9724
Ind. Eng. Chem. Res. 2008, 47, 9724–9727
Mechanism Analysis on the Two-Phase Flow Characteristics in Coalescence-Dispersion Pulsed-Sieve-Plate Extraction Columns Tang Xiaojin,* Luo Guangsheng, and Wang Jiading State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China
To investigate the influence of drop breakage and drop coalescence, a mechanism analysis of the liquid-liquid two-phase flow characteristics in the coalescence-dispersion pulsed-sieve-plate extraction column (CDPSEC) is performed. According to the experimental results, the operation of CDPSEC can be divided by a turning point into two regions: the drop breakage control region with higher holdup of the dispersed phase and the drop coalescence control region with lower holdup. The size of the maximum droplet in CDPSEC is estimated. It was found that, in the drop coalescence control region, droplets coalesce under the coalescence plate to produce the new large droplets, and the influence of drop coalescence is stronger than that of drop breakage. At the turning point, the new large droplets reach the maximum droplets and no larger droplets can be produced. From this point on, in the drop breakage control region, the droplets are so large that they break up into small droplets. Drop breakage becomes the main flow characteristic instead of drop coalescence. Introduction For extraction columns, the mass transfer performances were greatly influenced by the liquid-liquid two-phase flow characteristics, which conventionally were described by eq 1.1 In eq 1, the holdup of the dispersed phase was related to the operating conditions, including the superficial velocities of the two phases and energy input parameters. Under the logarithmic coordinates, eq 1 could be described as a linear flow characteristic curve with a positive slope coefficient. ud uc + ) uk(1 - φ)n (1) Vs ) 1-φ φ As an improvement upon the standard pulsed-sieve-plate extraction column (PSEC), the coalescence-dispersion pulsedsieve-plate extraction column (CDPSEC) was reported to have 200% throughput and 120% overall mass transfer efficiency over PSEC,2,3 but the two-phase flow characteristics in CDPSEC were quite different from other extraction columns. A special kind of plate made of Teflon, with a wetting ability toward the dispersed phase that is much stronger than that of the standard steel plate, is inserted into CDPSEC periodically over the column length. Thus, the droplets of the dispersed phase coalesce when they pass through the Teflon coalescence plates, and then the droplets break up to provide new interface area for mass transfer when they pass through the steel dispersion plates. It is the interface renewal effect, caused by the periodical drop coalescence and dispersion, that enhances the mass transfer in CDPSEC. Dai et al.4 suggested eq 2 to describe the two-phase flow characteristics with a 40-mm-diameter CDPSEC. Tang et al.5 reported that there was a turning point on the flow characteristic curve deducting the contribution of the local coalescence layer to the holdup of the dispersed phase in a 150-mm-diameter CDPSEC. When the holdup is higher than that at the turning point, the slope coefficient of the flow characteristic curve is positive, similar to that in other extraction columns. On the other hand, when the holdup is lower that that at the turning point, the slope coefficient of the flow characteristic curve is negative. * Corresponding author.
[email protected].
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86-10-62783870.
ud uc + ) uk(1 - φ)n(1 + φ)m (2) 1-φ φ In this study, the two-phase flow characteristics were investigated by considering the total holdup of the dispersed phase in CDPSEC. The influence of drop coalescence introduced by the coalescence (Teflon) plate on the flow characteristics was analyzed by determining the maximum droplet. Experimental Setup The CDPSEC was composed of a glass section, 150 mm in diameter, 2 m in effective height, and 25 mm in plate spacing. The combination of the dispersion (steel) sieve plates with one coalescence (Teflon) plate every four plates was applied. The experimental setup and the plate specifications were described in our previous work.5 The coalescence plate with 23 tongueshaped holes is show in Figure 1. The experimental system was 30% tributyl phosphate in kerosene-nitric acid-water with the organic phase as the dispersed phase. The physical properties of the experimental system are shown in Table 1. The holdup of the dispersed phase was measured by the volumetric replacement method. The drop size was measured by the infrared optical probe shown as Figure 2.6 Because of the difference of refractive index of the two liquid phases, the values
E-mail: Figure 1. Structure of the coalescence sieve plate.
