z ,of 0.24. 4 t this condition the first one or two rows upstream were inactive, but no evidence of dumping was detected. T h e effect of these dimensional changes on minimum tower capacity is shown on Figure 7 , A . The inlet weir extension pieces, valuable a t both low liquid and low vapor loads, prevent leakage of liquid around the sides of the inlet weir and thus aid in activating the tray initially. Tray leakage occurred in decreasing amounts u p to vapor rates of 307, of normal design rates (2000 to 2500 actual cubic feet per minute) because of the incorrect installation of the vapor end closure pieces. No appreciable leakage occurred when the trays were correctly installed. At liquid rates below 70 gallons per minute (equivalent to a head over the outlet weir of about 0.6 inches): the approach to tray activity was important. Vapor rates as high as 3070 of normal design were required to activate the tray initially, and then decreased to the desired vapor rate. Above 100 gallons per minute, there was no difficulty in activating the tray. T h e value of the notched outlet weir offered no advantage to tray stability a t low liquidrates. Downcomer Studies. The capacity of the tray was reached when the froth height on the tray reached a certain level and the degree of pulsation was enough to blow liquid against the bottom part of the tray above, causing excessive entrainment. It was supposed that when the liquid froth height in the downcomer backed u p above the next tray, the capacity of the tray was reached, and that flooding would occur with increased liquid or vapor rate, since a positive liquid head in the downpipe is necessary to overcome the total tray pressure drop. However, the present downcomer studies indicate that the density of the liquid or froth in the downcomer is an independent variable and that when the downcomer is full of froth the tray does not necessarily approach flooding. I n the initial wet tray studies (Table II), there was recorded the effective
ACFM
Figure 12. At lower vapor rates, restriction in the contraction area caused the downcomer to fill with froth, but it had no bearing on flooding
500
\ 4,000
400
'-4 2,000
300
5a
BW 200
A. Normal contraction area; D = 3I/z inches; B = 4 inches
100
B. Restriction in contraction area; D = '/a inch; B = 2l/2 inches
I
93 0 4 05 0 6 07 08 09 I O 03 04 0 5 0 6 07 08 09 I O DENS'TY DOWNCOMER
height in the downpipe as inches of clear liquid on a manometer, while the actual height of aerated liquid was measured visually. When the flooding point was approached, the level in the downcomer had backed u p to the next higher tray. A clear view of the downcomer of the top and middle tray was possible (Figure l ) , and the froth in the downcomer was a dense foam. Thus, it was believed that the full downcomer was limiting the capacity of the tray. T h e flow of liquid in the douncomer was then restricted by decreasing the contraction area [ D decreased from 3', 2 to 7 / 8 inch and B from 3l/2 to 2 inches (Figure l)]. T h e wet tray studies were continued with the vapor rate fixed at a low value of 2450 actual cubic feet per minute. The liquid rat? was gradually increased from 50 to 550 gallons per minute. T h e liquid level in tlle downcomer increased with liquid rate accordingly. and a t a vapor rate of 2450 actual cubic feet per minute and a liquid rate of 5.50 gallons per minute, the downcomer was full of liquid froth. T h e total tray pressure drop a t this condition was about 3.2 inches of clear liquid. I t was now anticipated that further increases to vapor flow and increased trav pressure drop would cause the liquid
E
to back u p across the tray resulting in a higher than normal froth height on the tray and high rates of entrainment a t equivalent lower vapor rates. However, this was not the case. .4s the vapor rate was increased. tray action, pressure drop and entrainment were all identical to the previous tests. Obviously. density of the froth in the downcomer varied a t different conditions of vapor and liquid flo~v. iVith the downcomer filled completely with froth, the tray did not approach flooding when the vapor rate was increased, but instead the apparent density of the froth became greater. The following data for two runs at low and high vapor rates. both with downcomer full of froth, are summarized to illustrate this phenomenon : Run No. 94 Gallons per minute 550 Actual cubic feet per minute 2450 Froth height in downcomer, in. 30 Liquid head in downcomer, in. 16 Apparent density of froth in downcomer 0.54 Froth height on tray, inches 6 Entrainment, per 100 Ib. vapor 0 Total tray pressure drop, inches clear liquid 3.2
99s 550 9000 30 20/25
0.80 20 10
10.0
Tray action and hydraulic characteristics were similar to runs 68 to 71 when liquid flow in the downcomer was not restricted. a
1 inmn.
