J O H N G R I S W O L D , J . W . M O R R I S 1, AND C . F . V A N B E R G 2
U niversity o f Texas, A u stin , Texas
BEVELLED FOR
Figure 1. Details of Construction of Experimental Column for Wire-Screen Plates (No. 1)
Development and performance o f three designs o f all- metal screen-plate fractionating columns for general laboratory and pilot-plant use are reported. The charac
teristics of six different screens were determined, and performance o f I.5-inch and 2-inch diameter columns are given with plate efficiency-rate data on n-heptane—methyl- cyclohexane at total reflux. The features o f these columns are ruggedness, ease of construction, and relatively high capacities, with maintenance o f efficiency at high rates characteristic o f plate-type columns. Maximum boil-up rates are 73 m l. per minute (1.2 gallons per hour) for a 1.5- inch column and 250 m l. (4 gallons) for a 2-inch column.
Maximum HETP occurs at maximum boil-up, and the corresponding values are 1.8 and 3.2 inches, respectively.
T
HIS article describes the development and performance of screen-plate columns that have been used for general- purpose hydrocarbon fractionations and in pilot-plant processes for separating pure hydrocarbons from petroleum* in the University of Texas laboratories during the past several years.Laboratory fractionating columns may be classified as packed, film-type, and plate-type. With the exceptions of Stedman embossed packing (1) and single-tum helices (Ą), high-efficiency film-type packings have been successful only in smaller sizes of laboratory columns. The efficiency of such columns is often sensitive to throughput and even to the operator’s technique.
These considerations led to the development of screen-tray columns having the desirable characteristic of relatively constant plate efficiency over a wide range of liquid and vapor velocities.
Bruun developed a 1-inch (25-mm.) diameter, all-glass bubble- tray column (2) that has found extensive use. Oldershaw re
ported comparable performance data on all-glass perforated- plate columns (5). Because of the complexities of constructional
1 Present address, Grasselli Chemicals Department, Oak Ridge, Tenn.
* Present address,--Humble Oil and Refining C om pany, B aytow n, Texas.
* Previous articles of this series appeared in Volume 35, pages 117-19, 247-51, 8 54-7 (1943).
1120 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 36, No. 12
Figure 2. Screen Piale and Its Parts (No. I)
details, small-scale all-metal bubble-tray columns have not found general usage. Palkin reported a column consisting of 40-mesh wire gauze cups mounted in a glass shell (7). Bubble- cap plates, perforated plates, and screen plates are fundamentally similar as to mechanism of vapor-liquid contacting. Screen plates are the easiest to fabricate in small sizes and should be most readily adaptable to all-metal bench-scale column construction.
D E V E L O P M E N T O F S C R E E N -T R A Y C O LU M N S
Preliminary observations, using a wide variety of screens mounted in a glass tube with air-kerosene as vapor and liquid, showed that the screen should be rather coarse but should have a relatively small free area (or be closely woven). Several designs of liquid overflow arrangements indicated that the most satis
factory was also the simplest— a plain tube, notched at the bot
tom and resting on the screen below. N o difficulty with liquid running through the screen at the bottom of the overflow pipe was encountered.
The construction of an all-metal test column is illustrated by Figures 1 and 2. The overflow pipes were merely press-fitted into the screens, projecting * /« inch above the upper screen sur
faces in order to maintain a shallow but definite liquid seal. The plates are punched from the screen, using plunger-and-ring dies made for the purpose. The thirty-six-plate test section was assembled from the single-plate elements cut from l ‘ /*-inch standard pipe and machined as shown in Figure 1. These provide a plate spacing of 1 inch. Each section constitutes both column wall and plate spacer, and leakage to the outside was prevented by sealing the joints with a molasses-graphite mixture.
The test setup is shown in Figure 3. The still was electrically heated, with other circuits for separate compensating heaters placed in the insulation of still and column. Heat loss determina
tions (as wattage) were made as blank runs over the temperature range existing in the tests. The losses were subtracted from the total heat 'inputs for the rate calculations. At any given pot temperature, heat loss deviation from run to run averaged 0.35 watt. The power going to vaporization ranged from 24 to 250 watts.
Te s t Pr o c e d u r e. The still charge was pure n-heptane and purified methylcyclohexane. The column was flooded, then operated under total reflux at a steady boiling rate for 4 hours
before the first set of samples was taken. Succeeding samples (at different vapor velocities) were withdrawn one hour after attainment of steady operating conditions for each new input setting. This procedure was found to give reproducible values with no change in sample compositions resulting from longer runs.
Refractive indices of top and bottom samples were taken at 20° C. with a Bausch & Lomb dipping refractometer. Mole percentages of n-heptane were read from a plot of the data given by Ward (S), and the number of theoretical plates was calculated by his procedure, except that the more accurate value of 1.083 was used as the relative volatility of n-heptane-methylcyclo- hexano (5). The number of theoretical plates was usually re
producible to one plate for a given screen at a fixed net heat in
put. The pressure drop was measured by the manometer shown,
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 1121
Ta b l e I. Wi r e Sc r e e n Sp e c i f i c a t i o n s
Figure 4.