10.1021/ie8002792 CCC: $40.75 2008 American Chemical Society Published on Web 11/08/2008
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9725 Table 1. Physical Properties of the Experimental System (25 °C)
continuous phase dispersed phase
density (kg/m3)
viscosity (Pa · s)
interfacial intension (N/m)
998.0 835.0
0.00105 0.00209
0.00995
of light intensity reflected from the probe tip are different when either liquid phase passes through the probe tip. Thus, the two liquid phases could be distinguished by the optic probe. Results and Discussion Two-Phase Flow Characteristics. Figures 3 and 4 are the photos of two-phase flow in CDPSEC under the same operating condition, for the coalescence plate and dispersion plate, respectively. It can be seen that there is a local coalescence layer of the dispersed phase under the coalescence plate, but it does not exist under the dispersion plate. When the droplets pass through the dispersion plate, the coalescence of the droplets is hardly observed, because the interaction of the dispersed phase with the stainless steel is much weaker than that with Teflon. Thus, the droplets coalesce and then break up periodically along the column in CDPSEC.
Figure 2. Scheme of the infrared optical probe system.
Figure 5. Two-phase flow characteristic curve. Table 2. Maximum Droplet in CDPSEC A (m) f (s-1) dmaxex (mm) dmaxca (mm)
0.010 1.0 11.4 11.4
0.015 1.0 11.4 8.4
0.020 1.0 5.7 6.8
0.005 2.0 10.0 11.4
The two-phase flow characteristic curve in CDPSEC is shown as Figure 5. Vs is the slip velocity between the two phases, and φ is the holdup of the dispersed phase. From Figure 5, it can be found that there is a turning point on the two-phase flow characteristic curve under the same energy input condition (fixed pulse amplitude A and pulse frequency f). When the holdup of the dispersed phase is higher than that at the turning point, the relationship between Vs and (1 - φ) agrees with eq 1 and the slope coefficient is positive. When the holdup is lower than that at the turning point, a negative slope coefficient of the flow characteristic curve is obtained. Since the existence of the coalescence plate leads to the sharp turning of the two-phase flow characteristic curve, it seems that the operation of CDPSEC should be divided into two regions: the drop breakage control region with higher holdup and the drop coalescence control region with lower holdup. In the coalescence control region, the droplets coalesce under the coalescence plate to form a local coalescence layer of the dispersed phase with the increase of the holdup, and then the new droplets are generated from the layer. When the holdup is high enough to produce the maximum droplets, the coalescence of the droplets should not go on anymore. From this point on, the two-phase flow characteristics are controlled by drop breakage instead of drop coalescence. Thus, the turning point is defined as the cutoff point between the drop breakage region and the drop coalescence region. Equation 3 is derived from the experimental results to calculate the holdup of the turning point. φt ) 0.64(A3f2)0.11
Figure 3. Dispersed phase passing the coalescence plate (A ) 1.0 cm, f ) 1.0 s-1, uc ) 0.00133 m/s, ud ) 0.00276 m/s).
0.025 1.0 5.7 5.7
(3)
Maximum Droplet. Equations 4 and 5 can be used to calculate the maximum droplet.7 dmax ≈ εm-0.25
(4)
εm ≈ C(Af)3/H
(5)
where εm is the energy dissipation, H is the plate spacing, and C is a constant. The maximum droplet measured by the infrared probe system in CDPSEC is shown in Table 2. Equation 6 is obtained by fitting the experimental drop size results, and the calculated results are also shown in Table 2. dmax ) 0.36(Af)-0.75 Figure 4. Dispersed phase passing the dispersion plate (A ) 1.0 cm, f ) 1.0 s-1, uc ) 0.00133 m/s, ud ) 0.00276 m/s).
(6)
Drop Coalescence at the Turning Point. Figure 6 shows drop coalescence under the coalescence plate with the increase of
9726 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 3. dt under Different Operating Conditions A (m) f (s-1) dmaxex (mm)
0.010 1.0 11.4
0.015 1.0 9.7
0.020 1.0 8.7
0.025 1.0 7.9
0.005 2.0 10.0
of dt under different operating conditions. From Tables 2 and 3, under the same operating conditions, the size of the new large droplets at the turning point agrees with the size of the maximum drop with average deviation less than 25%. Thus, in the drop coalescence control region, the droplets continuously coalesce under the coalescence plate to produce the new large droplets with the increase of the holdup, until the new large droplets reach the size of the maximum droplet at the turning point. In the drop breakage control region, the rest of the old droplets and the new large droplets, whose size is equal to the maximum droplet, pass through the dispersion plates and break up into small droplets. The influence of drop coalescence is restrained, and drop breakage takes the position of control and becomes more important than drop coalescence. Conclusions
Figure 6. Drop coalescence under the coalescence plate (A ) 0.010 m, f ) 1.0 s-1, uc ) 0.00133 m/s).
the holdup. With the increase of holdup, the thickness of the local coalescence layer under the coalescence plate increases, and that means the ratio of the droplets coalesced with the layer in the total droplets increases. Tang et al.8 suggested eq 7 to quantitatively calculate the ratio η of the local coalescence of droplets under the coalescence plate. The size of the new droplets generated from the layer increases with the increase of the holdup until these droplets become the maximum droplets. More importantly, the positions where the new large droplets are generated mostly focus on the tongues of the tongue-shaped holes. In this sense, the number of the new large droplets, which are much larger than the old droplets, can be obtained by the number of the tongue-shaped holes.
