U p p i n g Column Extruction Efficiency by
Pulse Application
...
- Sieve Columns
W. M. GOLDBERGER and R. F. BENENATI Polytechnic Institute of Brooklyn, Brooklyn, N. Y.
Mechanical agitating of extractor column liquids speeds up the slow separation process L i q u m - l i q u i d extraction is a relatively slow separation process, particularly in continuous countercurrent equipment where the sole driving force for flow of the liquids is provided by their density difference and is not sufficient to induce a highly turbulent action within the extractor, regardless of the internal column design. Because extraction rate increases with the degree of turbulence, methods have been developed for mechanically agitating extractor column liquids. Recently, the pulse column, a unit which develops high turbulence without using moving parts within the column proper, has become of interest. Pulse column studies have been made with the packed column (4, 6, 13) and a type of perforated plate column designed specifically for pulsed operation (3-5, 8, 70, 77). High extraction efficiencies and mass transfer rates reported in those studies establish pulse application as a means for improving extractor performance. I t was therefore expected that pulsation of the conventional sieve column extractor could improve its efficiency, and in the work reported here mass transfer and flow characteristics of this type of unit during pulsed operation were studied.
a phenolphthalein indicator. For the toluene phase titrations, 25-ml. aliquots of sample were added to approximately 200 ml. of deionized water, and this mixture was titrated under vigorous agitation. Blanks showed the tap water used to be slightly acid. This acid content was equivalent to about 4 X 10-6 1b.-mole of benzoic acid per cubic foot. Procedure. Before a run was started, the toluene-benzoic acid feed storage tank was charged with a solution having a n acid content of about 10.0 X lb-.mole of benzoic acid per cubic foot. The extractor, after washing, was filled with fresh tap water from the water feed tank and a water rate was established. T h e toluene-acid feed was then admitted a t the desired rate. T h e upper column interface was maintained 3 inches below the top header, by adjusting the flow resistance in the exit water line. Thirty minutes were allowed for the unit to attain equilibrium after a steady flow, condition was established; samples were then withdrawn from each stream a t three 20-minute intervals. This procedure was used for the pulsed and nonpulsed operation.
Mass Transfer Calculations. Two indices of extractor performance have been used to present the experimental data. A convenient measure of relative stage efficiency for a fixed water rate has been the per cent approach to equilibrium attained by the water phase. “Per cent of equilibrium” is defined as the percentage change in the water phase acid composition based on the change necessary if the water phase concentration were to reach equilibrium with the incoming toluene feed concentration, so that
The height of a transfer unit (HTTJ) has also been used to develop the extraction data, so that the experimental unit could be compared with units used in previous studies of similar nature. The procedure used to calculate HTU values has been simplified by use of the log mean concentration driving force, but is consistent with the determinations of other workers (7, 2, 6, 9, 72). When the solvents are assumed immiscible and no diffusional resistance is offered at the inter-
Roffinote
Woter In
Ex per imenta I Equipment. A simulated stage of a perforated plate extraction column was studied during the unpulsed as well as the pulsed operation. T h e unit consisted of two 12-inch sections of glass tubing having an inside diameter of 2w/32 inches, which were separated by a perforated plate. The perforated plate included a circular downcomer for continuous passage of the heavy liquid phase. Cyclic pulsations were developed by a reciprocating plunger operating within a cylinder, and controlled by a variablespeed motor with several gear reductions. Rotameters were used to measure the feed rate from constant-head storage tanks. The toluene-benzoic acid-water system was used throughout the extraction runs. For mass transfer calculations, data previously reported were used (9). Measurements. The acid concentration of each phase was determined by titrating feed and exit stream samples with standard sodium hydroxide, using
COLUMN ASSEMBLY
thread for tie rod flange bolt hole
The experimental column assembly consisted of two sections of glass tubing, separated b y a perforated plate
E x t r a c t Out
Toluene - A c i d I n locotad a s a h o u n On
in.
