Mass transfer studies in a horizontal pulsed sieve-plate column for uranium extraction by tri-n-octylamine using axial dispersion model

Progress in Nuclear Energy xxx (2017) 1e14

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Mass transfer studies in a horizontal pulsed sieve-plate column for uranium extraction by tri-n-octylamine using axial dispersion model Pouria Amani a, Jaber Safdari b, *, Ahmad Gharib c, Hossein Badakhshan c, **, Mohammad H. Mallah b a b c

School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box: 11155-4563, Tehran, Iran Nuclear Fuel Cycle Research School, Nuclear Science and Technology Research Institute, P.O. Box: 11365-8486, Tehran, Iran Energy Engineering and Physics Department, Amirkabir University of Technology, P.O. Box: 15875-4413, Tehran, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 October 2016 Received in revised form 23 February 2017 Accepted 25 February 2017 Available online xxx

In this work, uranium extraction from an aqueous sulfate solution with 0.25M concentration acid, by using 5% v/v tri-n-octylamine (TOA) solvent, 90% v/v kerosene diluent and 5% v/v decanol modifier is studied. The effect of operation parameters (i.e., phase flow rates and pulsation intensity) on the volumetric overall mass transfer coefficients and axial dispersion in a horizontal pulsed sieve-plate column is evaluated using axial dispersion model (ADM). According to the results, increasing pulsation intensity and phase flow rates leads to the enhancement of the mass transfer performance. The continuous phase axial dispersion is found to be significantly higher than that in the dispersed phase. Axial dispersion of a phase is found to be mostly influenced by pulsation intensity and the flow rate of the phase itself and is minor influenced by the second phase flow rate. The overall height of mass transfer unit (HTUoc) is obtained to be less than 0.2 m, indicating high performance of a horizontal pulsed sieve-plate column for solvent extraction of uranium. Furthermore, new correlations are proposed for prediction of Koca, Ec and Ed, which are in satisfactory agreement with the experimental data with AARE values of 1.03%, 3.35% and 1.89% respectively. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Axial mixing Mass transfer Uranium extraction Horizontal pulsed plate column Axial dispersion model

1. Introduction It is of importance to extract uranium from its containing ores due to its radioactivity and toxicity risks on surface and ground water. Uranium has become an important actinides element for the industrial applications and nuclear energy plans especially from environmental perspective (Karve and Rajgor, 2008; Lothongkum et al., 2009). Solvent extraction is a classical separation process that is conducted in most of industries including petroleum refining, biochemistry, nuclear fuel processing, pharmaceuticals, metal extraction, food industry, and waste management (Akhgar et al., 2017; Amani et al., 2017). It is because of the fact that solvent extraction offers various advantages including high selectivity, high product purity, high recycling capability, low cost, simplicity of process and high production capacity (Benedict et al., 1981).

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Safdari), [email protected] (H. Badakhshan).

Generally, pulsed columns compared to mixer-settlers are desirable from both safety and economic perspective, in particular their higher throughput, less space consumption, no internal moving parts, minimum leakage, and simplicity of design. The advantages are more beneficial especially while processing corrosive or radioactive solutions since the pulsing unit can be remote from the column. Pulsed columns can be divided into the vertical and horizontal types. The vertical columns meet the needs for industrial applications, but when height limitation, especially in indoor applications (i.e., nuclear industry), is a concern it is required to use the horizontal columns. The design and optimization of an extraction column require determination of independent parameters including the crosssectional area and the column length in order to accommodate required flows without flooding as well as meeting the desired extraction performance. In addition, the axial dispersion in the dispersed and continuous phases are among the important parameters which leads to the reduction of mass transfer performance and consequently the enhancement of the required length for meeting desired extraction (Pratt and Baird, 1983). Many

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Please cite this article in press as: Amani, P., et al., Mass transfer studies in a horizontal pulsed sieve-plate column for uranium extraction by trin-octylamine using axial dispersion model, Progress in Nuclear Energy (2017), http://dx.doi.org/10.1016/j.pnucene.2017.02.010

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models have been proposed for predicting the mass transfer in solvent extractors, such as the axial dispersion, plug flow, the mixing stages, the forward mixing, the backflow and drop population balance models (Panahinia et al., 2017b). Among these models, axial dispersion model, which considers all non-ideal parameters in a single parameter based on Fick's law and the concentration profile, has been widely employed for mass transfer characteristics in extraction columns (Asadollahzadeh et al., 2017). Taking into account the great applicability of horizontal extraction column in industrial applications that we previously investigated the hydrodynamic and mass transfer of such columns (Khajenoori et al., 2015a, 2015b, Panahinia et al., 2017a, 2017b), this article concerns the applicability of a horizontal pulsed sieve-plate extraction column in solvent extraction of uranium. In addition, the effect of operating parameters including pulsation intensity and flow rates of phases on mass transfer performance and axial mixing parameters has been evaluated using axial dispersion model.

achieved that the operating costs in solvent extraction columns are lower because of smaller losses of solvent. Therefore, the installation of the Bateman pulsed columns in order to purify the uranium solutions has shown that these columns provide a beneficial advantage over the mixer-settlers. The approximate capital expenditure of the pulsed column was about 20% lower than that of similar size plants employing mixer-settlers (Jahya et al., 2009; Movsowitz et al., 1997). The successful use of pulsed columns in the uranium industry is leading to more interest in pulsed columns in order to extract other metals such as cobalt, zinc, nickel and copper. For instance, applying pulsed columns in the GORO Nickel project in New Caledonia has exhibited the entrance of pulsed columns into the base metals industry, resulting in application of pulsed columns for other metals (Vancas, 2003). 2. Experimental 2.1. Extraction column description

