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Flooding rates in packed liquid extraction towers
Chemical En$neeringScience, 1058,Vol. 9, pp. 170to 175. PergsmonPressLtd.
Flooding
rates
in packed
liquid
extraction
M. RAJA RAO and C. VENKATA Department
of Chemical Technology,
Andhra University,
towers
RAO Waltair, Southern India
(Received 1 November 1957) Abstract-As incidental to our main studies on mass transfer, flooding rate data have been obtained in a packed liquid-liquid extraction tower with no solute transfer, with 1 in. Raschig rings, f in. copper rings and 6 mm glass beads as the different packing materials, using water as the continuous phase and each of the following solvents, toluene, Pegasol, kerosene and methyl tiobutyl ketone, which do not preferentially wet the packing, respectively as the dispersed phase. Visual observations, aided by photographic studies supplemented the measurements of the flooding velocities. The Aow rates at the flooding point have been satisfactorily fitted on the square root plot as a preliminary correlation, and have been well correlated by the method of DELL and PRATT. It was observed that flooding rates decreased with decreasing size of the packing for any given system, and that higher flooding rates are favoured by high density difference and low interfacial tension of the liquid systems. R&sum&--A l’occasion de leurs principales btudes sur le transfert massique, les auteurs ont obtenu des resultats experimentaux sur I’engorgement produit dans une colonne a garnissage pour l’extraction liquide-liquide, sans transfer% de solute. Les garnissages suivants ont ttC utilids: anneaux Raschig de 0.95 cm, anneaux de cuivre de 0.64 cm et billes de verre de 6 mm. La phase continue etait de l’eau, alors que la phase dispersee contenait un des solvants suivants : toluene, m&hyl&hylc&one, qui ne mouillent pas preferentiellement le materiau de Pegasol, kerosene, remplissage. Les mesures de vitesses d’engorgement ont &tC realisees de man&e visuelle ainsi que par des etudes photographiques. Une correlation prt%iminaire a permis aux auteurs de reproduire d’une maniere satisfaisante les debits ir l’engorgement par la methode des racines carrees. L’utilisation de la methode de DELL et PRATT a permis de correler les resultats avec succes. Les auteurs ont observe que les vitesses d’engorgement diminuent avec les dimensions du garnissage pour tout systeme, et que les grandes vitesses d’engorgement sont favoristes par une grande difference de densite et une faible tension interfaciale des systemes liquides. Zusammenfassung-Im Zuge unserer Untersuchungen iiber den Stoffaustausch wurden Flutungsgeschwindigkeiten in einer Ftillkiirperslule zur Fliissig-Fhissig-Extraktion ohne Ubertragung von Gel&tern ermittelt. Als Packungsmaterial dienten Raschigringe von 9.5 mm, Kupferringe von 6.4 mm und Glasperlen von 6 mm. Die geschlossene Phase war Wasser und die disperse Phase waren folgende Lijsungsmittel, Toluol, Pegasol, Kerosen und Methyl-lsobutylKeton, welche das Packungsmaterial vorzugsweise nicht benetzten. Visuelle Beobachtungen, unterstiitzt durch photographische Aufnahmen, erganzten die Messungen der Flutungsgeschwindigkeiten. Der Mengenstrom am Flutungspunkt wurde befriedigend durch eine quadratische Beziehung in vorl5ufiger Form wiedergegeben und konnte gut nach der Methode von DELL und Pa~rr dargestellt werden. Es wurde festgestellt, d,ass die Flutungsgeschwindigkeiten abfielen mit fallender Packungsgrasse fur ein gegebenes System und dam hijhere Flutungsgeschwindigkeiten begiinstigt werden durch hohe Dichtedifferenz und niedrige Grenzfliichenspannung der fliissigen Systeme. INTRODUCTION MUCH of the early work on flooding rates in packed
liquid-liquid columns was done incidentally to other investigations, and in this category came the reports
of RUSHTON [l], APPEL and ELGIN [2], SHERWOOD et at. [3], and Row et al. [PI. BLANDING
and ELGIN [5] described their investigations of the design and operation of spray and packed columns, pointing out the importance of the proper design distributor. During 170
recent
of
the
years,
entrance
sections
flooding rates
have
and been
Flooding rates in packed liquid extraction towers studied in packed liquid extraction towers by several investigators [6-111, and of these special mention should be made of DELL and PUTT, [9] who have made a rather extensive study of flooding rates -in columns of 3 in. and 6 in. inside dia., using a wide variety of liquid systems and packings. The above authors correlated their data by means of a theoretical equation :-
The value of 12 was found to be - l/4. The constant C has slightly different values for each type of packing used, and the values recommended for design are 0.68, 0.80, and 0.88 for Raschig rings, Lessing rings and Berl saddles respectively. Each of the flooding correlations so far developed for packed liquid-liquid columns is applicable over a particular range of the different operating variables, and as pointed by TREYBAL [12], it is necessary to exercise due caution in utilizing any of the flooding correlations to cases where the packing sizes, system properties and also column Table 1.