LIQUID RATE AT TOTAL REFLUX, M L ./M IN . IN 1.61 " 1.0. COLUMN
Over-all Plate Efficiencies for Usable Range of Rates
and the liquid holdup for each set of screens was determined by adding a small amount of nonvolatile mineral oil to the still and measuring its concentration before, during, and after test runs.
The procedure was to weigh a 10-cc. sample from the still, evapo
rate the volatile hydrocarbons on a steam plate, and weigh again.
Determinations before and after the run gave weights of oil within 0.1% of the value calculated from the amount added in each case. The holdup of all screens ranged from 3 to 6 ml. per plate, increasing with boiling rate. This compares favorably with Oldershaw’s values when the column diameters are con
sidered. It has been shown that the effect of holdup on sharp
ness of separation is not great unless the amount of a key com
ponent held up in the column is a large fraction of the amount of the same component in the still liquid (3).
Ch a r a c t e r i s t i c s o f Sc r e e n s. Complete efficiency-pressure drop-vapor rate curves were run on the six screens described in Table I, which had been selected on the basis of observations in the glass column noted earlier. Fourteen-mesh screen was the coarsest obtainable having a low-percentage free area. Since these six include all available coarse screens having low percent
ages of free area, the optimum screen must be among them.
The over-all plate efficiencies over the entire usable range of rates are shown in Figure 4. For five of the screens, a definite peak in efficiency occurred at some intermediate rate. Of the six screens tested, the size of opening appears to control the vapor capacity. At rates above the peaks, the efficiencies of screens 2 and 4 fell off much more rapidly than did the others.
Pressure drops ranging from 0.15 to 0.4 inch of water per plate were recorded. Highest pressure drops oc
curred on screens No. 2 and 4 at rates immediately below the flood points.
N o breaks were found in the pressure drop-rate curves to indicate load points such as exist in packed columns.
Definite surging or intermittent vapor flow was observed for each screen at low rates, but in no case did surging affect the efficiency appreciably. The surging mechanism is explainable on the basis of surface tension.
Study of the foregoing observations leads to postulations concerning the mechanism of the bubbling action, which were in part confirmed by visual observation in the preliminary glass apparatus. Since the efficiencies were high for such a small liquid seal, much if not most of the vapor-liquid interaction must occur in foam abovo
No. of Individual Free
8creen Mesh Wire Opening Area, Area,
No. per In. Diam ., In. Opening, In. Sq. In. X 10« % separation and the observed plate efficiency. Pressure drop would be expected to increase with foam height. Confirming these conjectures, screens No. 2 and 4 showed highest pressure
Figure 5. Comparison of Plate Efficiencies
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A1122 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 36, No. 12 doubled the efficiency of the 2-inch plates.
A 150-plate column was then made up in six sections of twenty- five plates each, using the design of Figure 6. It was planned to use ‘ /»-inch steel tubing for the overflow pipes; but since tubing was not obtainable, the pipes were made by boring out standard
’ /«-inch pipe with a ’ /«-in ch drill. The supply of No. 3 screen on hand was inadequate to make 150 plates, and no more was ob
tainable. The final column consisted of two sections (fifty plates) of No. 3 screen and four sections (one hundred plates) of No. 6 screen. The sections made with the No. 3 screen were in
stalled at the top and bottom of the column.
Preliminary tests on the 150-plate column usually gave a nearly pure product at one end of the column, which is not satis
factory from the analytical standpoint for calculating plate efficiencies. The procedure finally adopted was to superheat the top two sections and. introduce the reflux at the one hun
dredth plate from the bottom, thus testing 100 plates. Plate efficiencies of the 2-inch column were lower than those obtained with the 1.5-inch column, but the trend of plate efficiency with rate observed on the 2-inch column (Figure 9) corresponds in shape to a composite of the curves for screens 3 and 6, as evident from a study of Figure 4. The plate spacing of 1.25 inches on the larger column permitted a higher vapor velocity than was obtain
able with the 1-inch spacing in column 1. (This 2-inch column is hereafter designated “ column 2” .)
After about six months of intermittent use in a pilot plant using solvents, the maximum capacity of the column decreased as a result of accumulation of sediment at the bottom of the overflow pipes. An occasional stoppage necessitated reconstruction of an entire 50-plate section, since each individual tray was fuse-welded and an integral part of the section. T o avoid extensive loss of time from this source, a third design was evolved.
Further observations in a glass Bection with glass downflow pipes
drops, greatest falling off of efficiency as vapor rate increased, and relatively low flood points. The number of bubbles did not correspond to the number of screen openings; if so the finest screen would give the highest efficiency, and the 14-mesh screens would all give the same efficiency.
It is certain that at low rates not all of the mesh openings are active.
