Variables in Perforated Plate Column Efficiency and Pressure Drop
I
C. L. UMHOLTZl and MATTHEW VAN WINKLE University of Texas, Austin 12, Tex.
Effect of Hole Free Area, Hole Diameter, Hole Spacing, Weir Height, and Downcomer Area Plate efficiency of a perforated plate distillation column can be predicted with this information on the individual importance of mechanical design features
A N for the prediction of the plate efficiency in distillation EFFECTIVE METHOD
columns should include the physical properties of the mixture being distilled, the liquid and vapor rates, and the mechanical design features of the plate and column. Correlations in the literature (8, 9, 77, 22) have predicted the effect of some physical properties on plate efficiency, but in general, they do not include the effect of the mechanical design features of the plate and column. The mechanical design features affecting plate efficiency in a perforated plate distillation column are those factors that affect the area of contact and the time of contact of the vapor and liquid phases. With respect to the plate, these features are: weir height, hole pitch (closest distance between centers of holes, hole pattern, hole diameter, perforated area pattern, per cent free area (per cent perforated area), plate thickness, liquid flow pattern (baffle arrangement), inlet and outlet weir type and location, and downcomer type and location. The variables affecting over-all plate efficiency with respect to the column are: plate spacing, column diameter or liquid path length, and downcomer crosssectional area (liquid handling capacity). Although experimental data are available (2, 73, 75, 78, 79, 27, 27) on the effect of some mechanical design features on over-all plate efficiency and
pressure drop, no complete study using a single binary system to eliminate the effect of physical properties has been published. This investigation was undertaken to determine the individual importance of some of the mechanical design features on over-all plare efficiency in two small perforated plate distillation columns at total reflux, using one binary system. The mechanical design features studied were : hole diameter, per cent free area, weir height, downcomer cross-sectional area, and pitch-diameter (ratio of the distance between hole centers to the hole diameter in a triangular pattern). Per cent free area is defined as the actual hole area available for bubbling contact divided by the superficial cross-sectional area of the column and multiplied by 100. Per cent downcomer area is defined as the cross-sectional area of the downcomer on the plate divided by the superficial cross-sectional area of the column and multiplied by 100. The data were determined on columns of small diameter and the conclusions
Present address, Technical Service, Humble Oil and Refining Co., Baytown, Tex.
226
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Table I.
and observations based on these data are strictly applicable only to columns of small diameter. Equipment
The two distillation columns used in this investigarion were constructed similar in design, so that the data could be compared. Glass column sections were used in order to observe the plate operation during the various tests, as previous resulls had indicated that the data could not be properly interpreted without some visual observation. Schematic diagrams of the two columns and accessories are shown in Figures 1 and 2. The dimensions of the columns and the ranges of the variables studied are given in Table I. Holes were drilled in the 1/16-inchthick brass plates in such a pattern that the centers of the holes were at the apices of equilateral triangles. The resulting perforated area was extended to within a minimum distance of two hole diameters of the downcomer inlet and outlet
Dimensions of Columns and Variables
Column diameter, inches
1.83
3
No. of plates
5 3
5 6
Plate spacing, inches Plate thickness, inch Weir height, inch Hole diameter, inch yo free area Pitch-diameter yo downcomer area
‘116
0, 1 1 4 ,
‘!iJ
1/2,
3/a,
1
‘116
2.7,5.4,8.1,
1 ’116,
10.8, 13.5, 16.2
2, 3, 4, 5 1-08,2.96, 5.78
‘ / 8 , 5 / 8 1 $ ‘/I8
12.5, 16.2 2 3.55, 7.1
VOL. 49, NO. 2
FEBRUARY 1957
227
1
80 8 Y 8 T C M : f l - O C T A N C TGLULNC ATMOIPHCRIC PRCSSURE
,, 6 2
60'
w
E so w
a -I
x
'
GOLUMN DIAMCTCR NUMBER PLATES
30
w
20
- PLATE SPAGINQ W E I R HEIQHT
228
MOLL Y TGLUCNC ( M C b N l
5" I/4y
OHV
HOLE DIAMETER 1/16" Y f R E E AREA o.7 -PiicHiDiAMEitR o n D O W N G O M C R A R E A 1.00
sections of the plate. Circular downcomers were extended through the plates to form overflow weirs. Inlet weirs were considered unnecessary a t the liquid and vapor rates tested. A Ushaped downcomer was provided for the bottom plate t o make a liquid seal to prevent vapor bypassing. As the overhead vapor was totally condensed, the vapor rate was measured by a rotameter in the reflux return line.
I
I
l.BS" 6
~'
BOTTOMS-
0
14
4e
0
71
21
0
ee
I S
-
Condensation of the vapor in the vapor line from the head of the column to the condenser was minimized by installing a heating coil around this section and superheating the vapor slightly.