1.1. Previous work on uranium extraction Investigations conducted on uranium (VI) extraction from nitric acid solutions to ionic liquids can be classified into three distinctive mechanisms of the extraction (i.e., cation exchange, anion exchange (Chelating) and solvation) which depend on the nature of extractant, counterion concentration, structure of the ionic liquid as well as the aqueous phase composition (Dietz and Dzielawa, 2001; Wei et al., 2003). It has been revealed that liquideliquid extraction technique is the most effective separation method to extract and purify the solutions containing uranium (VI) in the nuclear industry (Billard et al., 2011). Various types of extractants have been used so far for purification of uranium which are given in Table 1. Most common extractants are di (2-ethyl hexyl) phosphoric acid (D2EHPA), tri n-butyl phosphate (TBP) and tri-n-octylamine (TOA) (Datta et al., 2016; Yan-Zhao et al., 2003; Zhijun et al., 2003). However, TOA is an environmentally-friendly extractant which has been used in many applications due to its high selectivity and efficiency especially for uranium recovery from various sources (Goldenberg and Abbruzzese, 1983). Mixer-settlers and columns are the only extraction equipment which have been used so far in the industrial applications for solvent extraction of uranium (Movsowitz et al., 1997). The only published type of columns applied in uranium solvent extraction is the Bateman pulsed column and only two reports are available in the literature which compare the performance of extraction between a pilot plant and industrial columns (Miller and Kleinberger, 2000; Movsowitz et al., 2000). Both reports deal with the Bateman pulsed columns: one involves in extraction of uranium in Olympic Dam, Australia which presented by Movsowitz et al. (2000) and the other one is focused on production of phosphoric acid by extraction in Haifa Chemicals, Israel, which proposed by Miller and Kleinberger (2000). According to Movsowitz et al. (2000), the performance of the columns was excellent and the extraction yield was similar to the 4 mixer-settlers in series - about 98%. It is

The experiments were conducted in a semi-industrial horizontal pulsed sieve plate column. The active part of column was a pipe housing an internal plate cartridge including 25 pair of sieve plates made from 304 stainless steel. Pulsations were provided by applying a fluctuated air pressure (controlled by two solenoid valves) to the air space above a liquid leg connected to the bottom settler of the extraction column. A 9 cm-diameter settler was installed on both ends of the column in order to separate two phases. The interface between the light and heavy phases in the upper settler was controlled by an optical sensor, which was connected to a solenoid valve in outlet streamline of downer settler. A schematic diagram of the setup is given in Fig. 1 and geometrical characteristics of the column are listed in Table 2. 2.2. Chemicals and reagents The aqueous solution was a leach liquor supplied from the Bandar Abbas Uranium Production Plant (BUP) with the concentration of 196 ppm. Inductively Coupled Plasma (ICP) used for analysis of the leach liquor that results is presented in Table 3. 5% v/ v tri-n-octylamine (TOA) solvent, 90% v/v kerosene diluent and 5% v/v decanol modifier were the dispersed phase. The commercial extractor tri-n-octylamine was prepared from Sigma Aldrich Company. Decanol and Kerosene (non-aromatic) were used as diluents without further purification. The heavy and light interfacial tension measurements were obtained via a Krüss tensiometer. Density and viscosity of organic and aqueous phases were measured by the picometer method and with DVI-Prime viscometer respectively. Physical properties of the phases are listed in Table 4. It should be noted that, under mass transfer conditions, a degree of uncertainty surrounds the estimation of physical properties (particularly interfacial tension), since these vary not only with the inlet solute concentrations, but also along the column. In the present research, the values of physical properties have been

Table 1 Types of extractants for uranium extraction. Cationic extractant

Anionic extractant

Neutral extractant

High density polyethylene b-diketones Cyanex 272 Cyanex 301 Cyanex 302 Hydroxyl oximes Thi-ocarbazones

Alamine 336 Tri-laurylamocum chbride Di etrimethyl hexyl ammonium chloride Aliquat 336 Tri-dentate diglycolamide 2-DGA Tri-n-octylamine

Tri-n-butyl phosphate Tri-n-octylphosphine oxide Tri-butylphosphine oxide Cyclic polyether di-benzo-18 -crown-6 (DBC) Cyanex 921 Cyanex 923 Cyanex 925

Please cite this article in press as: Amani, P., et al., Mass transfer studies in a horizontal pulsed sieve-plate column for uranium extraction by trin-octylamine using axial dispersion model, Progress in Nuclear Energy (2017), http://dx.doi.org/10.1016/j.pnucene.2017.02.010

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Fig. 1. A schematic diagram of the horizontal pulsed sieve plate column.

Table 2 Geometrical characteristics of the column used.

Table 4 Physical properties of the phases used in the system, at 25  C.

Column length (m) Column diameter (cm) Material of construction the column Upper and lower settler diameter (cm) Upper settler length (cm) Lower settler length (cm) Material used for plates, spacers and rod Plates thickness (mm) Holes diameter (mm) Holes pitch (mm) Plate spacing (cm) Average free area of the plates (%) a b

1.46 6.2 glass 9 60 30 Stainless steel 1 2 4 1 a , 5b 0.11

Liquid phase

r kg/m3

m  103 kg.s/m

s  103 N/m

Aqueous phase Organic phase

1016.8 739.8

1.02 1.476

6

uranium concentration was obtained by averaging the values obtained at the inlet and outlet of the column. The equilibrium uranium concentrations were experimentally measured through batch experiments. Potassium hydroxide (KOH) and sulfuric acid (H2SO4) supplied by Merck Company were used to adjust the pH.

Spacing between two individual plates in a cell. Spacing between two adjacent cells.

2.3. Experimental procedure Table 3 ICP analysis of the leach liquor supplied from the Bandar Abbas Uranium Production Plant (BUP). Elements

Concentration (ppm)

Uranium (U) Zinc (Zn) Chromium (Cr) Aluminum (Al) Molybdenum (Mo) Nickle (Ni) Copper (Cu) Cobalt (Co) Calcium (Ca) Beryllium (Be) Thorium (Th) Zirconium (Zr) Vanadium (V) Magnesium (Mg) Titanium (Ti) Tungsten (Ta) Niobium (Nb) Manganese (Mn) Thallium (Ta) Scandium (Sn) Platinum (Pt) Iron (Fe)

196 8.65 2.95 1155.1 0.825 3.47 9.25 6.55 895.6 <0.5 6.62 0.77 1.34 5971 1 2.5 2.5 1591.6 2.5 2.5 2.5 1060.6

assumed to correspond to the mean values of uranium concentration in the continuous and dispersed phases. The mean value of

