Factors Controlling Efficiency and Capacity of Sieve-Pla te Extraction Towers CYRUS PYLE, A.
P. COLEURN',
AND
H. R . D U F F E Y ~
E N Q l N E E R l N G RESEARCH LABORATORY-EN5lNEERING DEPARTMENT, E. I . D U P O N T DE N E M O U R S
e
COMPANY, INC., WILMINOTON. DEL.
T h e factors influencing the efficiency and capacity of sieve-plate extraction towers have been stud ed in a small glass extraction tower, using the system ethyl etheracetic acid-water. It has been found t h a t a t normal operating rates there i s little effect of hole size, within limits, on efficiency whereas a higher percentage of free area i s more advantageous than a lower percentage. Higher capacities are obtained when settling time of the two phases is rapid, and when down pipes are flush with the surface of the plate from which they drop.
I N T H E manufacture of cellulose acetate. one of the most
I important steps in $he process is the recokery of acetic acid
from aqueous mixtures containing about 25% acid by weight. Most manufacturers carry out the initial phase of the recovery by the uroces of liauid-liauid extraction. When large volumes of liquid must be iandled continuously and when k g h recovery efficiencies are desired, it is believed that plate-type towers are most satisfactory, and sieve-plate towers in particular seem to have marked advantages ( 4 ) . Although sieve extractors have been used for many years, designs for these extractors appear to have been evolved from experience and judgment, without benefit of detailed experimental investigation. Because of the increasing utility of extraction processes, a study of the factors controlling the efficiency and capacity of sieve-plate extracEXIT ETHER tors was inaugurated. This study was undertsken several COPPERyears ago on a plant actually PLATE recovering acetic acid by extraction. The system ethyl ether-acetic acid-water was T I E RODchosen for test purposes because all feed streams were conveniently available.
feed entered the column through the center supporting post and was fed to the top plate. The down pipes (or riser pipes) rose 1 inch above the plate to which they were attached, and extended to within 1 inch of the plate below. The spaces between plates and column wall were sealed by means of rubber tubing held in place by copper expansion rings. Rubber gaskets were used be-
~ $ & ~in$ the$ ~ top g ~ section ~ ~ ~ o fofl tthe::kt dcolumn d " , " to~ assist ~ r ~ ~ ~ $ f$ $ ~2 e: , in breaking emul-
sions of extract which were hard to separate. This separator was not employed in all r u m Connections were arranged for sampling the feed ether, feed acid saturated with ether, exit ether, and exit water. Thermometer wells were provided for determining the temperatures of feed ether, feed acid, exit ether. and exit water. Five setsof interchangeable plates were used, three sets having a constant hole size (0.110 inch) and varyingpercentage of hole area (1.77 to 5.05%) and two other sets having a varying hole size (0.0635 to 0.2010 inch) with a constant hole area (3.34%). The details of the plate construction are given in Table I. The drilled area of the plate amounted to 75.9% of RINOS the column area. For the runs in which the plate spacing was 2.5 inches, the down pipes were shortened by 2 inches, so that the space between the top of the plate below and the bottom of the down pipe was approximately the same as for the 4.62inch plate spacing ( 1 inch). PLATES Equilibrium Data. This study was undertaken primarily to find out how to design plant columns. The feed mixture is not one of two pure components, acid and water, but does contain other materials which probably influence the distribution F E E D E T H E R __f \ ' E X I T WATER coefficient. For this reason F E E D ACID distribution coefficients were measured on the liquids Figure 1. Semiworks Glass Extraction Column
-(I
EXPERIMENTAL STUDY
Apparatus. Details of construction of the experimental extraction column are shown in Figure 1. The ether entered the column through twelve 0.25inch holes in the bottom of a distributing ring and aased through about 5 incles of water phase before reachin the bottom plate. The acii 1 Present address, University of Delaware, Newark, Del. 8 Preaent address, Chemicals Department, The Quaker Oats Company, Chicago, 111.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Table 1.
Plate Speclfloations
--
(4 plates per set and 20 plates total)
No No' No: No No:
1 2 3 4 5
0 110-inoh holes on i/,-inch centers 0'110-inch holes on Wis-inch 0:llO-inch holes on 'a/:r-inch 0 201-inch holes on '/:*-inch 0.'0635-inch holes on 9/ta-inch
a e l
canters = centers centers centers =
-I
a 2 c W U
.s 0
actually employed. The standard method of expressing acetic acid concentrations at the plant was as grams of acetic acid per 100 ml. of solution. This standard was adhered to for this study and the distribution coefficients expressed as grams of acetic acid per 100 ml. of water phase divided by grams of acetic acid per 100 ml. of ether phase at 20" C. are shown in Table 11. Although these coefficients are lower than reported in the literature for pure components, they n-ere used as a basis for the equilibrium data, because they were obtained under conditions presumably similar to those encountered a t the plant. Had the more recently obtained equilibrium data of Major and Swenson (6)been employed, the calculated plate efficiencieswould have been slightly higher than those reported herein.