sizes are outside the range for which they are applicable. APPARATUS The extraction equipment consisted essentially of an extraction tower and the necessary accessories to maintain a steady flow of the two liquid phases. The design of the packed column was based on the recommendations of BLANDINGand ELGIN [5], and the apparatus used has been fully described elsewhere [13,14]. To facilitate visual observation of the column in operation, the column proper was constructed of Pyrex glass tubing, 1.88 in. inside dia., and two column lengths of 60 and 34 in. respectively were used in the experimental work. A dispersed phase disdistributor with 21 x & in. nozzles was used in the flooding rate studies. MATERIALS Of the dispersed solvents, kerosene and methyl isobutyl ketone were supplied by the BurmahShell Oil Co., Pegasol by the Standard Vacuum Oil Co., and toluene by the Bengal Chemical and Pharmaceutical Works, Calcutta. The physical properties of these solvents are recorded in Table 1. Tap water of negligible acidity, drawn
(a) Properties
of packing materials
Average size of units Material
Pocking
(in.)
a in. Raschig rings
Porcelain
0.375
Copper Glass
0.250
4 in. Copper
%Y
6 mm Glass beads
I.D.
O.D.
i
(in.)
Kerosene Pegasol Toluene Methylisobutyl ketone
1
0.273
(in.)
No. of units
per fL3
0.384
28,680
0.175 0.250 6 mm dia. sphere
81,060 163,000
(b) Properties Solvent
Len&h
a
ft.2 G
ST Measured
Calculated
178-l
oao
04w7
2154 198.0
0635 0.881
0.692 0.896
of solvents at 3OT
Density at 30°C (g/ems)
Viscosity in c.p.
0.778 O-785 O-859 0.800
1.08 0.854 0.556 0.520
171
Interfacial tension (dyn/cm)
89 29 26 10
M.IZlwa Rho and C.VENKATA RAO from the Andhra University mains supply was used in the experimental work. Three types of packing materials were used : (1)Q in. non-porous and unglazed porcelain Raschig rings, manufactured and supplied by the Maurice A. Knight Co., of Akron, (2) 4 in. copper rings, and (3) 6 mm glass beads. The column was filled with water, and the packing material dropped in from the top, a few pieces at a time, slowly and at random to the desired height. The packing was not shaken or tamped. The properties of the packing materials are listed in Table 1. With each of the three different packing materials, the liquid systems studied were : 1. toluene-water, 2. Pegasol-water, 3. kerosenewater, and 4. methyl isobutyl ketone-water, using water as the continuous phase and the organic solvent as the dispersed phase.