Figure 13. Froth density decreases with increasing vapor rate. Tray spacing, 24 inches
Hc
INCHES CLEAR LIQUID INCHES AERATED LIQUID
.GPM--.
,
,
,
,
\+----
9
+._
4000'
1
7 I -__ -I-
I
01
02
03
04
05
07
06
FROTH DENSITYTRAY = LIQ HEAD ON TRAY, INCHES CLEAR LIQ
HEAD OF FROTH ON TRAY, INCHES OF FROTH VOL. 51, NO. 5
M A Y 1959
631
I n Figure 12, the apparent density was calculated as the static head in the downcomer in inches of clear liquid measured by the manometer divided by the static head in the downcomer in inches of froth measured visually, or
ment was high (20 to 40%) and when the total tray pressure drop increased sharply with small increases in vapor or liquid flow. This high entrainment only occurred when the flooding point was approached. At this condition the froth height was greater than two thirds of the tray spacing and the amount of pulsation, or geysering, was suddenly increased. Because the rate of entrainment was low below the flooding point, the over-all tray eficiency of the Uniflux tray is expected to be high u p to the maximum capacity of the tray. The tray remained stable a t vapor loads as low as 127, of the maximum operating rate a t liquid rates above 2 gallons per minute per inch of outlet weir. An increase of tray spacing in the 5’/2 foot column from 24 to 30 inches increased the vapor-liquid handling capacity by 16%. Liquid backup in the downcomer, caused by an artificial restriction to liquid flow, had no effect on limiting tray capacity. T h e total tray pressure drop for the Uniflux trays in the 51/2 foot tower with the nitrogen-absorption oil system is the sum of three incremental pressure drops -namely, Ap,,,, @‘liquid and Aptriotion. Comparisons with pressure drop data in commercial towers of the same diameter indicate that the correlations check reasonably well. For smaller towers, the correlations obtained for Apd,, and APli,“id are valid up to moderate load-
Froth denSitydownoomer = head in downcomer, inches of clear liquid head in downcomer, inches of froth With minimum restriction to liquid flow, the apparent density varied from 0.4 to 0.8. Where the liquid flow through the contraction area was restricted, the range was from 0.5 to 0.8. T h e apparent density may vary differently in a tower as normally operated. h-evertheless these studies tend to de-emphasize the importance of such factors as residence time in the downcomer, and liquid backup in the downcomer. Instead, the studies indicate that the action between the vapor and liquid on top of the tray itself should be the prime consideration in designing a tray for highly loaded conditions.
Summary and Conclusions The Uniflux tray has a n operating capacity comparable to other fractionating trays, and if properly sealed, it has an operating range of about 8 to 1. The maximum tray capacity was approached when the rate of entrain-
Table Ill. Plate Stability and Tray Activity at Low Liquid and Vapor Rates [Dimensions at bottom of downcomer apron (Figure 1): E , 31/2 in.; D , 3’/z in.] Vapor, Liquid, Run Gal./ Cu. Ft.1 Min. No. Min. 55
101
56
1750
50
57
1750
15
58
1750
50
59
1570
50
72
2380
430
73 74 75 76 81= 82“
2100 1710 2310 2440 2300 1750
430 430 550 550 430 430
83b 84b
2300 1750 1450
430 430 550
lOOC
E.