Procedure For all the test runs made in this investigation, the columns were operated on the n-octane and toluene system a t total
INDUSTRIAL A N D ENGINEERING CHEMISTRY
reflux and atmospheric pressure. Before initiating this investigation, the columns were operated for a time at fixed conditions in order to establish the minimum run time. An "approach to steady state" graph was constructed by obtaining samples of the condensed overhead vapor and recording the change in composition with the time elapsing from the beginning of the run. The samplc withdrawn from the system was maintained as small as possible relative to the total charge to minimize the variation in composition as a result of vapor removal. Minimum run times of 1 and 3 hours, respectively, was selected for the 1.83and the 3-inch-diameter columns. A particular composition range of the n-octane and toluene mixture in the reboiler was selected after it was observed during the initial runs that the over-all plate efficiency, other factors remaining constant, varied with the composition. Figure 3 illustrates the variation of efficiency with the vapor rate a t various compositions. In order to eliminate this variation, a toluene reboiler composition range from 42 to 58 mole yo was selected for the operation of both columns. A single over-all plate efficiency curve as a function of vapor throughput rate resulted, other factors remaining constant, from maintaining the liquid composition in the reboiler within this range. In determining the change in over-all plate efficiency accredited to the various mechanical design features, the vapor throughput rate \vas varied by increasing the heat input to the reboiler, from a low
V A R I A B L E S IN P E R F O R A T E D PLATE COLUMN EFFICIENCY A N D PRESSURE D R O P rate, where only a few of the plates were operating, to the highest rate attainable. The upper limiting operating rate of the column was determined either by the flooding point of the column or by the total wattage available to the reboiler. As the overhead vapor was totally condensed, a liquid sample was taken from the reflux return line to determine the overhead vapor composition and a liquid sample was taken from the reboiler to determine the bottoms composition. After the compositions were determined by use of a refractometer, the number of theoretical plates needed to obtain the bottom and top compositions was calculated by Fenske’s formula i10).
LL
w
C”4
40
J
0
j 7 a
iiA!ETER
30
WEIBHT noLi DIAMETER U F R E t AREA ULDO*(ICOI(LR M C A I ” ’
w
:eo.
The relative volatility used in the Fenske formula was obtained from the published data of Berg and Popovac ( 4 ) . The reboiler theoretical plate was subtracted from the calculated theoretical plates in order to determine the overall plate efficiency exclusive of the reboiler. An over-all plate efficiency was then calculated by dividing the remaining theoretical plates by the number of actual plates in the column and multiplying by 100. Arithmetic average values of the variables were used in all cases where single valued variables were required. This was necessary to obtain an average column temperature and pressure, and an average vapor density, and molecular weight of the system. The vapor mass velocity reported on the figures was not corrected for the cooled reflux returning to the column. Using average values, the reported G should be multiplied by a factor of 1.26 to obtain the actual corrected G.
Results Per Cent Free Area. The results of the investigation of over-all pIate efficiency, as a function of per cent free area shown on Figure 4, indicate that efficiency is not affected, other factors remaining constant, by a change in per cent free area over the range tested. For all of the per cent free area increments tested, a dumping point was encountered. The dumping point, defined as the throughput rate at which the liquid on the plate drains off through the holes, resulting in a lower or negligible liquid level, is illustrated on Figure 4 as the point at which the over-all plate effi-
I
0.5
I/4‘
1/16. L. 7 1.00
I
I
I
I
I
I
I5
to
30
40
50
60
MASS VAPOR
0.1
1.0 noLE
5 4
8
2
VELOCITY,
1
1
--
3
I 1
I L
70 00 90 100
I
I
125
I50
L B , / MR.-SO. FT.
I
I
I
I
I
I
I
14
L
3
4
6
6
7
I I J I
0.6
e 0 0
I
I
6 - SUPERFICIAL
I
PITCM / DIAMETER
5 3.
-WEIR
IO
where B = bottoms ou = overhead vapor n r = total number of theoretical plates y = mole fraction a: = relative volatility
i.!...
NUMBER P L A T E S P L A T E SPACIUS
VAPOR
VELOCITY,
IJ 1
rT./sEc.
Figure 5. Effect of pitch-diameter ratio on over-all plate efficiency ciency begins to decrease rapidly as the throughput rate is decreased. Estimated flooding points are also indicated on Figure 4 for the per cent free area
tests in which they were encountered. The increase in over-all plate efficiency beginning a t a n F factor of 0.4 was caused by a peculiarity in the column
75 S Y S T E M : n-O C T A N E T O L U E N E ATMOSPHERIC PRESSURE
fl
*‘ 0
70
2
!!