S I E V E PLATE
Downcorner 0.497“l
VOL. 5 1 , NO. 5
M A Y 1959
641
7 0 / /
6
e / /
/
O
I
5
e
4
3
Y
(12)
I
2
A
.3
-5
.6
.7
,
3 4
5
(12) (I) (91 (I21 thi; study
0.1875
9.0
0.1875 0.1875 0.1250 0.1875
6.0
3
4.75
/
6 .O
I /
3.0 9.0
0.046
/e
V l =l2.3
6
.8 .9 1.0
2.5
2.0
1.5
2
5
Figure 1. Comparison of the unpulsed column with that of other investigations lower HTU values are probably due t o the smaller hole size used
-
Caw) dA
7
8
9
TOLUENE R A T E ,
Figure 2. coefficient
1 Vb
15
0 FT31.J/(HR.)(FTz2]
Effect of flow rates on mass transfer
Within the operable limits of water rate, only slight changer in mass transfer properties are noted
face, the basic mass transfer rate equation can be developed for the water phase across a differential element of column height, Vc dCaw = Kw(Caw*
6
If Kw is assumed constant within concentration range
(2)
then integrating over the limit of concentration change
The integral on the right of Equation 7 has been termed the number of transfer units. Use of the log mean driving force for its evaluation leads to
but dA = adV
Therefore,
Typical Experimental Data
for Sieve Plate Extractor during Pulsed Operation V/@
Tbi. T
Series
Run
I1
1A
Freq. Displ. 69.5 0
2A 3A 4A 5A
69.0 67.0 68.0 67.0
0 0
1A
68.0 75.0 80.0 58.0 62.0
180 180 180 180 180
64.0 71.0 63.0 69.0 62.0 67.0
[unpulsed]
IIP
4A 4A
5A 7A
I11 P
1A 2A 3A 4A 5A 6A
IVP
1A 2A 3A 4A
VP
1A 2A 3A 4A
a
H~O % Phase Equil.
Kwa
(HTU)ow
0.536
0.451
12.80
2.29
5.37
10.163
9.939
0.240
0.951
0.04
10.475 9.973 10.579 10.595
10.264 10.734 10.359 10.372
0.265 0.554 0.474 0.372
1.143 2.07 2.16 1.491
0.04 0.04 0.04 0.04
0.494 0.494 0.494 0.494
0.558 1.270 1.063 0.829
0.536 1.002 1.045 0.718
15.30 29.5 29.4 19.95
2.68 5.87 5.74 3.76
4.58 2.09 2.14 3.27
0.70 0.70 0.70 0.70 0.70
10.449 10.334 10.327 10.241 10.241
10.195 9.870 10.035 9.949 9.897
0.496 0.137 0.419 0.339 0.243
2.74 1.262 2.55 1.945 1.805
0.03 0.04 0.04 0.04 0.04
0.494 0.494 0.494 0.494 0.494
1.160 0.637 1.222 0.989 0.836
1.345 0.604 1.240 0.942 0.872
38.1 16.3 32.2 29.1 26.1
7.90 2.94 6.43 5.69 5.00
4.18 1.910 2.16 2.46
0.70 0.70 0.70 0.70 0.70 0.70
9.962 9.962 9.757 9.620 9.191 9.249
9.173 9.267 9.283 8.913 8.562 8.763
0.138 0.230 0.446 0.255 0.330 0.540
2.23 3.85 4.33 3.74 4.14 4.89
1.089 1.600 2.11 1.800 2.07 2.62
0.194 0.364 0.430 0.542
1.39 2.53 2.91 3.42
3.81 7.38 8.26 11.27
3.33 1.600 1.350 1.079
500 500 500 500
0.70 0.70 0.70 0.70
0.512 0.475 0.450 0.490 0.475 0.480 0.469 0.490
0.776 1.230 1.425 1.790
65.0 64.0 63.0 65.0
9.430 9.492 9.572 9.481 8.966 9.048 9.086 9.097
32.4 53.5 64.5 53.6 63.9 72.3 19.95 37.0 42.2 49.4
1.827 0.953 0.712 0.944 0.710 0.576
9.830 9.830 9.904 9.811 9.399 9.399 9.458 9.458
1.136 1.680 2.12 1.880 1.930 2.44 0.690 1.182 1.290 1.655
6.94 11.45 17.22 13.25 16.41 21.6
0.70 0.70 0.70 0.70
0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04
0.510 0.440 0.495 0.508 0.470 0.502
65.0 64.0 65.0 66.0
600 600 600 600 600 600 400 400 400 400
0.699 1.065 1.495 1.955
0.636 1.035 1.482 1.900
20.2 32.8 47.8 57.8
3.62 6.59 10.35 14.32
3.26 1.810 1.122 0.850
(Lb. moles/cu. f t . ) l O 3 .
642
Phase
VCb 0.494
0 0
... ... ... ... ...
Cu. ft./hr.
Catha
calo"o~
vdb
CWWC
C W i D C
0.1615 1.380 0.302 2.20 0.401 3.20 0.542 3.92
(Lb. moles/cu. f t . ) l O 4 .
INDUSTRIAL AND ENGINEERING CHEMISTRY
Lb. moles/hr.