For conducting the experiments, the active part of the column was firstly filled by aqueous uranium sulfate solution, followed by TOA þ kerosene þ decanol (dispersed phase) entering to the column. Next, flow rates of each phase as well as the pulse amplitude and frequency were set to the considered values. When the steady state condition was established after about 60e90 min depending on the phase flow rates, 10e20 ml samples were taken from 7 sampling valves which were equidistant from each other and distributed along the column. The amount of uranium extracted by organic phase was measured after recovery by sodium carbonate 1M in order to analyze the uranium concentration in both phases by inductively coupled plasma-atomic emission spectroscopy (ICPAES). Regarding the measurement of the drop sizes, they were determined by a photographic approach using Nikon D5000 digital camera. Drop dimensions were measured with AutoCAD software. Nearly 800 drops were measured in each experiment to guarantee the statistical significance of the determined Sauter mean drop diameter. The observed drops have mainly spherical shapes, but in some cases ellipsoidal shapes were observed which were determined by their major axis (dH), and their minor axis (dL). Therefore, in order to measure the drop distortion, the drop diameter with an equivalent sphere was determined by Eq. (1).

di ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 dH;i 2 dL;i :

(1)

The Sauter mean drop diameter, d32, was then calculated by Please cite this article in press as: Amani, P., et al., Mass transfer studies in a horizontal pulsed sieve-plate column for uranium extraction by trin-octylamine using axial dispersion model, Progress in Nuclear Energy (2017), http://dx.doi.org/10.1016/j.pnucene.2017.02.010

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using the following relation:

Pn

d32 ¼ Pi¼0 n i¼0

ni d3i ni d2i

;

(2)

where ni is the number of drops with diameter di. Fig. 2 shows two photo of dispersed phase drops in the column. The following objective function, the Average Absolute Relative Error (AARE), was used to calculate the fitted parameters:

AARE ¼

   dx dx Sð1  4Þ  rc uc x þ Ec Sð1  4Þ dz zþdz dz z    rc Koc a x  x* Sdz



rc uc x þ Ec

X jXi ðexpÞ  Xi ðtheoÞj 1 NDP ; NDP i¼1 Xi ðexpÞ

(3)

where NDP represents the number values of data points and Xi ðexpÞ and Xi ðtheoÞ represents the experimental and theoretical data, respectively. For measurement of the dispersed phase holdup, displacement approach was used. At steady state condition, pulsation and flow rates were stopped and the volume fraction of the organic phase was determined after complete settlement of each phases by calculating the arc length of the part of the circumference wetted by the phases. The cord length at the interface (Li) was determined from Eq. (4):

   1 s ; Li ¼ 2ri cos p 2 ri

(4)

where ri indicates the radius and s represents the circumference. Therefore, the dispersed phase holdup was then measured from the cord length by using Eqs. (5) and (6):



1 Li Ao ¼ pri2  ri2 cos1 2 2ri



Ao 4o ¼ 2 : pri

1=2 L  i ri2  x2 ; 2

¼ 0;

(7)

   dy dy S4  rd ud y þ Ed S4 dz z dz zþdz   þ rd Koc a x  x* Sdz



rd ud y þ Ed ¼ 0;

(8)

where x and y represent the weight fraction of solute concentrations in the continuous and dispersed phases respectively. Ec and Ed are the continuous and dispersed phases axial mixing coefficients. Qc and Qd are the volumetric flow rates of the continuous and dispersed phases. Koc represents the overall mass transfer coefficient of extraction based on the continuous phase. x* denotes the continuous phase solute concentration in equilibrium with the organic phase, S is the column cross-sectional area, and a is the specific interfacial area determined by Eq. (9):

a¼

64 d32

(9)

The last terms in Eqs. (7) and (8) represent the solute mass transfer from the continuous phase to the dispersed phase. Rearrangement and normalization of Eqs. (7) and (8) generate the following second order ordinary differential equation (ODE) set:

1 d2 X dX  NTUoc ðX þ Y  1Þ ¼ 0 ; þ Pec dZ 2 dZ

(10)

1 d2 Y dY NTUoc   ðX þ Y  1Þ ¼ 0 ; Ped dZ 2 dZ U

(11)

(5)

(6) where X and Y are dimensionless solute concentrations of the continuous and dispersed phases respectively. It should be noted that in Eqs. (10) and (11) U ¼ KD Qd rd =Qc rc , Pec ¼ uc H=Ec , Ped ¼ ud H=Ed and NTUoc ¼ H=HTUoc ¼ HKoc a=uc . The following open-open boundary conditions were used for the second order equation sets of (10) and (11) at the both end of the column:

3. Axial dispersion model Based on the axial dispersion model and material balance in the column, the equation set for the continuous and dispersed phases was established as follows (Pratt and Baird, 1983):

Z ¼ 0 ; / X ¼ 0 and Y ¼ Yout

Fig. 2. Two photos of droplets for the steady state conditions.

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Z ¼ 1; / X ¼ Xout and Y ¼ 0

should be noted that the characteristic velocity is a pivotal factor for determination of flooding velocities in extraction columns.

The central finite difference approach was performed to numerically solve the set of Eqs. (10) and (11). In the present work these calculations were done using a code developed on MATLAB Software.

4.1. Effects of operating parameters on the steady state dimensionless concentration profile along the column length

4. Results and discussion As previously mentioned by Melnyk et al. (1992), three operating regimes can be characterized in horizontal pulsed sieve-plate column: mixer-settler, pseudo-dispersion and emulsion, with respect to the pulsation intensity. When pulsation intensity is low, column operates in mixer-settler regime. With further increase in pulsation intensity, shear forces increase and the column tends to operate in dispersion regime. However, at high pulsation intensities, uniform distribution of dispersed phase drops occurs in each compartment of the column and drops retardation approaches to almost zero underneath sieve plates. Since the most efficient stable regime for extraction columns is the dispersion regime, in this study all the experiments are carried at in this regime. Exploratory investigations are conducted to determine the hydrodynamics and flooding conditions under different operating parameters including pulsation intensity and flow rates of continuous and dispersed phases. The dispersed phase holdup data are used for evaluation of slip velocity which is a key parameter for determination of characteristic velocity of drops and can be calculated by Eq. (12) as follows (Logsdail and Thornton, 1957):