COLUMN
BEHAVIOUR
A good number of photographs were taken of the column during operation with a view to interpreting the flooding data in terms of drop size, holdup and coalescence of the dispersed phase. In general, it was observed with each of the packings and liquid systems ,used that the streams of the dispersed phase droplets worked their way up the column, deforming and following a torturous path while passing up through the interstices of the packing. With increased flow rate of the dispersed phase its holdup was markedly increased, and the drops became very closely packed up in the column. As the flooding rate was closely approached the drops were‘ observed to coalesce, forming slugs which gradually tended to travel down the column. The flooding rate was taken at the fust appearance of a thin layer of the dispersed phase below the packing support. As compared with Q in. PROCEDURE Raschig rings, the column was rather densely Each of the liquid phases was first saturated packed with + in. rings, and more particularly with the other, as this is particularly necessary with 6 mm beads. With the latter packing the when the solvent and water have appreciable drops showed a greater tendency to coalescence, mutual solubility, as in the case of water and and for any liquid system the flooding rates were methyl isobutyl ketone in the present investigalower than those for ring packings. tion. The continuous water phase was first It was observed that smaller and finer drops admitted into the column and then set at the were formed with solvents like methyl isobutyl desired flow rate. The dispersed solvent phase ketone, and comparatively larger drops, showing was then slowly introduced, and its flow rate was increased tendency to coalesce, with kerosene and gradually increased in 5 to 10 steps until the Pegasol. No attempt was made to determine flooding point, as indicated by the first appearance either pressure drop or holdup at the flooding of a thin layer of the dispersed phase underneath point. the packing support, was reached. In a few runs, RESULTS AND CORRELATION when the dispersed phase rate was high, the Altogether over 50 runs have been taken on procedure was reversed by first establishing the flooding rates with the different liquid systems rate of the dispersed phase and then gradually increasing the continuous phase rate up to the and packings, and the data are presented in Table 2. Maximum flooding rates were obtained flooding point. The flooding results obtained by with 9 in. Raschig rings with the methyl the two methods agreed quite well. isobutyl ketone-water system, and the least In all the runs extra care was taken to maintain the liquid-liquid interface in the upper end with kerosene-water and Pegasol-water systems. The larger drops, and their increased tendency to section at the same level, and during each run, coalesce observed with the last two systems, as the flow rates were not only checked, but also the exact rates of the solvent and water phases compared with the relatively smaller drops of the were obtained by timing the exit streams for a ketone-water system, may perhaps be due to the fact that the interfacial tension (u) of the ketoneknown interval of time and measuring their water system is only 10 dyn/cm, which is much volumes. 172
Flooding rates in packed liquid extra&ion towers
.
Table 2.
Eqerimental jlooding data : I.88 in. i.d. COIWM,21 x & in. nox2;ledistributor. Continuous phase : wuter. Dispersed phase : sol&&t
Series No.
Run No.
-_ I
1 2
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2.5 26 27 28 29 30 31 32 33
II
III
IV
V
VI
VII
z 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
VIII.
IX x
XI
Sfple7Pt
Packing Material
M.I.B.K.-water 1, 3, ,> >f
3 in. Raschig rings ,I 3, I> ,> >, >I ,I 3, 1, 3, ,, ,t >, >, ,t
Kerose&-water ,, 3, f, 2, Toluene-l&ater ,, >, 2,
a’ in. Co&r
Toluene&ter ,, ,> ,f ,> ,,
rings
St I, ,, i>
93 >,
Kerose&water $3 ,,
‘> ,I ,I >,
Pegasoi~water II ,,
3, I’ I,
,9
M.I.B.<.-water t, 3, ,, 3, >, ff
>,
a, >.
6 mm
Kerose&water ,I Pegasol%tter ,I ,1 >> Tolue&-water
G&s beads ,’ If ,, 1, 3, ‘I 1. 3, ,, ,? I. 3, ,, ,,
UC Whr) two
47.9 25.9 84.2 11.6 105.6 64.9 98.5 42.9 79.8 18.2 30.3 17.6 23.7 29.7 35.2 59.4 26.5 32.5 19.3 11.0 44.6 49.0 54!*0 23.7 12.7 20.3 36.4 26.5 49.6 15.4 64.5 8.8 17.4 24.2 40.8 22.6 149 23.1 11.0 16.5 25.9 12.7 15.4 26.5 39.1 29.8 22.6 25.9 12.1 16.5
UD
fft/h) 49.0 64.9 103.4 27.5 143.0 20-4 38.5 12.7 64.4 31.4 115.5 88.2 44.6 27.0 17.1 13.8 9.4 46.3 43.0 56.2 76.0 30.3 26.5 22.6 21.5 33% 30.9 12.1 71.6 46.3 84.3 34.2 79.3 56.2 43.5 28.1 16.0 19.0 15.4 21.5 35.3 26.2 38.6 40% 29.2 16.0 23.1 28.7 12.7 31-5 25.5
smaller than those of kerosene-water (0 = 39 dyn/cm) and Pegasol-water (a = 29 dyn/cm). In general, flooding rates decreased with decreasing size of the packing. These observations are in quite good agreement with those of the The flooding rate data earlier investigators. (Table 2) show clearly that higher flooding rates
are favoured by high density difference and low interfacial tension of the system. A general type of plot which has been utilized by previous investigators in preliminary correlation of flooding data is to plot the square root of flow rate of the dispersed phase against that of the continuous phase at flooding. On such a plot 173
M. RAJA Rno and C. VENKATA RAO
FIG. 1. Flooding
correlationof DELL and PRATT.