1.555
SIEVE COLUMN EXTRACTOR
-+
501
70
I
I
I
!
WATER RATE, VAs12.3
/
+
I
I
-
*
8’
a,’
40
,a
0 U N P U L S E D COLUMN
/-
0
/
-3’
30 6
I
I
33 P U L S E S P E R M I N . 0.68 G.C. P E R P U L S E
I
I
I
I
-
I
8 9 IO I I 12 13 14 15 3 2 TOLUENE RATE , V b FT/(HRS(FT.) 7
Figure 3. Extractor performance during unpulsed and Type I pulsed operation shows no operational difference
b Figure 4. Comparison of extraction performance a t various frequencies of pulsation shows marked improvement over that of the unpulsed column The 600 pulse per minute line indicates the high efficiency possible with Type 111 dispersion
and (HTU),, = H/(NTUjow =
vc’
I;, X a
(9)
Results and Discussion The first extraction runs were made to establish the unpulsed column performance (Figure 1). Use of a smaller hole size has probably resulted in the correspondingly lower H T U values obtained for this work as compared with prior studies. Within the operable limits, the water rate had little effect on the mass transfer coefficient (Figurc 2), and was then maintained constant during the following pulsed extraction runs. T h e first pulsed runs, made with a pulse displacement of 0.675 ml. and a frequency of 33 pulses per minute, showed no significant change in column performance (Figure 3). Random data were then taken a t higher frequencies, but not before a frequency of 150 pulses per minute was used was a performance change apparent. At a water rate of 12.30 cubic feet per hour per square foot, using frequencies of 180, 400, 500, and 600 pulses per minute with a pulse displacement of 0.70 ml., a marked improvement in the stage performance was noted, particularly at 600 pulses per minute (Figure 4). Extraction performance was unchanged below 150 pulses per minute, because no change occurred in the nature of the phase dispersion. Above 150 pulses per minute, however, the flow characteristics changed and column operation was similar to that described by Sege and W‘oodfield (70). Again, three distinct and stable types of phase dispersion behavior occurred which flow rates
and the degree of pulsation established (Figure 5). Type I. Flow is entirely similar to that of the unpulsed column. with the continuous phase traveling across the plate and down the downcomer. The dispersed phase coalesces to form a layer beneath the plate and issues from the perforations almost unaffected by the pulsation. The resistance to flow is increased by the pulsation, as evidenced by an increased thickness of dispersed phase layer beneath the plate compared with thickness during the unpulsed operation ( 7 ) . Type 11. The flow of the continuous phase is affected by the pulsation, so that part of the flow continues through the downcomer but part is pulled through the perforations on each downstroke. Neither phase can strictly be termed continuous: as each is dispersed within the other on alternate cycles of the pulse. The lighter liquid still coalesces beneath
the plate, and the thickness of this layer depends on the degree of pulsation as well as the flow rates of the phases. This type of operation is similar to the ‘.mixer-settler” pattern described in pulse perforated column studies (70. 13). Drop size becomes irregular as large drops form a t the start of a pulse, and smaller ones form by breaking up of the jet as the stroke reverses. Type 111. When the degree of pulsation is such that the product of the pulse displacement and frequency far exceeds the light liquid feed rate. coniplete coalescence of that liquid into a layer beneath the plate no longer occurs. The light liquid drop size is irregular and much smaller than that observed in Type I or Type I1 dispersion, because the phases no longer alternate in their passage through the perforations; both liquids are present during flow in either direction. Because of the extremely small drops formed and
TYPE I I I
UPSTROKE
Figure 5.