Vs ¼

Vd Vc þ ¼ V0 ð1  4Þ; 4 14

(12)

where Vs and V0 are slip and characteristic velocities. Characteristic velocity can be generally obtained from experimental Vc, Vd and 4 by plotting Vd þ Vc 4=ð1  4Þ versus 4ð1  4Þ. The characteristic velocity plot for some experiments is illustrated at Fig. 3, by varying continuous phase flow rate from 0.3  106 m3/s to 1.38  106 m3/ s at constant dispersed phase flow rate of 0.58  106 m3/s and pulsation intensity of 0.95 cm/s. A linear plot through the origin between Vd þ Vc 4=ð1  4Þ and 4ð1  4Þ is achieved which indicates the validity of the Thornton equation. The characteristic velocity V0 is calculated to be 0.0017 m/s from the slop of the line in Fig. 3. It

Fig. 3. Characteristic velocity plot for the system water/uranium/TOA/kerosene under varying continuous phase flow rate from 0.3  106 m3/s to 1.38  106 m3/s at constant dispersed phase flow rate of 0.58  106 m3/s and pulsation intensity of 0.95 cm/s.

The influence of the ratio of continuous phase to dispersed phase flow rate along with the effect of pulsation intensity on the dimensionless concentration profile of uranium along the column length are shown in Figs. 4 and 5. Smoot and Babb (1962) studied the effect of operating parameters on mass transfer and solute concentration gradients in a vertical pulsed sieve-plate column using methyl isobutyl ketone-acetic acid-water and 1,1,2trichloroethane-acetone-water systems and observed the longitudinal concentration profile in the column. According to Henschke and Pfennig (2002), the overall impact of the continuous phase counter flow through the perforations of plates and the shear forces along with drops breakage increases the mass transfer performance in extraction column. Moreover, Lade et al. (2014) compared the effect of varying flow rate of continuous and dispersed phase on mass transfer performance and revealed that the influence of continuous phase flow rate is more profound in comparison with that of dispersed phase flow rate due to increasing continuous phase axial mixing with increasing its flow rate which is proved to be a key parameter on mass transfer performance. 4.1.1. Effect of continuous to dispersed phase flow rate ratio (R ¼ Qc/ Qd) The influence of the ratio of continuous phase to dispersed phase flow rate on the variation of uranium concentration along the column length is shown in Fig. 4 and discussed in two parts. First is the study of concentration profile under different dispersed phase flow rate (0.3  106 to 1.38  106 m3/s, i.e., R ¼ 1.91e0.42) at constant flow rate of continuous phase (0.58  106 m3/s) and pulsation intensity (0.95 cm/s). When the dispersed phase superficial velocity increases from 0.3  106 to 1.38  106 m3/s, the efficiency of uranium extraction enhances from 94.0% to 96.8%. Conversely, it is obtained that when phase flow ratio decreases from 1.91 to 0.42, the mass transfer performance will increase. As can be seen in Fig. 4a and b, the uranium concentration profile along the column length shows overlapping trend and becomes undifferentiated for different dispersed phase flow rates. It demonstrates that the dispersed phase flow rate has an insignificant impact on the axial dispersion in the column. The comparison between the dispersed and continuous phase axial mixing coefficients are presented in Section 4.2. Second, the variation of concentration under different continuous phase flow rate (0.3  106 to 1.38  106 m3/s, i.e., R ¼ 1.91e0.42) at constant superficial velocity of dispersed phase (0.58  106 m3/s) and pulsation intensity (0.95 cm/s) is presented in Fig. 4c and d. It should be noted that the uranium output concentration in the dispersed phase remains constant at the operating conditions and tends to obtain equal amount of uranium from the continuous phase. In fact, an increase in the continuous phase flow rate leads to the reduction of extraction efficiency and consequently decreasing the uranium concentration gradients. The variation of uranium concentration along the column length is more monotonous with the enhancement of continuous phase flow rate, which indicates profound impact of the continuous phase axial mixing on mass transfer performance. 4.1.2. Effect of pulsation intensity The uranium concentration variation along the column length under different pulsation intensities (0.80e1.30 cm/s) at a constant continuous to dispersed phase flow ratio (R ¼ 1) is illustrated in

Please cite this article in press as: Amani, P., et al., Mass transfer studies in a horizontal pulsed sieve-plate column for uranium extraction by trin-octylamine using axial dispersion model, Progress in Nuclear Energy (2017), http://dx.doi.org/10.1016/j.pnucene.2017.02.010

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Fig. 4. Effect of continuous to dispersed phase flow rate ratio (R ¼ Qc/Qd) on the variation of uranium concentration in (a), (c) continuous phase and (b), (d) dispersed phase, along the column length.

Fig. 5. Effect of pulsation intensity on the variation of uranium concentration in (a) continuous phase and (b) dispersed phase, along the column length.

Fig. 5. At pulsation intensity of 1.30 cm/s the uranium concentration of the continuous phase declines from 192.1 to 2.18 ppm, while at a lower pulsation intensity of 0.80 cm/s the uranium concentration decreases from 193.3 to 13.16 ppm along the column length. In fact, when pulsation intensity increases, mass transfer performance enhances due to the variation of drop sizes and holdup along with axial mixing, which is thoroughly discussed in Section 4.2. Therefore as can be seen in Fig. 5, the higher pulsation intensity, the steeper decline in the uranium concentration of the continuous phase can be observed, which is due to the higher deformation and coalescence of the dispersed phase drops at higher pulsation intensities. The formation of smaller drops leads to some mixing within the droplet including circulation and recirculation, and consequently more disturbance in the radial concentration

gradient, which enhances the mass transfer performance along the column. 4.2. Effect of operating parameters on volumetric mass transfer and axial mixing coefficients The objective of applying ADM is to simulate the uranium concentration profile along a horizontal pulsed sieve-plate column, corresponding to the impact of the operating parameters on the uranium extraction along the column length. In this study, the effect of operating parameters including pulsation intensity and flow rates of the continuous and the dispersed phases on mass transfer and axial mixing coefficients is investigated. The operating conditions of the experiments performed for mass transfer modelling