a series of almost parallel straight lines was obtained, one for each set of conditions. The slope of all these lines was found to be close to - l, the range being - 1.0 to - 1.1. An attempt has been made to fit the data by the well-established correlation of DELL and PRATT, and such a plot is represented in’ Fig. 1. The flooding data are quite satisfactorily fitted by the dimensionless equation and the computed values of the slope, IZ, and the constant C are found to be. - f and 0.66. These values are in quite good a~eement with the values of - + and 0.68 obtained by the above authors for Raschig rings. The excellent fit of the flooding data on DELL and PRATT’Splot of correlation shows that, with the different packings used, the packing constant C in the equation is almost identical and constant at 0.66, probably because of the closer size range of the packings. In view of the excellent correlation of the flooding rate data by the method of DELL and PRATT no other correlation
methyl isobutyl ketone which do not preferentially wet the packing, as the dispersed phase. Visual observations aided by photographic recordings of the cohunn in operation supplemented the measurement of flooding rates, Maximum flooding rates were obtained with Qin. Raschig rings as packing material and methyl isobutyl ketone-water system, and the least rates with kerosene-water and Pegasolwater systems. The flooding data have been fitted satisfactorily on the square root plot as a preliminary eorrelation, and have been well correlated by the method of DELL and PRATT with a value of - & for n and O-66 for C. It was observed that flooding velocities decreased with decreasing size of the packing for any given system, and that low interfacial tension of the system and high density difference tended to give higher flooding rates by favouring the formation of smaller drops.
has been attempted.
NOTATION a = interfacial contaot area per unit tower
in the packed extraction tower, as incidental to our main studies on mass transfer, with 2 in. Raschig rings, $ in copper rings and 6 mm glass beads, using water as the continuous phase and each of the solvents toluene, Pegasol, kerosene and Flooding
rate data
have been obtained
volume f@/fts. n, C = constanta in DELL and PRATT’sequation. g = acceleration due to @v&y. V, or U, = flow rate of continnous water phase in ft8/(ft2) (hr). V, or U, = flow rate of dispersed solvent phase in ft8/(ft2) (hr.)
174
Flooding l
rates in packed
liquid extraction
or F = void fraction of the packing material, ft8/ft8. pC = density of the continuous phase in lb/f@. dD = density of the dispersed phase in lb/f@.
towers
A,, = density difference of the phases in lb/ft.8 pC = viscosity of the continuous phase in lb/W WI. (I or y = interfacial tension in dyn/cm.
REFERENCES [l]
RUSETON J. H. Ind. Engng.
Chem. 1937 29 309.
[2] [3]
APPEL F. J. and ELGIN J. C. Ind. Engng. Chem. 1937 29 451. SHERWOODT. K., EVANS J. E. and LONGCORJ. V. A. Trans. Amer. Inst. Chem. Engrs. 1939 35 597.
[4]
Row S. B., KOFF~LT J. H. and WITHROW J. R. Trans. Amer. Inst. Chem. Engrs.
[5]
BLANDING F. ‘H. and ELGIN J. C. Trans.
[6]
BRECKENFELDR. R. and WILKE C. R. Chem. Engng.
[7]
BALLARD J. H. and PIRET E. L. Ind. Engng.
Amer. Inst.