Table
II.
Equilibrium Data a t 20" C. Distribution Coefficient
G. Acetic Acid/lOO M1. Ether Layer 0.225 0.460 0.954 2.035 3.245 4.590
G. Acetic Acid/lbO
7.80 9.65 11.78 14-20 16.9 20.0
k2.0 14.0 16.0 18.0 20.0 22.0
6.10
MI. Water Layer 0.5 1.o 2.0
4.0 6.0 8.0
10.0
G . ACid/1m MI*Ether LiYer G. Aoid/100 Ml. Water Layer
0
*
2
* u
ui al
8 12 eo OMS. ACETIC ACID/ IOOCC. W l T t R LAYER
4
0
Figure 2.
Typioal Extractiorr Diagram Plate ef?lciency, Run 24A
EFFICIENCY TESTS
0.477
0.737 0,788 0.846
0.911
Method of Calculation. The two most promising methods for calculation of plate efficiencies in extraction appear to be: use of the triangular diagram as described by Hunter and Nash ( 6 ) and the absorption process method described by Sherwood (7). Careful study of both methods led to the choice of a combination of the two as suitable for the purpose. The method actually used in the calculation of the results of the experimental runs was as follows: The absorption method was used with the concentrations plotted in grama of acetic acid per 100 ml. of ether layer us. grams of acetic acid per 100 ml. of water layer, as shown in Figure 2. Curvature of the operating line was determined by converting terminal compositions to weight percentage, with use of the triangular phase diagram] constructed in the same units, for computation of various points on the operating line. Material balances based on uantitiea of acetic acid in and out of the system were computed ?or most of the runs. In nearly all cases the inlet quantity checked the amount leaving within 10%. Where material balances were obviously in error, the runs were not emplo ed for construction of the graphs. $he plate efficiency was determined by the Baker-Stockhardt ( I ) procedure of drawing on the equilibrium diagram a series of curves, each representing a definite percentage of the driving force and by stepping off four plates between the operating line and the several "percentage" curves until the experimentally observed separation was obtained by four ste s (when four plates were employed in the test). In each case afTowance was made for extraction taking place between the distributor ring and the bottom plate. The proper correction had previously been measured as a unction of ether flow rate, and was found to be 60% of a theoretical plate at an ether rate of 550 gallons/(hour)(sq. foot) and 43% of a theoretical plate a t the lower ether rates. A graph illustrating run 24A is shown as Figure 2.
EOB, 0.28
The results of this study are shown graphically in Figures 3 to 8 and the experimental data are included 88 Table 111. The results may logically be divided into plate efficiency measurements and capacity measurements.
0.450 0.459
0.509 0.541 0.574 0.610 0.649 0.689
z4
Reproducibility. The first three runs were carried out in duplicate in order to determine how nearly reproducible the results were. I t was observed that the difference between the efficiencies at the two lower rates was less than 1%,whereas the difference between the efficiencies a t the high rates waa 2%. I t is believed that,all efficiencies could be reproduced with a difference of about 2% in the check run in each case. Effect of Per Cent Free Area. These testa were made on plates having 0.110-inch holes, I-inch overflow pipes, and 4.62inch plate spacing. The results are shown in Figure 3. The efficiencies are all near 30%, but there is a tendency at high rates for the plate with the high percentage of free area to give higher efficiencies. It is believed that a minimum depth of 0.25 inch of
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sac 400 wo CTMCR RATC, O A L . / l M R - ) ~ S o . IT.)
mo
wo
Effeot of Per Cent Free Area
G8llcns of ether par p8llon of feed acid, 8pproxirnably 2.4 PI8te spacln 4 62 Inches Hole size, 0.RO inch
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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ether below the plate is desirable to ensure good distribution of the ether and that the maximum depth of ether below the plate should be 0.75 to 1 inch in order to avoid too great a reduction in the depth of water layer through which the ether passes. Effect of Hole Size. Using plates with 3.34% free area, there was little effect of hole size on plate efficiency between the limits 0.0635 and 0.201 inch, although the smaller holes (which gave a greater depth of ether below the plate) show a higher efficiency (Figure 4). It is possible that with the 0.201-inch holes, the drops were not as large as 0.2 inch in diameter, inasmuch as the effect of water and ether flow may be such as to reduce the sise of the drops.
