Ind. Eng. Chem. Res. 1995,34, 4110-4117
4110
Wettability Effects of Plate Materials on Hydrodynamics in a Pulsed Perforated-Plate Extraction Column of Pulser Feeder Type Hidematsu Ikeda**'and Atsuyuki Suzukis Research Center for Nuclear Science and Technology, and Department o f Quantum Engineering and Systems Science, Faculty o f Engineering, The University o f Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113, Japan
Liquid-liquid-plate contact angles of several plate materials were measured using the preferential wetting method proposed by the authors. The work of adhesion (IsL),which was calculated using these contact angles, was utilized to evaluate plate wettability. Experimental results indicated that different plate materials affect plate wettability. Both required pulse velocity and required hole diameter for a basic design of a perforated-plate were obtained. The plate wettability of stainless steel was observed to be different in an aqueous phase compared t o a n organic phase. The axial holdup distribution in a pulsed column using several perforatedplate materials and types was also measured. An empirical correlation of overall holdup was obtained as a function of the plate wettability. The nozzle effect was explained by the different proportionality-constants in the empirical correlation. The pulsed perforated-plate extraction column (i.e., pulsed column) is an example of a process of continuous periodic operation; however, the details of hydraulic characteristics have not been studied. An application of a periodical pulsation to the column provides the additional energy necessary to force the two liquid phases through the plates. The small plate perforations in the pulsed column provide high fluid velocities and small droplets and thus create the turbulence and transfer area necessary for high extraction rates. However, the most desirable operational condition for the pulsed column can be qualitatively achieved only if the plate is preferentially wetted by the continuous phase. Wetting the plate surface with the dispersed phase causes a reduction in the driving force for liquid-liquid dispersion in the pulsed column. In order to observe the effects of plate wettability, several different plate materials, types, and hole diameters were utilized in the pulsed column. For aqueouswettable plates, preferentially wetted by the aqueous phase, stainless steel (mainly used for plate material), brass (Miyauchi and Oya, 19651, and aluminum plates (Shirotsuka et al., 1958) were used. For organicwettable plates, preferentially wetted by the organic phase, polytetrafluoroethyene (PTFE/Teflon, Geier, 1957; Kim and Baird, 1976) and plastic plates were used (Rouyer et al.,1974). Geier (1957) studied the general aspects of pulsed column for use in the Purex process for reprocessing of nuclear spent fuel to recover uranium and plutonium. In the Purex process, the aqueous phase is the dispersed phase to the extraction column; the organic phase becomes the dispersed phase to the stripping column. He found that the Teflon plates indicated adequate capacity and efficiency under the operating condition with the dispersed aqueous phase. These Teflon plates were suspect, however, because of possible changes and/ or damage occurring on prolonged exposure to high level radiation such as the co-extraction column in the Purex process. This led to the use of the stainless steel nozzle plates. When the nozzle type plates were used, the flooding rate become greater than when the sieve type
* Corresponding author telephone:
' Research Center for Nuclear ' Faculty of Engineering.
03-3812-2111,ext 2939. Science and Technology.
0888-5885195f2634-4110$09.00/0
plates were used. He also found that dirty stainless steel plates, which were partially wetted by the organic phase, led to unfavorable effects. Rouyer et al. (1974) reported that slip velocity increases with flow ratio when the plates are not wetted by a continuous phase solution, and thus the throughput is increased as the flow ratio increases. Teflon plates, plastic plates, and stainless steel plates were used in their study. Both researchers found that the plate wettability, which is dependent on both plate material and dispersed phase, affects the flooding rate and the throughput. These studies did not however show the relation that exists between plate wettability and flooding rate. In this study, the wetting characteristic of several perforated-plate materialdtypes and its effect on axial holdup were studied by utilizing a concept of plate wettability, which was determined using the liquidliquid-plate contact angles and the work of adhesion performed. Both stainless steel and fine ceramic plates were prepared for the perforated-plate. The fine ceramic plate was designed in anticipation of functional materials such as acid resistance, radiation resistance, hydrophobic, and hydrophilic. The liquid-liquid-plate contact angle was measured using the preferential wetting method proposed by the authors (19921, and an empirical correlation of overall holdup was reported in the paper. The work of adhesion, which was expressed as a function of the contact angle, was utilized to evaluate plate wettability.