Screen No. 3 is regarded as the optimum for general use. Figure 5 is an efficiency comparison between it and other tray types of laboratory columns. The screen plates have somewhat lower plate effi
ciencies than the glass perforated plate and bubble cap columns. (This 1.5-inch diameter column with screen 3 is here
after designated "column 1” .) T W O -IN C H D IA M E T E R C O LU M N S A column having a greater throughput than was obtainable with the 1.5-inch diameter design was desired. Sections constructed of 2-inch standard pipe with No. 3 screen were made up and tested. These gave relatively >ow effi
ciencies of only about 25%. They also had larger downflow pipes, and it was surmised that the liquid was running directly across the screens without adequate vapor con
tact. T o avoid such a condition, the down
flow pipes were squeezed to an oval shape in a vise, and narrow baffles were in
stalled at the centers of the screens. The
Ta b l e III. Pe r f o r m a n c e Fi g u r e s f o r Sc r e e n- Tr a y Co l u m n s
Figure 7. Construction Details of Wire-Screen Plates and Assembly (No. S)
1123
UPPER FLANGE „
V s " X 4 3/8", 6 HOLES ON 3
\
CIRCLE ORILLED a TAPPED ^
USS 18 THREAD
‘/ 8 NIPPLE FOR INLET OR OUTLET
0 5 * SLANT UPWARD
FIRST INSULATION LAYER
3 THICKNESSES ASBESTOS PAPER
SECOND INSULATION LAYER
| 6 Jj“ SAME AS FIRST
HEATER WINDING
2 5 TURNS/F00T , No. 2 0 NICHROME WIRE 2 WINDINGS PER COLUMN UNIT SHOWN ( I FOR EACH 2 5 -P L A T E SECTION)
JU N C T IO N S IL V E R -S O L D E R E D
TO P LA T E
PORCELAIN IN S U LA TO R
A R E A -T Y P E THERMOCOUPLE
D ETAIL
2 " HEAVY ELECTRICAL CONDUIT _ _ - w . TUBING
NEEDLE VALVE INLET OR OUTLET X ' P IP E , « • SLANT
LOWER FLANGE HOLES DRILLED
(SEE UPPER FLANGE)
Figure 8. Column Shell Unit for Two 25-Plate Sections
showed that the plates must be perfectly level, and also that vapor actually bubbled up through the overflow pipe while the column was in normal operation. Earlier attempts to use ring-type spacers that snugly fitted the inner wall of the column gave very low efficiencies because of liquid flowing down between spacer and column wall.
Spacers that cleared the wall by */» inch indi
cated satisfactory performance, and flow of vapor through the overflow pipes was avoided by the use of a small bottom plate that partially sealed each pipe. The latter feature was found to increase the flood point of the column greatly.
A third modification was to mount the downflow pipes flush with the surface of the screen. This reduced liquid holdup considerably while reduc
ing plate efficiency only slightly at low rates and not at all at high rates in comparison with column 2.
Design details are shown in Figure 7. Center holes are drilled in the plates, and twenty-five plates are mounted on a central rod or stringer.
Two of these assemblies are mounted in each of three flanged column sections of standard 2-inch diameter electrical conduit tubing, making a column of 150 plates (column 3). In this column a section can be removed, cleaned, and put back into operation within 2 hours.
Details of thermal insulation, electrical wind
ing, and double thermocouple arrangement for maintaining adiabatic operation as used in column 3 are shown in Figure 8. Differential thermocouples mounted similarly to these are in successful use in a number of laboratories.
However, these couples were separately con
nected to the potentiometer and used to de
termine temperature level as well as to detect temperature gradients through the insulation.
Test data for the three columns are sum
marized in Table II. Performance figures are given in Table III. Plate efficiencies plotted against liquid rate (using n-heptane-methylcy- clohexane at total reflux) are given in Figure 9 for all three columns.
A CK N O W LE D G M E N T
J. W. Morris held a University Research Assistantship during 1943-44. 0 . F. Van Berg held a University Advanced Fellowship during 1942-43. Assisting in the construction and opera
tion of the columns were H. H. Hurmence (Gulf Fellow, 1943-44), B. R. Randall, and others.
L IT E R A T U R E C ITED (1) Bragg, L. B., Trans. A m . Inst. Chem. Engrs., 37, 19 (1941).
(2) Bruun, J. H ., In d. En g. Ch e m., An a l. Ed., 8, 224 (1936).
(3) Colburn and Stearns, Trans. A m . Inst. Chem. Engrs., 37, 291 (1941).
(4) Fenske, Lawroski, andTongberg, In d. En g. Ch e m., 30,297 (1938).
(5) Griswold, John, Ibid., 35, 247 (1943).
(6) Oldershaw, C. F „ In d. En g., Ch e m., An a l.Ed., 13, 265 (1941).
(7) Palkin, S., Ibid., 3, 377 (1931).
(8) Ward, C. C., Bur. Mines, Tech. Paper 600, 9, 14 (1939).
Figure 9. Efficiency v s . Liquid Rate for n-Heptane- methylcyclohexane at Total Reflux
Column I.D ., Plate Spacing, Screen No. Plates in
N o, In. In. N o. T est Section
1 1 .6 1 1 . 0 3 36
2 2 .0 7 1 .2 5 3 <fc 6 100
3 2 .0 7 1 .2 5 6 160