Please cite this article in press as: Amani, P., et al., Mass transfer studies in a horizontal pulsed sieve-plate column for uranium extraction by trin-octylamine using axial dispersion model, Progress in Nuclear Energy (2017), http://dx.doi.org/10.1016/j.pnucene.2017.02.010

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along with uranium extraction efficiency for each run are presented in Table 5 and the values of mean drop size and dispersed phase holdup as well as the height of mass transfer unit (HTU) for each experiment are presented in Table 6. Accordingly, an attractive low value of HTUoc less than 0.2 m is achieved, which demonstrates high performance of a horizontal pulsed sieve-plate column for uranium extraction. Typical variation of measured concentration of continuous and dispersed phases versus those measured by ADM is also illustrated in Fig. 6 for a constant continuous to dispersed phase flow ratio (R ¼ 1) and pulsation intensity (Af ¼ 0.95 cm/s), which shows satisfactory agreement between the experimental data can those calculated by ADM. 4.2.1. The effect of pulsation intensity on mass transfer and axial dispersion The effect of pulsation intensity on volumetric overall mass transfer coefficient (Koca) and axial dispersion coefficients of the continuous and dispersed phase is shown in Fig. 7(a, b and c) at constant dispersed and continuous phase flow rate of 0.58  106 m3/s. According to Table 6, high pulsation intensity results in the reduction of mean drop size and dispersed phase holdup due to the intense frequency of drops collisions with internal plates. Since, decrease in holdup has more profound impact on interfacial area according to Eq. (9), the interfacial area available for mass transfer slightly decreases when pulsation intensity increases. However, more turbulence mixing environment, narrower drop size distribution and considerably improvement of mass transfer driving force at higher pulsation intensities compensate the slight reduction of interfacial area and leads to the enhancement of volumetric overall mass transfer coefficient via increasing pulsation intensity. It is in accordance with many research in various extraction columns. For instance, Harikrishnan et al. (1994) for a reciprocating-plate column and Jiao et al. (2013) for a vertical pulsed sieve-plate column revealed that overall mass transfer coefficient will increase with increasing power input, but with further increase in pulsation intensity, it is achieved that Koca will decreases after passing a maximum value because of appreciable axial mixing (Safari et al., 2012). The influence of pulsation intensity on continuous and dispersed phase axial dispersion is shown in Fig. 7(b and c). It is observed that when pulsation intensity increases, the axial dispersion of dispersed and continuous phase increase. It is because of high population density of drops and the vortex of the continuous phase inside the column in more turbulence conditions (Srinikethan et al., 1987). Moreover, from comparing the values of continuous and dispersed phase axial dispersion coefficients, it can be obtained that the continuous phase axial mixing is considerably higher than the dispersed phase axial dispersion which is in qualitative agreement with previous research on various extraction columns (Asadollahzadeh et al., 2017; Kumar and Hartland, 1988; Tang et al., 2004).

7

Table 6 The values of mean drop size and holdup with the AARE of obtained data by ADM. Run

1 2 3 4 5 6 7 8 9 10

Af (cm/s)

0.95 0.95 0.95 0.95 0.95 0.95 0.80 0.95 1.10 1.30

Qc/Qd

0.52 1.71 2.38 1.91 0.58 0.42 1.00 1.00 1.00 1.00

d32 (mm)

1.0 1.1 1.2 1.0 1.2 1.3 1.1 1.0 0.8 0.7

Holdup

0.16 0.21 0.23 0.20 0.29 0.32 0.26 0.22 0.17 0.13

HTUoc (m)

0.050 0.115 0.144 0.107 0.057 0.048 0.088 0.078 0.071 0.065

%AARE X

Y

12.5 7.3 8.7 7.8 8.4 12.5 9.6 8.2 9.3 10.3

11.5 8.1 6.8 7.0 5.8 12.0 6.7 7.6 8.4 11.9

Fig. 6. Solute concentration variations in two phases versus column height (Qc ¼ 0.58  106 m3/s, Qd ¼ 0.58  106 m3/s, Af ¼ 0.95 cm/s).

4.2.2. The effect of dispersed phase flow rate on mass transfer and axial dispersion The influence of the dispersed phase flow rate on volumetric overall mass transfer coefficient and axial dispersion is shown in Fig. 8 at constant pulsation intensity of 0.95 cm/s and continuous phase flow rate of 0.58  106 m3/s. Generally, mean drop size and holdup increase with the enhancement of flow rate of the dispersed phase (see Table 6), due to intense drops coalescence and increasing drag forces between the bulk phase and dispersed phase droplets. However, the influence of holdup growth on the specific interfacial area is dominant than the effect of mean drop size based on Eq. (9), which leads to an increase in the interfacial area and mass transfer driving force with increasing dispersed phase flow

Table 5 Operating conditions, uranium concentrations and extraction efficiency. Run

Af (cm/s)

Qc  106 (m3/s)

Qd  106 (m3/s)

Cc,in (ppm)

Cc,out (ppm)

Cd,in (ppm)

Cd,out (ppm)

% Extraction efficiency

1 2 3 4 5 6 7 8 9 10

0.95 0.95 0.95 0.95 0.95 0.95 0.80 0.95 1.10 1.30

0.31 1.00 1.39 0.58 0.58 0.58 0.58 0.58 0.58 0.58

0.58 0.58 0.58 0.31 1.00 1.39 0.58 0.58 0.58 0.58

193.0 192.8 194.0 195.0 191.2 193.3 193.3 194.5 192.1 192.1

8.2 11.1 13.2 11.6 7.6 6.1 10.0 10.4 8.9 8.5

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

135.7 430.5 595.5 471.6 146.3 107.3 251.9 253.0 251.8 252.3

95.75 94.25 93.20 94.05 96.02 96.84 94.82 94.65 95.37 95.57

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Fig. 7. Variation of (a) volumetric overall mass transfer coefficient, (b) continuous phase and (c) dispersed phase axial dispersion coefficients as a function of pulsation intensity.