Experimental Apparatus and Procedures Pulsed Column and Liquids. A schematic diagram showing the experimental pulsed column of pulserfeeder type is shown in Figure 1. This apparatus is similar to that used previously by Ikeda et al. (1987a,b) and Ikeda and Suzuki (1989, 1992). Liquid pulsation was generated by using a proportioning pump that was connected to a motor by a variable stroke-length regulator so as to obtain a variable output flow rate from 0 to 60 L/h. The pulse frequency was constant at 1.2 Hz. The column was made of Pyrex glass pipe, 5 cm i.d., 5.5 cm o.d., and 1 m in total length. The disengaging sections at the top and the bottom of the column were also made of Pyrex glass. The cartridge geometry is given in Table 1. The tie rod that supports the plate cartridge assembly was 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 4111 PLATE
A
Figure 2. Measurements of liquid-liquid-plate contact angle [Plate is wetted by heavy phase, h 0 (left), and by light phase, h < 0 (right)].
Figure 1. Schematic diagram of the experimental pulsed perforated-plate extraction column. Table 1. Cartridge Geometry ~~
column diameter effective height of column tie rod spacer plate spacing plate thicknesses sieve plate (SUS 304) nozzle plate (SUS 304) alumina plate silica plate hole diameter free area nozzle depth of nozzle plate
PLATE
5.5 cm 0.d. 5.0 cm i.d. 100.0 cm 0.6 cm 0.d. 1.0 cm 0.d. 0.64 cm i.d. 4.0 cm 8.0 cm 0.1 cm 0.1 cm 0.15 cm 0.15 cm 0.3 cm 20.0% 0.14 cm
attached to the column and was threaded into the plates and the spacers. The spacers were axially located t o support the plates. Both the tie rod and the spacers were made of stainless steel. The physical properties of liquids are summarized in Table 2. Dyestuffs. In order to obtain a direct visual observation, two types of dyestuff were used as visualization materials: oil red (C24H20N404) and methylene blue (C16HlsClN3S.nH20). The oil red was used for visualizing the dispersed organic phase, and the methylene blue was used for visualizing the dispersed aqueous phase. The dyestuffs only dissolved in their respective phase. The concentrations of the oil red and the methylene blue dyestuffs used were 3.1 and 1.6 mg/L, respectively. This concentration of the methylene blue was not observed to adsorb on the plate surface, while a blue coating of this dyestuff was reported to adhere on the plates (Prvcic et al., 1989). When a case of high concentration of the methylene blue was used as a tracer with both aqueous and organic phases, this chemical material has a trend to adsorb on the aqua-wettable materials, but the mechanism was not clear. Perforated-Plates. Both sieve and nozzle type plates were used for perforated-plates. The sieve plates with each hole indented becomes the nozzle plates, and the indentations cause each hole to act as a tiny jetting nozzle. The stainless steel sieve plates (sieve plates), the stainless steel nozzle plates (nozzle plates), the alumina sieve plates (alumina plates), and the silica
sieve plates (silica plates) with the same cartridge geometry were used in the column. The alumina plates were fine ceramic plates made from 92% A1203 alumina powder. The silica plates were made from a mixture of fine ceramic powders (Si02 50% and A1203 40%). Crystallization and formation of these fine ceramic powders were processed by a hot-press at for a certain time, i.e., both alumina and silica plates were formed using the hot-press under the condition of 14.7 GPa at 1600 "C. All the plates had 3 mm diameter circular holes arranged in a triangular pitch, resulting in a free area of 20%. Measurement of Contact Angles. Liquid-liquidplate contact angles were measured using the vertical plate method by means of analyses of photographs of the partially immersed plates. Figure 2 shows the reformed vertical plate method (Ikeda and Suzuki, 1992) applied to measure the liquid-liquid-plate contact angle. Using this reformed method for the liquidliquid-solid system, it is difficult t o accurately estimate the value of the contact angle, 8, directly from the photograph due to difficulties in correctly locating the tangent to the curved liquid-liquid-plate interface (dashed lines on Figure 2). Instead, enlarged photographs were used to obtain the value of the wet-line height, h , which then was applied t o the calculation of the contact angle. The contact angle measured by the vertical plate method is evaluated as follows: sin e = 1 - ( ~ ~ g h ~ / 2 a , , )