3500
Good tray action, less bubbling in first row of slots; no dumping; slight leakage from bottom end closures; good functioning of inlet weir extensions; 5’ sections too close to side of tower for evaluating wing extension Good tray action, but activity decreased in first row of slots; no dumping but increased leakage No tray activity; liquid dumping in upstream rows and vapors raising through downstream rows No tray activity until vapor i s increased to about 2700 cu. ft./ min. and then brought back down to 1750 cu. ft./min. This i s the minimum liquid and vapor rate possible for some tray activity with indicated B and D dimensions. Plate only half active (three rows bubbling); no appreciable dumping; high leakage because vapor end closure pieces are not flush with tray ring Minimum vapor for complete tray activity at 430 gal.!min. ; leakage occurring through space at bottom of vapor end closure piece First row of slots inactive; no dumping First row of slots inactive ; leakage increased Same as run 73 Same a s run 72 Good tray action with all rows active Same as run 72 ; moving the outlet weir to downstream side of first row improved tray action Same as run 81 Same as runs 72, 76, and 82 Same as runs 72,76, 82, and 84. Increasing the restriction at bottom of downcomer apron appreciably improves tray activity
Relocated inlet weir. E , at downstream side of first row. B = 2 in.; D = 7 / g in. ~
632
~~
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
Completely removed inlet weir,
ings; however, a t the higher loadings, the relation for APfriotion does not hold. Nomenclature
A ca
= superficial vapor area or total
tower cross-sectional area minus downcomer area supplying liquid to and withdrawing liquid from the tray, sq. ft. ( A c a = 19 sq. ft. for S1/2-ft. experimental tower) F = superficial F factor = u , P , ” ~ = height of the crest over the how outlet weir as calculated by Francis weir formula = specific gravity of liquid 5g us = vapor velocity based on superficial active tray area A,,, ft./sec. = vapor density, lb./cu. ft. Pi. = dry tray pressure drop or APdry pressure necessary to force dry vapors through the tray itself, inches clear liquid = liquid tray pressure drop or lpliquid the effect of the hydrostatic head of liquid flowing across the tray .1PiriCtlon = the added pressure drop caused by the added friction loss at high vapor loadings lPtOt,l = total pressure drop across one tray Acknowledgment
The authors wish to express their appreciation to H. S. Myers, C. C. Boyer, and the operating staff of the research department of C. F. Braun and Co., Inc., for their part in obtaining the data for these studies, and to V. 0. Bowles of the Socony Mobil Oil Co., Inc., for his suggestions during progress of the work. References
(1) Bowles, V. O., Chem. Eng. 61, 174-5 (1954). (2) Bowles, V. O., Petrol. Refiner 33, 197-8 (1954). (3) Cicalese, J. J., Davies, J. A., Harrington, P. J., Hougland, G. S., Hutchinson, A. J. L., Walsh, T. J., Petrol. Refiner 26, 431-95 (1947). (4!, Coulson, J. M., Richardson, J. F., Chemical Engineering,” vol. I, pp. 66-73, McGraw-Hill, New York, 1957. (5) Gerster, J. A., Bonnet, W. E., Hess, I.. Chem. Eng. Progr. 47, 523, 621 (1951). ( 6 ) Gerster, J. A,, Colburn, A . P., Bonnet, M’.E., Carmody, T. W., Ibid., 45, 716 (1949’1.
(7j-Ger$ter, J. A,, Foss, A. S., Ibid., 52, 28-5 (1956). (8) Hutchinson, M., Buron, H., Miller, W.,“.4erated Flow Principle Applied to Sieve Plates,” .4m. Inst. Chem. Engr., Los .4ngeles, Calif., March 1949. (9) Sherwood, T. K., Jenny, F. J., IND. END.CHEW27, 265 (1935). RECEIVED for review September 19, 1957 ACCEPTED September 2, 1958
Division of Industrial and Engineering Chemistry, 132nd Meeting, ACS, New York, N. Y., September 1957.