rate. As a consequence, the Koca will increase. However, it should be noted that according to Kumar and Hartland (1999) and a review conducted by Yadav and Patwardhan (2008) the most effective operating parameters on mass transfer performance are pulsation intensity and dispersed phase flow rate and in most research it is achieved that the impact of pulsation intensity is more profound, while in this research, the effect of dispersed phase flow rate is greater on overall mass transfer coefficient which is due to the inverse variation of holdup in a horizontal pulsed sieve-plate column compared to other types of extractors when power input increases. The effect of dispersed phase flow rate on the axial dispersion coefficients is presented in Fig. 8(b and c). It is observed that increasing dispersed phase flow rate leads to the enhancement of dispersed phase axial mixing, while it has negligible impact on continuous phase axial dispersion. Insignificant influence of dispersed phase flow rate on Ec is in agreement with previous research on various extraction columns (Kumar and Hartland, 1988; Sehmel and Babb, 1964; Srinikethan et al., 1987). Moreover, it should be noted that most investigations have been conducted on continuous phase axial dispersion and the impact of dispersed phase axial dispersion is assumed to be neglected because of its insignificant value. However, Din et al. (2008) and Srinikethan et al. (1987) are some the researchers who have studied the axial dispersion of the dispersed phase in a vertical pulsed sieve plate extraction column and a reciprocating plate column and their results also revealed that increasing dispersed phase flow rate leads to the reduction of dispersed phase axial dispersion. 4.2.3. The effect of continuous phase flow rate on koca and Ec The influence of continuous phase flow rate on volumetric overall mass transfer coefficient and axial dispersion is illustrated

in Fig. 9(a, b and c) at constant pulsation intensity of 0.95 cm/s and dispersed phase flow rate of 0.58  106 m3/s. The Sauter-mean drop diameter and the dispersed phase holdup increase with the enhancement of the continuous phase flow rate due to the increment of drag force between the continuous and dispersed phases. According to Eq. (9), the impact of increasing holdup is greater than that of mean drop size on the specific interfacial area which consequently results in the enhancement of interfacial area, thereby increasing mass transfer performance. Moreover, it should be noted that regarding the results from the work of Jiao et al. (2013), with further increase in Qc, Koca reaches a maximum value due to the fact that the faster the continuous phase flow rate, the shorter the phases contact. As a consequence, the aggravation of the turbulence induced by the higher continuous phase flow rate causes a tendency of the significant enhancement of the continuous phase axial dispersion and the reduction of maximum throughput. Therefore, the overall volumetric mass transfer coefficient will decline with further increase in the continuous phase flow rate. The comparison between the variation of overall mass transfer coefficient with dispersed and continuous phase flow rate reveals that the impact of flow rate of continuous phase is less than that of dispersed phase on Koca, which is in accordance with previous investigations (Kumar and Hartland, 1999). It can be referred to the variation of mean drop size and holdup under different dispersed and continuous phase flow rate, since it can be observed in Table 6 that dispersed phase flow rate has more profound impact on dispersed phase holdup and mean drop size compared to that of continuous phase and according to Eq. (9) it leads to higher enhancement of interfacial area for mass transfer. Furthermore, the effect of continuous phase flow rate on axial dispersion in continuous and dispersed phase is shown in Fig. 9(b and c). It is observed that increasing continuous phase flow rate

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P. Amani et al. / Progress in Nuclear Energy xxx (2017) 1e14

9

Fig. 8. Variation of (a) volumetric overall mass transfer coefficient, (b) continuous phase and (c) dispersed phase axial mixing coefficients versus dispersed phase flow rate.

leads to the reduction of continuous phase flow rate, while it has insignificant impact on the dispersed phase flow rate. Generally, it is obtained that the increment of continuous phase flow rate is found to provide more uniformly formation of the dispersed phase droplets which results in a decrease of axial dispersion in continuous phase. In fact, the enhancement of the mass transfer coefficient can be attributed to the reduction of continuous phase flow rate. Wang et al. (2006) for a pulsed disc & doughnut column and Din et al. (2008), Sehmel and Babb (1964), Srinikethan et al. (1987) and Kumar and Hartland (1988) for a vertical pulsed sieve-plate column also revealed that continuous phase axial dispersion varies inversely with the continuous phase flow rate.

4.3. Comparison of previous correlations for prediction of mass transfer and axial dispersion coefficients Firstly, it should be noted that no correlation of volumetric overall mass transfer coefficient is available for a horizontal pulsed sieve-plate column in the literature and all of correlations are proposed for vertical columns. However, a number of correlations for Koca in vertical columns reported in the literature are taken from the work of Smoot et al. (1959), Logsdail and Thornton (1957), Smoot and Babb (1962), Luo et al. (1998), He et al. (2004) and Safari et al. (2012) in order to make a comparison with the experimental data. Fig. 10 is provided for comparison of experimental results of Koca with those calculated by the correlations presented in Table 7. The Average Absolute Relative Error (AARE) is used to evaluate the accuracy of current work. According to Table 7, it can be attained that none of the previous correlations are suitable for determination of Koca in a horizontal pulsed sieve-plate column.

The disagreements between calculated and experimental data are due to the fact that all previous studies available in the literature were conducted in other types of extractors and there is no published investigation on the applicability of uranium extraction in a horizontal column. In addition, Yadav and Patwardhan (2008) have revealed that none of reported correlations for vertical columns are even accurate for calculation of Koca beyond the set of data in other vertical pulsed columns. In fact, higher values of experimental data can be referred to the fact that horizontal pulsed sieve-plate column operated attractively efficiently with HTUoc values less than 0.2 m which consequently offers high values of Koca compared to that of vertical columns. Moreover, some correlations reported in the literature for prediction of axial mixing coefficients were taken from the work of Kagan et al. (1973), Miyauchi and Oya (1965), Tung and Luecke (1986), Kumar and Hartland (1988), Sehmel and Babb (1964), Srinikethan et al. (1987) and Safari et al. (2012) in order to compare with the experimental data and represented in Table 8. According to Table 8, it is observed that none of the previous correlations are suitable for determination of Ec and Ed in a horizontal pulsed sieve-plate column. Poor predictive ability of these correlations can be referred to the fact that the many correlations proposed for prediction of continuous and dispersed phase axial dispersion parameters have limited applications, as they apply only to the particular systems and column geometries for which they were formulated and there is no available correlation proposed for horizontal pulsed sieve-plate columns. Furthermore, by comparing data from the work of Kagan et al. (1973), Nemecek and Prochazka (1974), Baird (1974), and Matsumoto et al. (1989), who have all studied the axial dispersion in pulsed sieve-plate column, it is

