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O) emulsion by hydrocyclone
Accepted Manuscript Demulsification of the phosphoric acid−tributyl phosphate (W/O) emulsion by hydrocyclone Yuqing Cao, Yang Jin, Jun Li, Da Zou, Xi Chen PII: DOI: Reference:
S1383-5866(15)30407-X http://dx.doi.org/10.1016/j.seppur.2015.12.038 SEPPUR 12763
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
9 May 2015 17 December 2015 21 December 2015
Please cite this article as: Y. Cao, Y. Jin, J. Li, D. Zou, X. Chen, Demulsification of the phosphoric acid−tributyl phosphate (W/O) emulsion by hydrocyclone, Separation and Purification Technology (2015), doi: http://dx.doi.org/ 10.1016/j.seppur.2015.12.038
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Demulsification of the phosphoric acid‒tributyl phosphate (W/O) emulsion by hydrocyclone
Yuqing Cao, Yang Jin*, Jun Li, Da Zou, Xi Chen Department of Chemical Engineering, Sichuan University, Chengdu 610065, PR China
*Corresponding author. Tel/Fax: +0086−028−85468288 E-mail address: [email protected] (Y. Jin). 1
Abstract: A demulsification process of phosphoric acid-tributyl phosphate emulsion by using a liquid-liquid hydrocyclone was investigated. In this study, the droplet size distributions and average droplet sizes of the emulsions pre and post demulsification were measured and were used to evaluate the demulsification performance. The effects of the inlet flow rate and the structural parameters such as large cone part angle, small cone part angle, cylindrical part height, inlet inner diameter and overflow pipe inner diameter of the hydrocyclone on the demulsification performance were investigated. The optimum inlet flow rate and structural parameters were obtained. Under the optimum conditions, average droplet size of the emulsion increased from 4.78 to 55.07 μm, the difference was 50.29 μm. The average droplet size differences were associated with these above-mentioned parameters through a dimensionless model. The comparison of experimental and calculated data showed that this model was reliable to predict the average droplet size difference in most demulsification conditions by hydrocyclone. Keywords:
Hydrocyclone; Demulsification; Phosphoric acid; Tributyl phosphate;
Emulsion
1. INTRODUCTION Solvent extraction is an effective and economical method for purifying phosphoric acid (PA), in which tributyl phosphate (TBP) can serve as an effective extractant because of its immiscibility with the aqueous solutions and good selectivity to phosphoric acid [1−3]. Reciprocating sieve plate column and mixer settler are usu2
ally applied in the extraction process. This process suffers from the fact that the extraction rate is slow and the residence time of material is long, which makes the extraction equipment and extractant circulation volume large. In order to greatly improve the extraction rate and shorten the extraction equilibrium time, we emulsify the mixture of PA and TBP using high-speed shearing to prepare a W/O emulsion to obtain a large contact interfacial area between the two phases. However, the follow-up demulsification process is needed to separate the two phases. In the demulsification process, chemical methods [4−6] have been widely applied. They have several disadvantages, such as significant cost and environmental impact [7]. Some researchers also reported about the application of biological methods [8−10] for the demulsification. Because the growth requirements for demulsifying microorganisms are very strict, the biological methods can not be applied broadly. Physical methods for demulsification include heating [11,12], ultrafiltration [13], precoat filtration [14], membrane process [15] and centrifugation [16]. Centrifugation is especially preferred in some cases. As inexpensive, convenient, efficient and simple in designing devices, liquid-liquid hydrocyclones (LLHC) are used in various industries to separate two immiscible liquids of different densities with the aid of the strong centrifugal force created by the swirling flow. Fig. 1 shows a schematic diagram of LLHC. The emulsion is injected tangentially with a pump through the inlet into the cyclonic body. The fast movement of liquid causes an intense whirling motion and generates centrifugal force within the device. There is a forced vortex in the region close to LLHC axis and a free-like vortex in the outer region [17]. 3
Previous studies have shown that LLHC can be used for two-phase separation of the emulsion systems. N. Kharoua et al. [18] reviewed the performance parameters affecting the de-oiling hydrocyclone operation. Noroozi et al. [19] investigated the effects of drops breakage and coalescence on de-oiling hydrocyclone performance. Guo and Deng [20] researched the dispersed oil droplets breakage and emulsification in the dynamic oil and water hydrocyclone. Husveg et al. [21] described the performance of a de-oiling hydrocyclone during variable flow rates. However, these studies focused only on the oil-in-water emulsions, wherein oil is the dispersed phase and water is the continuous phase. Few people studied the application of LLHC in water-in-oil emulsion system. It is exactly the same as PA-TBP emulsion system. When this emulsion passes LLHC, oil migrates towards the centre as the heavier water droplets are forced towards the wall. Therefore, most of the water droplets concentrate in the pseudo free vorticity region. In this region, the processes of the water droplets movement and drop‒drop coalescence are effected by some forces. These forces are centrifugal force (FC), centripetal buoyancy (FB), Stokes' drag (FD), Magnus force (FM). The changes of fluid tangential velocity and the Magnus force are very small in the pseudo free vorticity region. Therefore, this force can be neglected. Meanwhile, the breakage effect is neglected. Droplet with different diameter has different resultant force's magnitude of the above-mentioned forces (FB, FC and FD, neglect FM) and different centrifugal sedimentation velocity. A big droplet has a higher centrifugal sedimentation velocity than a small droplet, which means there is a relative radial velocity between the droplets. In the axial direction, the water droplets that are simultaneously 4