Chem. Engrs. 1942 33 305. Progr.
1950 46 137.
Chem. 1950 42 1033.
[3]
CRAWFORDJ. W. and WILKE C. R. Chem. Engng.
[D]
DELL F. R. and PRATT H. R. C. Trans.
Inst.
1941 37 559.
Progr.
1951 47 423.
Chem. Engrs.
(Land.)
1951 29 39.
[lo]
SAI&ADIS B. C. and JOHNSONA. L. Ind. Engng.
[ll]
HOF~NG E. H. and LOCKHARTF. J. Chem. Engng.
Chem. 1954 46 1229.
[12]
TREYBAL R. E. Ind. Engng.
[13]
Fl~o M. RAJA, D.Sc. Thesis, Andhra University (India), 1956.
[14]
B.AO M. RAJA and RAO C. VENEATA, Communicated to Chem. Engng.
Progr.
1954 50 94.
Chem. 1955 47 539.
176
Sti.
October 1957.
Flooding
rates
in packed
liquid
extraction
M. RAJA RAO and C. VENKATA Department
of Chemical Technology,
Andhra University,
towers
RAO Waltair, Southern India
(Received 1 November 1957) Abstract-As incidental to our main studies on mass transfer, flooding rate data have been obtained in a packed liquid-liquid extraction tower with no solute transfer, with 1 in. Raschig rings, f in. copper rings and 6 mm glass beads as the different packing materials, using water as the continuous phase and each of the following solvents, toluene, Pegasol, kerosene and methyl tiobutyl ketone, which do not preferentially wet the packing, respectively as the dispersed phase. Visual observations, aided by photographic studies supplemented the measurements of the flooding velocities. The Aow rates at the flooding point have been satisfactorily fitted on the square root plot as a preliminary correlation, and have been well correlated by the method of DELL and PRATT. It was observed that flooding rates decreased with decreasing size of the packing for any given system, and that higher flooding rates are favoured by high density difference and low interfacial tension of the liquid systems. R&sum&--A l’occasion de leurs principales btudes sur le transfert massique, les auteurs ont obtenu des resultats experimentaux sur I’engorgement produit dans une colonne a garnissage pour l’extraction liquide-liquide, sans transfer% de solute. Les garnissages suivants ont ttC utilids: anneaux Raschig de 0.95 cm, anneaux de cuivre de 0.64 cm et billes de verre de 6 mm. La phase continue etait de l’eau, alors que la phase dispersee contenait un des solvants suivants : toluene, m&hyl&hylc&one, qui ne mouillent pas preferentiellement le materiau de Pegasol, kerosene, remplissage. Les mesures de vitesses d’engorgement ont &tC realisees de man&e visuelle ainsi que par des etudes photographiques. Une correlation prt%iminaire a permis aux auteurs de reproduire d’une maniere satisfaisante les debits ir l’engorgement par la methode des racines carrees. L’utilisation de la methode de DELL et PRATT a permis de correler les resultats avec succes. Les auteurs ont observe que les vitesses d’engorgement diminuent avec les dimensions du garnissage pour tout systeme, et que les grandes vitesses d’engorgement sont favoristes par une grande difference de densite et une faible tension interfaciale des systemes liquides. Zusammenfassung-Im Zuge unserer Untersuchungen iiber den Stoffaustausch wurden Flutungsgeschwindigkeiten in einer Ftillkiirperslule zur Fliissig-Fhissig-Extraktion ohne Ubertragung von Gel&tern ermittelt. Als Packungsmaterial dienten Raschigringe von 9.5 mm, Kupferringe von 6.4 mm und Glasperlen von 6 mm. Die geschlossene Phase war Wasser und die disperse Phase waren folgende Lijsungsmittel, Toluol, Pegasol, Kerosen und Methyl-lsobutylKeton, welche das Packungsmaterial vorzugsweise nicht benetzten. Visuelle Beobachtungen, unterstiitzt durch photographische Aufnahmen, erganzten die Messungen der Flutungsgeschwindigkeiten. Der Mengenstrom am Flutungspunkt wurde befriedigend durch eine quadratische Beziehung in vorl5ufiger Form wiedergegeben und konnte gut nach der Methode von DELL und Pa~rr dargestellt werden. Es wurde festgestellt, d,ass die Flutungsgeschwindigkeiten abfielen mit fallender Packungsgrasse fur ein gegebenes System und dam hijhere Flutungsgeschwindigkeiten begiinstigt werden durch hohe Dichtedifferenz und niedrige Grenzfliichenspannung der fliissigen Systeme. INTRODUCTION MUCH of the early work on flooding rates in packed