(1)
Equation 1is derived from the vertical plate method (Neumann, 1964), which is widely used to calculate the air-liquid-plate contact angle. The representative value of h used in the above equations were taken as being the average of the values measured at both surface and back surface of the perforated-plate. The accuracy of measurement of h by this method was enhanced by the use a reference scale mounted on the test plate; the scale also enabled correction of any distortion in the apparent linear distances that may have resulted from takJng the photographs through the curved glass wall of the column. The plate surfaces were cleaned by a similar treatment as is used for homogeneous surfaces because contact angles are easily affected by possible contamination of the plate surfaces. The procedure for the plate surface treatment, using the method outlined in Luke et al. (1987), is given in Table 3. Measurement of Axial Holdup Distribution. After reaching steady-state flow, i.e., when the continuous and the dispersed phases flow countercurrently, all of
4112 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 Table 2. Physical Properties of Liquids at 20 "C liquids density (kg/m3) viscosity (mPa.s)a 30% TBP in kerosene tributyl phosphate (TBP) kerosene acetic acid deionized water 30% TBP in kerosene-water
839.0 977.0 776.0 1049.3 998.0
1.84 3.41 1.40 1.22 1.00
surface tension (rnN/mlb
interfacial tension (mN/m)c
23.4 11.0 25.6 27.4 72.8 42.3
a Measured by the Cannon-Fenske viscometer (flow up type). Measured by a Kyowa CBVD-A3 surface tension meter. Measured by the drop weight method.
Table 3. Cleaning Procedure for Plate Surface Treatment all plates had both surface (1)scrubbed with soft brush in tap water with ABS (sodium alkylbenzenesulfonatej for 5 min ( 2j scrubbed in deionized water for 1 min (3) washed in an ultrasonic bath with ethyl alcohol for 5 min (4)washed in an ultrasonic bath with deionized water for 1 h (5) dried in an electric oven a t a temperature of 150 "C (Teflon plates temperature was 50 "C) (6) contact angles were measured within 3 h following the above cleaning procedure, while the clean surfaces stain easy by the organic contaminants in the atmosphere. All plates were kept in a desiccator following cleaning and prior to use.
Table 4. Preferential Wetting Method for Liquid-Liquid-Plate Contact Angle Case 1
(1)applied to the dispersed aqueous mode (2) plate soaked in the continuous organic phase for 1 h (3) plate then set upright in the continuous organic phase with the center of the plate placed a t the interface (4) liquid-liquid-plate contact angle measured by using the vertical plate method ( 5 )wet-line shape became downward-convex form, and the wet-line height had negative value, i.e., h < 0 (6) if the wet-line shape became upward-convex form, then the plate wettability changed fro? continuous organic phase to dispersed aqueous phase Case 2 (1)applied to the dispersed organic mode (2) plate soaked in the continuous aqueous phase for 1 h (3) plate then set upright in the continuous aqueous phase with the center of the plate placed a t the interface (4) liquid-liquid-plate contact angle measured by using the vertical plate method ( 5 ) wet-line shape became upward-convex form, and the wet-line height had positive value, i.e.,h > 0 (6) if the wet-line shape became downward-convex form, then the plate wettability is changed from continuous aqueous phase to dispersed organic phase
the inlet and outlet valves were then closed, and photographs were taken so as to measure the dispersed phase holdup. The perforated-plate of 0.3-cm-diameter hole size was used in this study, and then the phases do not flow through the plates after the isolation valves were closed. The experimental axial holdup in the mixer-settler region was defined as a percentage value of a stage volume between two perforated-plates occupied by the dispersed phase at steady-state operation, and the plateto-plate holdup was measured by direct visual observation with photographs. Data of axial holdup distribution for both alumina and silica plates were measured in this study, while the data for both sieve and nozzle plates were obtained in Ikeda et al. (1987a). Both dyestuffs, oil red and methylene blue, were used for this purpose.