Please cite this article in press as: Amani, P., et al., Mass transfer studies in a horizontal pulsed sieve-plate column for uranium extraction by trin-octylamine using axial dispersion model, Progress in Nuclear Energy (2017), http://dx.doi.org/10.1016/j.pnucene.2017.02.010

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P. Amani et al. / Progress in Nuclear Energy xxx (2017) 1e14

Fig. 9. Variation of (a) volumetric overall mass transfer coefficient, (b) continuous phase and (c) dispersed phase axial mixing coefficients versus continuous phase flow rate.

axial dispersion coefficients in a horizontal pulsed sieve-plate column is one of the aims of the current study. Therefore, the following correlations for calculation of Koca, Ec and Ed as a function of operating parameters including pulsation intensity as well as continuous and dispersed phase superficial velocities are proposed by dimensionless analysis method:



mc Koc a Af ¼ 2:3  105 2 uc rc ðAf Þ Ec rc uc

s

¼ 1:562  104

0:288  0:564   Af s 0:545 ; ud Af mc

(13)

 0:537  0:069   Af Af s 2:192 ; uc ud Af mc (14)

Ed rd ud

s

Fig. 10. Comparison of experimental Koca with previous models for pulsed columns.

achieved that their experimental data were also dissimilar and some data were scattered one. 4.4. Predictive correlation for axial mixing and mass transfer coefficients Development of new correlation for overall mass transfer and

 ¼ 51:697

Af uc

0:029 

Af ud

0:734 

s Af mc

1:556

:

(15)

The predictive anility of represented correlations are illustrated in Fig. 11. Accordingly, Eqs. (13), (14) and (15) reproduce the experimental data with the AARE values of 1.03%, 3.35% and 1.89% respectively, which show high accuracy of proposed equations. It should be noted that these correlations are derived for uranium extraction with the concentration of 196 ppm and further investigations are needed to evaluate the influence of different uranium concentrations on the mass transfer performance. In addition, physical properties of the chemical system (particularly interfacial tension) vary not only with the inlet solute concentrations but also along the column. Therefore, it is expected that the proposed

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P. Amani et al. / Progress in Nuclear Energy xxx (2017) 1e14

11

Table 7 The AARE values between the predicted values of Koca obtained by the previous correlations and the experimental data. Reference (Smoot et al., 1959)

Equation  Koc ah uc

¼ 0:20

Afdo rd

0:434  1:04 

"  0:33 (Logsdail and Thornton, 2 Koc a ¼ uc b gmrc 2 1957)

(Luo et al., 1998)

s mc uc

0:096  0:636  0:317  4:57 1 ud uc

Dr rc

u0 3 ð14Þ rc 3

ud uc

mc md

D h

!2n=3   2ðn1Þ=3  0:5 

mc g

c

(Smoot and Babb, 1962)

md rd Dv

Dr rd

amd

0:865 

uc 3 rc gmc 43

0:33 #1

 0:4  0:43  0:56  0:62 1 uc uc do Koc a ¼ 504  uc h Af dmo rd ud Af h d  0:26 

1  1=3 0:17 2 2 mc mð1þmd =mc Þ mc rc 2 s3 uc 1 4 Dr g Koc a ¼ HTU ¼ K 4ð14Þ ; K ¼ K1 =0:52K1 ¼ 33mmd þ2 n 60:00375 n ¼ 0:67 þ 0:028 m 225 r m þ3m m 4 Drg oc d

c

c

c

d

(He et al., 2004)

u0:2404 ðAf Þ0:5536 ; Dc ¼ 0:6mKoc a ¼ 1:4296 u0:6685 u0:2404 ðAf Þ0:5536 ; Koc a ¼ 5:1705 u0:6685 c c d d

(Safari et al., 2012)

u0:2404 ðAf Þ0:5536 ; Dc ¼ 0:1m Dc ¼ 0:05mKoc a ¼ 1:8125 u0:6685 c d  0:09  1:62 1:78 uc Koc a ¼ 1:9  108 rdc us Re 2 Af ud us

c

ð14Þ

32

Column type

AARE

Pulsed sieve plate column

75.0%

Pulsed sieve plate column

78.7%

Pulsed sieve plate column

92.4%

Pulsed sieve plate column

73.8%

Pulsed sieve plate column

98.1%

Pulsed packed column

99.1%

Table 8 The AARE values between the predicted values of Ec and Ed obtained by the previous correlations and the experimental data. Reference (Kagan et al., 1973) (Miyauchi and Oya, 1965) (Miyauchi and Oya, 1965) (Tung and Luecke, 1986) (Kumar and Hartland, 1988)

Equation Ec ð1  4Þ ¼

c Ec ¼ 0:250 ahu 1:30

Ec Dr

198.7%

Pulsed sieve plate column

146.1%

Pulsed sieve plate column

98.5%

Pulsed sieve plate column

74.2%

j¼



Pulsed sieve plate column

89.1%

Pulsed sieve plate column

151%

Pulsed sieve plate column

212%

 0:36  0:47

Pulsed packed column

99.3%

 0:62  0:81

Pulsed packed column

226%

 0:565  0:606

Af ðAf Þm ðAf Þm

!0:61 

3 

(Safari et al., 2012) (Safari et al., 2012)

do h

u m 0:11  m 0:37 c d c

Drh r * h*

2

md

s

0:36 

Af ðAf Þm ðAf Þm

1:05

!0:33 1=4 a ðAf Þm ¼ 9:69  103 sDr 3=4 m d

      mc 1:45 t 0:70 h 0:68 Ec ð1  4Þ ¼ 0:171 uc do do t r c uc t      0:36  0:07 ud rc t 0:30 src t 0:42 f rc t 2 A  mc mc t mc 2  0:3  0:2  0:45 mc Af Ec do 0:22 ¼ 3:65 uc a hu u dr h c