liquid-liquid columns was done incidentally to other investigations, and in this category came the reports
of RUSHTON [l], APPEL and ELGIN [2], SHERWOOD et at. [3], and Row et al. [PI. BLANDING
and ELGIN [5] described their investigations of the design and operation of spray and packed columns, pointing out the importance of the proper design distributor. During 170
recent
of
the
years,
entrance
sections
flooding rates
have
and been
Flooding rates in packed liquid extraction towers studied in packed liquid extraction towers by several investigators [6-111, and of these special mention should be made of DELL and PUTT, [9] who have made a rather extensive study of flooding rates -in columns of 3 in. and 6 in. inside dia., using a wide variety of liquid systems and packings. The above authors correlated their data by means of a theoretical equation :-
The value of 12 was found to be - l/4. The constant C has slightly different values for each type of packing used, and the values recommended for design are 0.68, 0.80, and 0.88 for Raschig rings, Lessing rings and Berl saddles respectively. Each of the flooding correlations so far developed for packed liquid-liquid columns is applicable over a particular range of the different operating variables, and as pointed by TREYBAL [12], it is necessary to exercise due caution in utilizing any of the flooding correlations to cases where the packing sizes, system properties and also column Table 1.
sizes are outside the range for which they are applicable. APPARATUS The extraction equipment consisted essentially of an extraction tower and the necessary accessories to maintain a steady flow of the two liquid phases. The design of the packed column was based on the recommendations of BLANDINGand ELGIN [5], and the apparatus used has been fully described elsewhere [13,14]. To facilitate visual observation of the column in operation, the column proper was constructed of Pyrex glass tubing, 1.88 in. inside dia., and two column lengths of 60 and 34 in. respectively were used in the experimental work. A dispersed phase disdistributor with 21 x & in. nozzles was used in the flooding rate studies. MATERIALS Of the dispersed solvents, kerosene and methyl isobutyl ketone were supplied by the BurmahShell Oil Co., Pegasol by the Standard Vacuum Oil Co., and toluene by the Bengal Chemical and Pharmaceutical Works, Calcutta. The physical properties of these solvents are recorded in Table 1. Tap water of negligible acidity, drawn
(a) Properties
of packing materials
Average size of units Material
Pocking
(in.)
a in. Raschig rings
Porcelain
0.375
Copper Glass
0.250
4 in. Copper
%Y
6 mm Glass beads
I.D.
O.D.
i
(in.)
Kerosene Pegasol Toluene Methylisobutyl ketone
1
0.273
(in.)
No. of units
per fL3
0.384
28,680
0.175 0.250 6 mm dia. sphere
81,060 163,000
(b) Properties Solvent
Len&h
a
ft.2 G
ST Measured
Calculated
178-l
oao
04w7
2154 198.0
0635 0.881
0.692 0.896
of solvents at 3OT
Density at 30°C (g/ems)
Viscosity in c.p.
0.778 O-785 O-859 0.800
1.08 0.854 0.556 0.520
171
Interfacial tension (dyn/cm)
89 29 26 10
M.IZlwa Rho and C.VENKATA RAO from the Andhra University mains supply was used in the experimental work. Three types of packing materials were used : (1)Q in. non-porous and unglazed porcelain Raschig rings, manufactured and supplied by the Maurice A. Knight Co., of Akron, (2) 4 in. copper rings, and (3) 6 mm glass beads. The column was filled with water, and the packing material dropped in from the top, a few pieces at a time, slowly and at random to the desired height. The packing was not shaken or tamped. The properties of the packing materials are listed in Table 1. With each of the three different packing materials, the liquid systems studied were : 1. toluene-water, 2. Pegasol-water, 3. kerosenewater, and 4. methyl isobutyl ketone-water, using water as the continuous phase and the organic solvent as the dispersed phase.