injected into the cyclonic body move to the wall in a spiral-like trajectory. Because of the relative radial velocity, there must be a coincidence point on the trajectories. Then collision occurs on this point (Fig. 2a), which provides the chance of coalescence [22]. Fig. 2b shows the coalescence process of the water droplets in the hydrocyclone. Generally, the coalescence process has been assumed to divided into three manageable subprocesses: (1) two droplets collide under the action of the resultant force of the above-mentioned forces, trapping a small amount of continuous phase between them; (2) droplets keep in contact till the liquid film drains out to a critical thickness; (3) the critical film thickness between two droplets ruptures resulting in coalescence, i.e. the coagulated droplets combine and become bigger droplets [23–25]. The increase of droplet size is propitious to accelerate the sedimentation and separation rate of the two phases. However, few people studied the change of droplet size in LLHC or demulsification performance of emulsion by using LLHC. In this research, a demulsification process of PA-TBP emulsion by using LLHC was investigated. This research focused on the effects of the inlet flow rate (Qi), large cone part angle (α), small cone part angle (β), cylindrical part height (h), inlet inner diameter (Di) and overflow pipe inner diameter (Do) of LLHC on the demulsification performance of this emulsion system. The drop size distribution (DSD) and average droplet size (d43) of emulsion pre and post demulsification were measured to evaluate the demulsification performance. A model was proposed to correlate the average droplet size difference (d43) and the above-mentioned parameters (Qi, α, β, h, Di, Do) by dimensional analysis. 5
2. EXPERIMENTAL SECTION 2.1 Materials Phosphoric acid (analytically pure, 85%) was supplied by Du Jiangyan Jiahong Phosphoric Acid Plant (Sichuan, China). TBP (purity ≥ 98.5%) and kerosene were provided by Sichuan Zhongcui Chemical Co. (Sichuan, China). TBP was used without any further purification. Kerosene was washed with concentrated sulfuric acid, then neutralized with 5% Na2CO3 solution, and finally washed with water until the pH was neutral and distilled at 185−255 °C. Deionized water was used to prepare the aqueous phase. 2.2 The structure of hydrocyclone In this experiment, when the emulsion passed through LLHC, the structure parameters of LLHC had a great influence on its DSD and d43. Five important structure parameters were selected namely, the angles of large cone part (α) and small cone part (β), the height of cylindrical part (h), and the inner diameters of inlet (Di) and overflow pipe (Do). The structure parameter ranges were given in Table 1. 2.3 Emulsion preparation Experiments were carried out at 50 ± 0.2 ℃. A known amount of TBP was diluted in kerosene with the volume ratio of 3:2 (VT/Vk) to prepare the oil phase while 85% PA was diluted to 45% by changing the mass percentage of H3PO4 with deionized water to prepare the aqueous phase. Appropriate amount of aqueous phase and oil phase were mixed with the phase ratio of 1:5 (Va/Vo) in a beaker. The mixture was stirred vigorously with a high shear emulsification machine (Youyi Instruments Co., 6
Ltd., Fluko JRJ-300-I, Shanghai, China) to obtain a homogenization emulsion. In order to reduce the experimental error, the total volume of the emulsion (Vtot) was fixed at 3000 mL, the stirring frequency (fs) and time (ts) were kept constant at 3000 r/min and 30 s, respectively. Previous work [26] demonstrated that the two liquids can form a water-in-oil emulsion, wherein PA was the dispersed phase and TBP was the continuous phase. 2.4 Procedure Measurements of DSD and the average droplet size (the average droplet size of emulsion pre and post demulsification, d43i and d43u, respectively) of the emulsion were
performed
in
a
particle
size
analyzer
(Sympatec
GmbH,
OPUS,
Clausthal-Zellerfeld, Germany), which had a measuring method based on the principle of ultrasonic attenuation. The average particle size d43 was calculated from the equation [27]: n
d 43 d 4j j 1
n
d j 1
3 j
(1)
The density distribution of drop size (q3 ln(x)) was calculated by the equation: q 3 ln( x)
Q3 ( xi x) Q3 ( xi ) 2.3 ln
( xi x) xi
(2)
where Q3 is the function of volume cumulative distribution, xi is the lower limit of the divided range of DSD, x is the length of the divided range of DSD. Before demulsification, the average droplet sizes of four runs emulsion prepared with the same homogenization and physical conditions were measured to obtain d43i. The emulsion was pumped into LLHC by a gear pump (Shenyang Pumps Manufac7
turing Co., Ltd., 20CQB-15, Wenzhou, China). We adjusted the gear rotational speed of the pump to change Qi. TBP with a small amount of PA flowed out from the overflow, and separated into two layers obviously. PA with a large amount of TBP flowed out from the underflow, and existed in an emulsion form in a long time. Meanwhile, compared with the overflow amount, the underflow amount was large. Therefore, after passing through LLHC, about 150 mL underflow was charged in a beaker and transferred into the particle size analyzer
to measure d43u immediately.