[
]
1
1.68(Af)-0.37 V y ud K ln K ) 15.9A0.25f -0.03 -0.1 f ) 1.0/s V) 0.11 f ) 2.0/s 1.5 f ) 1.0/s y) 1.29 f ) 2.0/s
η)
{ {
(7)
Nomenclature A ) pulse amplitude, m d ) drop diameter, mm f ) pulse frequency, s-1 H ) plate spacing, m N ) number of tongue-shaped hole u ) superficial velocity, m s-1 uk ) characteristic velocity, m s-1 S ) cross-sectional area, m2 Vs ) slip velocity between the two phases, m s-1 εm ) energy dissipation, m2 s-3 η ) ratio of drop coalescence under the coalescence plate φ ) holdup of the dispersed phase Superscripts
According to eq 3 and the experimental results of the holdup, when the superficial velocity of the dispersed phase ud is 0.00476 m/s, the operating state of CDPSEC approaches the turning point. From eq 7, the size of the new large droplets at the turning point dt can be obtained by eq 8. dt ) ηtutS/N
A mechanism analysis on the liquid-liquid two-phase flow characteristics in CDPSEC is performed to investigate the influence of drop breakage and drop coalescence. The operation of CDPSEC can be divided into two regions: the drop breakage control region and the drop coalescence control region by a turning point. In the drop coalescence control region, where the holdup of the dispersed phase is lower than that at the turning point, droplets coalesce under the coalescence plate to produce the new large droplets, and the influence of drop coalescence is stronger than that of drop breakage. At the turning point, the new large droplets reach the maximum droplets and drop coalescence cannot produce larger droplets anymore. From this point on, in the region of higher holdup, drop coalescence is restrained, and the droplets are so large that they break up into small droplets. Drop breakage becomes the main flow characteristic instead of drop coalescence.
(8)
where ut and ηt are the superficial velocity of the dispersed phase and the ratio of the local coalescence at the turning point, respectively. S is the cross-sectional area of CDPSEC, and N is the number of the tongue-shaped holes. Table 3 shows the values
ca ) calculated data ex ) experimental data Subscripts c ) continuous phase d ) dispersed phase max ) maximum droplet t ) turning point
Literature Cited (1) Wang, J. D.; Shen, Z. Y.; Wang, C. F. Study on two-phase flow characteristics in a liquid-liquid pulsed-sieve-plate extraction column. J. Chem. Ind. Eng. (Nanjing, China) 1965, 215.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9727 (2) Lei, X.; Dai, Y. Y.; Shen, Z. Y.; Wang, J. D. Study on two-phase flow characteristics and mass transfer in pulsed column with dispersion-coalescence type cartridge. In Proceedings of CIESC/AICHE Joint Meetings of Chemical Engineering; Chemical Industry Press: Beijing, 1982; pp 538-557. (3) Li, H. B.; Luo, G. S.; Fei, W. Y.; Wang, J. D. Mass transfer performance in a coalescence-dispersion pulsed sieve plate extraction column. Chem. Eng. J. 2000, 78, 225. (4) Dai, Y. Y.; Lei, X.; Zhu, S. L.; Wang, J. D. Two-phase flow characteristics in a pulsed-sieve-plate extraction column. J. Chem. Ind. Eng. (China) 1988, 422. (5) Tang, X. J.; Luo, G. S.; Li, H. B.; Wang, J. D. Two-phase flow characteristics in a coalescence-dispersion pulsed-sieve-plate extraction column. Chin. J. Chem. Eng. 2004, 12, 1.
(6) Tang, X. J.; Luo, G. S.; Wang, J. D. Optical probe technique for two-phase flow measurements in an extraction column. AIChE J. 2005, 51, 1565. (7) Gourdon C.; Casamatta G.; Muratet G. Liquid-Liquid Extraction Equipment; Wiley & Sons: New York, 1994. (8) Tang, X. J.; Luo, G. S.; Wang, J. D. Local coalescence effect on extraction column performance. In Proceedings of the International SolVent Extraction Conference; Tsinghua Tongfang Press: Beijing, 2005.
ReceiVed for reView February 18, 2008 ReVised manuscript receiVed August 28, 2008 Accepted October 06, 2008 IE8002792