Results and Discussion Plate Wettability of Liquid-Liquid-Plate Systems. A potential difficulty in measuring the liquidliquid-plate contact angle arises from the need to place the dry plate surface with respect to the liquid-liquid interface so as to ensure that the plate surface is wetted by the continuous phase prior to measuring the liquidliquid-plate contact angle using the procedure given below. This method is called the "preferential wetting method" by the authors (Ikeda and Suzuki, 1992). The liquid-liquid-plate contact angle was measured similarly to the air-liquid-plate contact angle by using the vertical plate method, although the wet-line shape
became a downward-convex form (Figure 2a) when the plate was wetted by the light phase (Table 4, case 1). The wet-line height, h, is thus negative in value in this case. When the plate was wetted by the heavy phase (Table 4, case 21, the wet-line shape became an upwardconvex form, and h became positive (Figure 2b). Thus, using the present method, the sign of h indicates the direction of the wet-line, i.e., as either an upward- or a downward-convex form. The contact angle is independent of the sign of h after eq 2. The liquid-liquid-plate contact angle was evaluated using eq 2 with the aid of photographs. The work of adhesion of the liquid-liquid-plate system was evaluated as follows:
Equation 2 is derived from the work of adhesion for the air-liquid-solid system (Neumann, 1964). The physical meaning of the work of adhesion, ISL,is the magnitude of the surface free energy that overcomes the interfacial tension to renew the solid surface and the liquid surface, i.e., the work of adhesion is the separation work to disassociate the liquid phase from the plate surface. Representative results are shown in Figure 3. In this figure, it was determined that the experimental values of the work of adhesion were larger for case 2 than for case 1. The time dependency for stability of plate wettability is also more prominent in case 2 when compared with case 1. For example, the wet-line height of the stainless steel plate was reduced by about 24.1%
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 4113 I
..
'
'."'I
.
'
'
'.."I
.
'
'
30%TBP in kerosene-water system
i.i 3
v)
-o-
Stainless steel - t - - Alumina - v Silica
Case 1
-
*- *---4-4 *-*-
4 ---:
2
*e-,
70
60
-
= =
All plates were wet by continuous organic phase: he0
........
I
1
........ I
.
. . .
consisting of a stainless steel plate cartridge is best suited for operation with the continuous organic phase. The alumina plate is also best suited for operation with the continuous aqueous phase. Required Pulse Velocity for Liquid-Liquid Dispersion. The required pulse velocity, i.e., the pulse power necessary to remove the liquid from the plate surface (''wettug phase"), is determined by the amplitude/ frequency, the physical properties of the liquid system, and the work of adhesion. From the motion of the wetting phase, which results from a sinusoidal input pulse into the column, the maximum pulse power, FSL, of a single pulse can be expressed as follows (see Appendix for derivation):
The required pulse velocity is then determined when this condition occurs: 90
t
1
(4)
Sample Calculation of Required Pulse Velocity.
-+- Stainless stee -*-Alumina - o - Silica -~
Case 2 'Or
6o
h>O: alumina and silica were wet by continuous aqueou
: h