-
0 LL LL W
65
60
W
I-
U
i
55
-I -I
2 W
50
>
/
0
46
I
2
3 4 5 PITCH / D I A M E T E R
6
Figure 6. Effect of pitch-diameter ratio on, over-all plate efficiency at constant vapor rate Column diameter, inches
No. of plates Plate spacing, inches Weir height, inch Hole diometer, inch
% free a r e a % downcomer a r e a
1.83 5 3
‘14 1/16
2.7 1.08
F 0.00 0.04 0.02 VOL. 49, NO. 2
Rate G
127 64 32
V 6.7 3.4
1.7
FEBRUARY 1957
229
construction and normally should not be encountered at these throughput rates. The 1.83-inch column contained a glass rod extending into the reboiler which positioned the plate sections in the column. At mass velocities in the neighborhood of 600 pounds per square foot per hour, a partial flooding action in the. constructed column area (caused by the rod) was noted. This constricion retained some of the liquid and intro-
bp
duced a partial bubble contact area amounting to part of a plate. Pitch-Diameter Effect. Kirschbaum (79) postulated that when the centers of the holes in a perforated plate are at the apices of equilateral triangles, the ratio of the pitch to the diameter of the hole is the deciding factor in the interchange efficiency between the forming bubble and the liquid on the plate. When the holes are far apart, the liquid between the holes has little contact with
the vapor. When the holes are close together, the bubbles coalesce and form large bubbles with less relative surface area available for contact with the liquid. The results of this part of the investigation are illustrated on Figures 5 and 6 at constant pitch-diameter ratio and constant throughput rate. respectively. Theoretically a maximum over-all plate efficiency should exist as a function of the pitch-diameter ratio, but it was not encountered in the range of ratios tested. I t is possible that the effect of the pitchdiameter ratio on over-all plate efficiency is very small except in the range of ratios approaching 1 and very large values. Weir Height Effect. Since the interchange efficiency between the vapor bubble and the liquid surrounding the bubble is some function of the vapor rate as well as the amount of liquid available for contact on the plate, it is apparent that an important mechanical design feature is the weir height, which fixes the quantity of liquid retained on the plate for contact. This should not imply that a fixed static liquid level as high as the weir is maintained on the plate a t all vapor rates. Until the vapor rate is great enough to prevent the liquid from draining through the holes (dumping point), the weir does not function and the liquid level o n the plate varies with the vapor velocity. A s the vapor velocity is increased beyond the dumping point, the liquid level on the plate rises to the overflow level of the weir and the height of liquid on the plate
100 90 80
*' 0 70
2
-0 60 w
k w
50
E 40 4
-I
a
-I -I
30
9
K W
20
230
0.015
0.02
0.03
0.04
INDUSTRIAL A N D ENGINEERING CHEMISTRY
0.06 0.08 0.1
0.15
0.2
0.3
0.4
0.5
V A R I A B L E S IN P E R F O R A T E D P L A T E C O L U M N E F F I C I E N C Y A N D PRESSURE D R O P
Figure 9.
becomes a function of the weir height as well as the vapor and liquid rates. T h e over-all plate efficiency, shown on Figure 7 as a function of the vapor throughput rate, was greatest at or near the dumping point for each weir height investigated. The operating throughput range of the perforated plate between the dumping and flooding points decreased slightly, other factors remaining constant, as the weir height was increased. This was anticipated because the dumping point should be found at higher vapor throughput rates a t higher weir heights (higher static heads) and the flooding point correspondingly should be found at lower throughput rates. This is indicated on Figure 7 by the dumping and flooding lines. The efficiency curves are parallel and approximately a n equal distance apart, except for the 0-inch weir which is located at a much lower over-all plate efficiency as a function of vapor throughput rate. The large difference in overall plate efficiency between the 0-inch weir curve and the l/d-inch weir curve was probably the result of a combination of factors. Two of these are: The low liquid level allowed a considerable amount of vapor to bypass the plate through the unsealed downcomer, and insufficient liquid was present on the plate to maintain a good bubbling contact with the vapor. Both factors result in lower over-all plate efficiencies. Downcomer Area Effect. The effect of this design feature on plate efficiency
Effect of hole diameter on over-all plate-efficiency
is connected directly to the liquid handling capacity; any increase in downcomer cross-sectional area, other factors remaining constant, should increase the capacity of the plate and column and correspondingly affect the over-all plate efficiency. The over-all plate efficiency curves are shown on Figure 8 as a function of throughput rate with parameters of per cent downcomer cross-sectional area relative to the column crosssectional area. ‘ T h e over-all plate efficiency decreased, other factors remaining
PLATE THICKNESS HOLE DIAMETER P I T C H I DIAMETER
constant, as the per cent downcomer area increased and the range of plate operation was approximately doubled for a twofold increase in the per cent downcomer area. Some factors - that could result in this decrease are: a shorter liquid path length on the plate with large diameter down-comers, a shorter vapor disengaging area around the downcomer, and a lower aerated liquid buildup on the plate with the larger downcomers. Hole Diameter Effect. The effect of
1/16. 1/16‘ 0
ER AREA
HOLE
I e0
30
I
I
40 50 60 G - SUPERFICIAL
002
00s F
Figure 10. columns
VAPOR
I I I I I
-
om
355
VELOCITY, FT./SEC.