COLUMN
BEHAVIOUR
A good number of photographs were taken of the column during operation with a view to interpreting the flooding data in terms of drop size, holdup and coalescence of the dispersed phase. In general, it was observed with each of the packings and liquid systems ,used that the streams of the dispersed phase droplets worked their way up the column, deforming and following a torturous path while passing up through the interstices of the packing. With increased flow rate of the dispersed phase its holdup was markedly increased, and the drops became very closely packed up in the column. As the flooding rate was closely approached the drops were‘ observed to coalesce, forming slugs which gradually tended to travel down the column. The flooding rate was taken at the fust appearance of a thin layer of the dispersed phase below the packing support. As compared with Q in. PROCEDURE Raschig rings, the column was rather densely Each of the liquid phases was first saturated packed with + in. rings, and more particularly with the other, as this is particularly necessary with 6 mm beads. With the latter packing the when the solvent and water have appreciable drops showed a greater tendency to coalescence, mutual solubility, as in the case of water and and for any liquid system the flooding rates were methyl isobutyl ketone in the present investigalower than those for ring packings. tion. The continuous water phase was first It was observed that smaller and finer drops admitted into the column and then set at the were formed with solvents like methyl isobutyl desired flow rate. The dispersed solvent phase ketone, and comparatively larger drops, showing was then slowly introduced, and its flow rate was increased tendency to coalesce, with kerosene and gradually increased in 5 to 10 steps until the Pegasol. No attempt was made to determine flooding point, as indicated by the first appearance either pressure drop or holdup at the flooding of a thin layer of the dispersed phase underneath point. the packing support, was reached. In a few runs, RESULTS AND CORRELATION when the dispersed phase rate was high, the Altogether over 50 runs have been taken on procedure was reversed by first establishing the flooding rates with the different liquid systems rate of the dispersed phase and then gradually increasing the continuous phase rate up to the and packings, and the data are presented in Table 2. Maximum flooding rates were obtained flooding point. The flooding results obtained by with 9 in. Raschig rings with the methyl the two methods agreed quite well. isobutyl ketone-water system, and the least In all the runs extra care was taken to maintain the liquid-liquid interface in the upper end with kerosene-water and Pegasol-water systems. The larger drops, and their increased tendency to section at the same level, and during each run, coalesce observed with the last two systems, as the flow rates were not only checked, but also the exact rates of the solvent and water phases compared with the relatively smaller drops of the were obtained by timing the exit streams for a ketone-water system, may perhaps be due to the fact that the interfacial tension (u) of the ketoneknown interval of time and measuring their water system is only 10 dyn/cm, which is much volumes. 172
Flooding rates in packed liquid extra&ion towers
.
Table 2.
Eqerimental jlooding data : I.88 in. i.d. COIWM,21 x & in. nox2;ledistributor. Continuous phase : wuter. Dispersed phase : sol&&t
Series No.
Run No.
-_ I
1 2
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2.5 26 27 28 29 30 31 32 33
II
III
IV
V
VI
VII
z 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
VIII.
IX x
XI
Sfple7Pt
Packing Material
M.I.B.K.-water 1, 3, ,> >f
3 in. Raschig rings ,I 3, I> ,> >, >I ,I 3, 1, 3, ,, ,t >, >, ,t
Kerose&-water ,, 3, f, 2, Toluene-l&ater ,, >, 2,
a’ in. Co&r
Toluene&ter ,, ,> ,f ,> ,,
rings
St I, ,, i>
93 >,
Kerose&water $3 ,,
‘> ,I ,I >,
Pegasoi~water II ,,
3, I’ I,
,9
M.I.B.<.-water t, 3, ,, 3, >, ff
>,
a, >.