I
TYPE I l l
DOWNSTROKE
Three stable types of phase dispersion may occur VOL. 51, NO. 5
M A Y 1959
643
16
I
WATER
-
I
RATE,
Nomenclature
1 1 -
= acid concentration in fresh
VL= 12.3 F?I(HR)(F3/
=
=
-
=
TYPE I
=
F i g u r e 6. The three regions of phase dispersion are established b y the flow rates and degree of pulsation
II
TYPE
=
= =
solvent water, 1b.-moles/ cu. ft. acid concentration in exit water, 1b.-moles/cu. ft. acid concentration in toluene feed, 1b.-moles/cu. ft. acid concentration in exit toluene, 1b.-moles/cu. ft. acid concentration in water phase in equilibrium with exit toluene acid concentration, Ib.-moles/cu. ft. acid concentration in water phase in equilibrium with toluene feed acid concentration, 1b.-moles,'cu. ft . change in water phase acid concentration. 1b.-moles ' cu. ft. caw,,, - c,,,, log mean concentration difference, 1b.-moles/cu. ft. (Caw,* - Call',,) (Caw,*
100
200
300
400
FREQUENCY, the high degree of turbulence above and below the plate, the light liquid holdup is increased and local flooding occurs if the pulsation level becomes excessive. T h e column behavior is similar to the "emulsion" type operation of the pulse perforated plate unit. The data shown in Figure 6 were taken to define the zones of operation at the pulse displacement and water rate used throughout the pulsed extraction runs. The plate smearing tendency of water, particularly noticeable at the transition point from Type I to Type I1 dispersion, made it difficult to detect the exact transition frequency, but these data indicate the condition of the stage for any of the extraction runs. T h e only series of runs made completely within the Type I11 region were those of 600 pulses per minute (7). Marked increases were found in the extraction rates for this series, which can be attributed to the finer drop size and the highly turbulent liquid action which occurs during this type of dispersion. Although considerable similarity exists in the pulsed operation of this unit and the performance of the pulse perforated plate column, there is a major difference. Because of the absence of a downcomer in the latter design, pulsation is necessary to obtain counterflow of the phases. Therefore, flooding of this unit can be induced by inadequate pulsation. T h e more versatile conventional design can be operated with or without pulsation. The flooding noted in Type I11 disper-
644
500 PULSES
/
600 MIN.
sion is important because it establishes the maximum limit to which the level of pulsation may be increased. Flooding may be induced at any given set of flow conditions within the normal unpulsed operating range by applying sufficiently high pulsation. I n effect, the operating range of the pulsed column is somewhat lower than that of the unpulsed unit. However, the stability of operation below the flooding point is unaffected. I n addition, system pressure losses are increased in pulsed operation by increased frictional resistance of both phases in passage through the column. Conclusions
Mass transfer rates may be significantly improved by applying pulsation to the conventional sieve column extractor, if the degree of pulsation (pulse displacement times frequency) is sufficient to alter the mechanism of the phase dispersion. Three stable types of phase dispersion may occur. The type established is dependent upon flow rates and degree of pulsation. Mass transfer rates increase with increasing degree of pulsation and become extremely high when the pulsation level is sufficient to prevent the light liquid from coalescing into layers beneath the plates of the column. T h e stability of column operation is unaffected by pulse application, although local flooding may occur at lower flow rates. Pulsation, however, causes added flow resistance to each phase, which results in greater pressure losses.
INDUSTRIAL AND ENGINEERING CHEMISTRY
-
C.%",,+) ...
In[(Caw,*- Cawin)/ (Caw,* - ~ ~ W , , J l = acid transfer rate, 1b.-moles/ hr. = over-all mass transfer coefficient based on water phase, 1b.-moles/(hr.) (sq. ft.) (AC,,) = water rate, cu.ft./hr. = water rate, cu.ft./(hr.) (sq. ft.) = effective column height, feet (NTU)., = number of transf& units based on lvater phase calculations (HTU),, = height of transfer unit based on water phase calculations literature Cited (1) Allerton, J., Strom, B. O., Treybal, R. E., Trans. Am. Inst. Chem. Engrs. 39. 361 (1943). (2) Appe1,'F. J.', Elgin, J. D., IND.ENG. CHEM. 29, 451 (1937). (3) Beyer, G. H., Edwards, R. B., Nuclear Engineering & Science Congress, Cleveland, Ohio, Dec. 21, 1955. (4) Chantry, W. A., Von Berg, R. L., Wiegandt, H. F., IND. ENG. CHEM. 47, 1153 (1955). ( 5 ) Cohen, R. M., Beyer, G. H., Chem. Eng. Progr. 49, 279 (1953). (6) Feick, G., Anderson, H. M., IKD. EXG.CHEW44, 404 (1952). ( 7 ) Goldberger, W. M., master's thesis
in chemical engineering, Polytechnic Institute of Brooklyn, June 1956. (8) Li, W. H., Ph.D. thesis in chemical engineering, Georgia Institute of Technofogy, 19-52. (9) Row, S. B., Koffolt, J. H., Withrow, J. R., Trans. Am. Inst. Chem. Engrs. 37, 559 (1941). (10) Sege, G., Woodfield, F. W., Chem. Eng. Progr. 5 0 , 396 (1954). (11) Stevenson, R., Ibid., 49, 340 (1953). (12) Treybal, R. E., Domoulin, F. E., IND.ENG.CHEM. 34, 709 (1942). (13) Wiegandt, H. F., Von Berg, R. L., Chem. Eng. 61, 183-8 (July 1954).
RECEIVED for review June 16, 1958 ACCEPTED February 2, 1959