I
I
60 1 0 0 150 200 MASS VAPOR VELOCITY,
004 FACTOR
1.06
I
I
I
I I I I I
300 400 600 LB./ HR.-SQ FT.
008 0.1 0.15 OP (FT / SECJ (LE. I CUBIC FT
d
03
0.4
I500
600 1000
06
OB
IO
Over-all plate efficiency relation between 1.83- and 3-inch diameter
VOL. 49, NO. 2
FEBRUARY 1957
231
0.8
U DOIUCOYCR
Q3 0.4
Figure 11.
ARC4 1
1
l
I
7.1
I
1
1.5
2
-
I
0.6 08 1.0
3
4
5 6 7 8 9 10
15
20
30
Column pressure drop as function of hole diameter
hole diameter on over-all plate efficiency as a function of the vapor throughput rate (Figure 9) is not clearly defined. The fact that the curves with a parameter of hole size are not parallel would seem to indicate that other factors are involved than those maintained constant during this study. Some variation in hole size could result in indefinite conclusions. However, this variation was believed small enough to be considered negligible. The results of the 1/16-inch-diameter hole test indicates a much longer throughput range of operation than those
obtained from the larger hole sizes tested. The effect of surface tension in preventing the dumping of liquid through the holes would be more pronounced on the hole plates of smaller diameter. Column Relationship. The 1.83-inch and 3-inch columns were compared by operating both columns under the same conditions and using the same mechanical design features. The mechanical design features that varied between the columns were: the plate spacing of 3 and 6 inches, the per cent downcomer area of 1.08 and 3.5570, and the column
diameters of 1.83 and 3 inches. The results of this comparison are shown on Figure IO. The two curves of overall plate efficiency as a function of the vapor throughput rate overlap throughout the throughput range of the smaller diameter column. The range of the smaller column was shorter because the per cent downcomer area was less. The difference in plate spacing does not seem to have a noticeable effect on the over-all plate efficiency in the range of vapor velocities investigated. Pressure Drop. Column pressure drop as a function of the vapor throughput rate is plotted on Figure 11 for the 3-inch column. A single curve was plotted for the ‘/aq, 5/32-, and 3,’~6-inchdiameter hole plates, but the I / L ~ inch hole plate produced a separate pressure drop curve as a function of the vapor throughput rate a t the lower vapor velocities. This curve explains the variation in operating range of the various hole diameters on Figure 9. As the throughput rate was decreased. the plate retained a liquid level (providing a contact medium for the rising vapor) until the dumping point was attained. Then the liquid dropped through the holes in the plate, resulting in a lower pressure drop as well as a lower over-all plate efficiency. The ultimate result of the over-all research project, of which this work is a small portion, is to predict efficiencies in distillation columns. A great deal of work, some of which may seem obvious or unimportant, must be done before a project of this type can be completed.
Variables in Perforated Plate Column Efficiency and Pressure Drop
Effect of Plate Thickness and System Properties
I
P. D. JONES1 and MATTHEW VAN WINKLE
F O R evaluation of the effect of plate thickness and system properties of efficiency and pressure drop in a 3-inch-perforated plate distillation column, the column and accessories are shown in Figure 2, with slight modifications in heating provisions, so that throughputs in the neighborhood of 2000 pounds per I Present address, Shell Chemical Co., Deer Park, Tex.
232
hour per square foot of free cross-sectional area could be attained. The plates were made of brass and were machined to the exact thickness required: 0.054, 0.083, 0.125 (‘/a), 0.1875 (31’18), and 0.250 inch (l/4), varying at the most ~ k 0 . 0 0 2 inch. Thickness-hole diameter ratios ( 7 / ’ D ) were, respectively, 0.46, 0.66, 1.0, 1.5, and 2.0. Seventy-two l/*-inch holes totaling 12.5yG free area were drilled,
INDUSTRIAL AND ENGINEERING CHEMISTRY
as shown in Figure 12, using triangular pitch of ‘14 inch for a pitch-hole diameter ratio of 2. Holes were drilled from the bottom side of the plate, so that the sharpest edge of the orifice formed would face the vapor flow direction. Downcomers were made of ‘j2-inch hard-drawn copper tubing and totaled 7.1y0 free area. They were soldered to the plate, forming I-inch over-flow weirs which extended to within 3/a inch of the