(Srinikethan et al., 1987)

Af uc

do h

¼ 46:15  exp½0:8  j

mc ðsDrhÞ1=2



(Srinikethan et al., 1987)

97.6%

Pulsed sieve plate column

  uc Ec ¼ doaAfh 1 þ 2Af ð14Þ ð1  4Þ D2=3   do Afh2=3 ud Ed ¼ aD2=3 1 þ 2Af 4 4 

Ed hud Ec

c

Af ud

0:6

Re yc ¼ 1:48 ð14Þ8:5

Ed

c

 0:25  0:13

¼ 1:513

Re yd ¼ 2:15 ð14Þ6:8

AARE

Pulsed sieve plate column

2=3

mc

(Sehmel and Babb, 1964)

Column type 1:26104 A1:2 f 1:35 ðud þuc Þ1:4

do h

Af us

Af us

a0:22

ud uc

ud uc

equations can be used to predict the mass transfer rate and axial dispersion at engineering applications, particularly in solvent extraction of uranium from its low grade ores. 5. Conclusion In this work, the performance of a horizontal pulsed sieve-plate column is investigated for uranium extraction from an aqueous sulfate solution with 0.25M concentration acid, by using TOA solvent, kerosene diluent and decanol modifier of 5%, 90% and 5% v/v. Uranium removal efficiency is determined from 93.22% to 96.62% under pulsation intensity of 0.8e1.3 cm/s, continuous phase flow rate of 0.3  106 to 1.38  106 m3/s and dispersed phase flow rate of 0.3  106 to 1.38  106 m3/s. Moreover, As can be seen in Appendix A (Table A1), the presence of large amount of impurities such as Al and Fe do not affect the performance of solvent extraction of uranium by TOA.

Satisfactory prediction of the mass transfer rate in the column is observed by using the axial dispersion model. Accordingly, the effects of operating parameters including pulsation intensity and continuous and dispersed phase flow rates are considered for investigation of mass transfer and axial mixing in the column. The following results are observed:  There is an enhancement in column performance with an increase in pulsation intensity from 0.8 to 1.3 cm/s and in continuous and dispersed phase flow rate from 0.3  106 to 1.38  106 m3/s.  The effect of dispersed phase flow rate is found to be more profound on mass transfer coefficient.  For the range of operating parameters investigated in this study, axial dispersion is observed to vary from 2.01  105 to 5.61  105 m2/s in the continuous phase and from 4.07  106 to 6.60  106 m2/s in dispersed phase, which indicated that

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P. Amani et al. / Progress in Nuclear Energy xxx (2017) 1e14

Fig. 11. Comparison of the experimental values of (a) overall mass transfer coefficient, (b) continuous phase and (c) dispersed phase axial dispersion coefficients with those calculated by the correlations developed in the current study.

axial dispersion in continuous phase is significantly higher than that in dispersion phase.  The axial dispersion of a phase is strongly influenced by the pulsation intensity and the flow rate of the phase itself, and slightly influenced by the second phase flow rate.  The overall height of mass transfer unit (HTUoc) is found to be less than 0.2 m, demonstrating high performance of a horizontal pulsed sieve-plate column for uranium extraction. Furthermore, since previous correlations are not able to accurately predict Koca, Ec and Ed, new correlations are proposed for these parameters with AARE values of 1.03%, 3.35% and 1.89% respectively. The proposed correlations in this research are applicable in low grade uranium ores applications. Nomenclature a A Af D do dp d32 E f g h

Specific interfacial area, m2/m3 Amplitude of pulsation, m Pulsation intensity, m/s Column diameter, m Hole diameter, m Pitch of the holes, m Sauter mean diameter, m Axial dispersion coefficient, m2/s Frequency of pulsation, Hz Acceleration due to gravity, ¼ 9.81 m/s2 Plate spacing, m

H K Q t u0 uc ud us S x y z

Column length, m Mass transfer coefficient, m/s Volumetric flow rate, m3/s Plate thickness, m Characteristic velocity, m/s Superficial velocity of continuous phase, m/s Superficial velocity of dispersed phase, m/s Slip velocity, m/s Column cross-section area, m2 Mass fraction of uranium in continuous phase Mass fraction of uranium in dispersed phase Axial dimension of column, m

Dimensionless symbols NTUoc Overall number of transfer unit, ¼ HKoc a=uc Pe Peclet number, ¼ HV=E Re Reynolds number, ¼ d32 us rc =mc X Continuous phase concentration, ¼ ðx  xin Þ=ðx*out  xin Þ Y Dispersed phase concentration, ¼ ðy  yin Þ=ðy*out  yin Þ Z Dimensionless height, ¼ z=H Greek symbols a Fractional free area ε Power dissipated per unit mass of fluid, W/kg 4 Holdup y Kinematic viscosity, m2/s m Viscosity, kg/m.s r Density, kg/m3

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P. Amani et al. / Progress in Nuclear Energy xxx (2017) 1e14

r s U

Density of mixture of phases, kg/m3 Interfacial tension between two phases, kg/s2 ¼ ðmQd rd Þ=ðQc rc Þ

Subscripts c Continuous phase d Dispersed phase o Overall based on (c or d) Superscripts * Equilibrium Appendix A The extraction efficiency of different elements of leach liquor supplied from the Bandar Abbas Uranium Production Plant (BUP) in a horizontal pulsed sieve-plate column is presented in Table A1.

Table A1 Removal efficiency of different elements of leach liquor used in this work in run 10 Element

Ci,in

Ci,out

Removal efficiency (%)

Uranium (U) Iron (Fe) Calcium (Ca) Magnesium (Mg) Manganese (Mn) Aluminum (Al)

192.1 1060.9 895.6 1591.9 5971 1155.1

8.5 962.5 867.5 1549 5827.5 1138

96.62 9.3 3.1 2.7 2.4 1.4

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Please cite this article in press as: Amani, P., et al., Mass transfer studies in a horizontal pulsed sieve-plate column for uranium extraction by trin-octylamine using axial dispersion model, Progress in Nuclear Energy (2017), http://dx.doi.org/10.1016/j.pnucene.2017.02.010