6 mm
Kerose&water ,I Pegasol%tter ,I ,1 >> Tolue&-water
G&s beads ,’ If ,, 1, 3, ‘I 1. 3, ,, ,? I. 3, ,, ,,
UC Whr) two
47.9 25.9 84.2 11.6 105.6 64.9 98.5 42.9 79.8 18.2 30.3 17.6 23.7 29.7 35.2 59.4 26.5 32.5 19.3 11.0 44.6 49.0 54!*0 23.7 12.7 20.3 36.4 26.5 49.6 15.4 64.5 8.8 17.4 24.2 40.8 22.6 149 23.1 11.0 16.5 25.9 12.7 15.4 26.5 39.1 29.8 22.6 25.9 12.1 16.5
UD
fft/h) 49.0 64.9 103.4 27.5 143.0 20-4 38.5 12.7 64.4 31.4 115.5 88.2 44.6 27.0 17.1 13.8 9.4 46.3 43.0 56.2 76.0 30.3 26.5 22.6 21.5 33% 30.9 12.1 71.6 46.3 84.3 34.2 79.3 56.2 43.5 28.1 16.0 19.0 15.4 21.5 35.3 26.2 38.6 40% 29.2 16.0 23.1 28.7 12.7 31-5 25.5
smaller than those of kerosene-water (0 = 39 dyn/cm) and Pegasol-water (a = 29 dyn/cm). In general, flooding rates decreased with decreasing size of the packing. These observations are in quite good agreement with those of the The flooding rate data earlier investigators. (Table 2) show clearly that higher flooding rates
are favoured by high density difference and low interfacial tension of the system. A general type of plot which has been utilized by previous investigators in preliminary correlation of flooding data is to plot the square root of flow rate of the dispersed phase against that of the continuous phase at flooding. On such a plot 173
M. RAJA Rno and C. VENKATA RAO
FIG. 1. Flooding
correlationof DELL and PRATT.
a series of almost parallel straight lines was obtained, one for each set of conditions. The slope of all these lines was found to be close to - l, the range being - 1.0 to - 1.1. An attempt has been made to fit the data by the well-established correlation of DELL and PRATT, and such a plot is represented in’ Fig. 1. The flooding data are quite satisfactorily fitted by the dimensionless equation and the computed values of the slope, IZ, and the constant C are found to be. - f and 0.66. These values are in quite good a~eement with the values of - + and 0.68 obtained by the above authors for Raschig rings. The excellent fit of the flooding data on DELL and PRATT’Splot of correlation shows that, with the different packings used, the packing constant C in the equation is almost identical and constant at 0.66, probably because of the closer size range of the packings. In view of the excellent correlation of the flooding rate data by the method of DELL and PRATT no other correlation
methyl isobutyl ketone which do not preferentially wet the packing, as the dispersed phase. Visual observations aided by photographic recordings of the cohunn in operation supplemented the measurement of flooding rates, Maximum flooding rates were obtained with Qin. Raschig rings as packing material and methyl isobutyl ketone-water system, and the least rates with kerosene-water and Pegasolwater systems. The flooding data have been fitted satisfactorily on the square root plot as a preliminary eorrelation, and have been well correlated by the method of DELL and PRATT with a value of - & for n and O-66 for C. It was observed that flooding velocities decreased with decreasing size of the packing for any given system, and that low interfacial tension of the system and high density difference tended to give higher flooding rates by favouring the formation of smaller drops.
has been attempted.
NOTATION a = interfacial contaot area per unit tower
in the packed extraction tower, as incidental to our main studies on mass transfer, with 2 in. Raschig rings, $ in copper rings and 6 mm glass beads, using water as the continuous phase and each of the solvents toluene, Pegasol, kerosene and Flooding
rate data
have been obtained
volume f@/fts. n, C = constanta in DELL and PRATT’sequation. g = acceleration due to @v&y. V, or U, = flow rate of continnous water phase in ft8/(ft2) (hr). V, or U, = flow rate of dispersed solvent phase in ft8/(ft2) (hr.)
174
Flooding l
rates in packed
liquid extraction
or F = void fraction of the packing material, ft8/ft8. pC = density of the continuous phase in lb/f@. dD = density of the dispersed phase in lb/f@.
towers
A,, = density difference of the phases in lb/ft.8 pC = viscosity of the continuous phase in lb/W WI. (I or y = interfacial tension in dyn/cm.
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