Demulsification performance was evaluated by comparing with the values of d43 which was calculated by the equation: Δd 43 = d 43u d 43i
(3)
Microscopic images of the emulsion at the inlet and underflow outlet of LLHC were captured using an inverted microscope (Chongqing photoelectric instrument Co., Ltd., XDS-1B, Chongqing, China).
3. RESULTS AND DISCUSSION 3.1 Emulsion properties DSDs of four runs emulsion prepared with the above-mentioned homogenization and physical conditions were measured, and yielded opaque emulsions with DSDs in the range of 0.10 to 31.62 μm (Fig. 3) and with d43 in the range of 4.53 to 5.51 μm. It is observed that all of these profiles exhibit almost coincident unimodal distributions. It means that the emulsions obtained with the same procedures have almost the same homogenization degree. It also indicates that the measurement errors of the instrument 8
are in a permitted range. The average value 4.78 μm of d43 of the four runs emulsion is taken as d43i for calculating d43 of the rest experiments. 3.2 Effect of inlet flow rate Based on the two groups of different experimental conditions, the effects of inlet flow rate (Qi) on DSD and d43 are presented in Figs. 4 and 5. The experimental conditions of group A: h, Di and Do were 40, 6, and 4 mm, while α and β were π/12 and π/90 radian, respectively. The experimental conditions of group B: h, Di and Do were 60, 8 and 4 mm, while α and β were π/9 and 13π/300 radian, respectively. Fig. 4 shows that DSD varied greatly with the increase of Qi. Under the experimental conditions of group A, it is observed that the DSDs are in the range of 2.51 to 63.10 μm (9.52 L/min, 12.80 L/min, 14.27 L/min), 3.16 to 79.43 μm (11.01 L/min), and 0.10 to 79.43 μm (15.16 L/min, 16.81 L/min). With Qi increasing from 9.52 to 14.27 L/min, the main peaks of the curves offset upward, and an evolution towards larger droplets takes place, which results in a higher value of average droplet size. From 14.27 to 16.81 L/min, the main peaks move downward and DSDs are more uniform. Under the experimental conditions of group B, the main peaks of the curves offset upward as Qi increases from 9.52 to 12.80 L/min, and then move downward as Qi increases from 12.80 to 16.81 L/min obviously. Fig. 5 shows that in the group B, d43 at Qi of 12.80 L/min is significantly larger than those at other Qi. While for the group A, although d43 at Qi of 14.27 L/min is bigger than the one at Qi of 12.80 L/min, in order to reduce the energy and the wear of pump, Qi of 12.80 L/min is the best selection. 9
The results show that Qi has a significant impact on DSDs of emulsions and d43 of droplets. In LLHC, the centrifugal sedimentation velocity of the droplets can be described by the Stokes equation [28,29]:
va
d 2j 18
v2 d 2j 18 r
Qi2
(4)
where, v: centrifugal sedimentation velocity of the droplets, : density difference between the oil phase and aqueous phase, a: acceleration of the droplets. From Eq. (4), we know that v increases with the increase of Qi, which increases the chance and force of collision between the droplets and then strengthens the coalescence process. However, while Qi exceeds a value, the droplet size decreases. The cause of this phenomenon is that the vortex and the shear force which cause size reduction of the liquid droplets [30] increase as Qi increases. The breakage of the droplets occurs when the shearing crushing effect exceeds the bear ability of interfacial tension and interfacial film strength, and then weakens the coalescence process of the droplets. Therefore, d43 increases to the maximum and then decreases with the increase of Qi. 3.3 Effect of large cone part angle The effect of large cone part angle (α) on DSD under the conditions: Qi = 12.80 L/min; β = π/90 radian; h = 40 mm; Di = 6 mm; Do = 4 mm was studied. The results are presented in Fig. 6. DSDs range from 3.16 to 79.43 μm (π/12 and π/9 radian) and 0.10 to 50.12 μm (π/6 radian). It is observed that there is a little difference at α of π/12 and π/9 radian. An evolution towards larger droplets takes place as α decreases. We observe that at α of π/6 radian, the emulsion has a flattened DSD with diameters ranging from 0.10 to 50.12 μm, which means d43 is smaller than those of the π/12 and 10
π/9 radian. Corresponding to Fig. 11, we can see that d43 at α of π/12, π/9 and π/6 radian are 40.92, 39.56 and 12.42 μm, respectively. As α increases, the vortex and shear force increase, and this results in the weakening of coalescence process and the aggravation of droplets breakage. Furthermore, the residence time of droplets in large cone part with a large α is shorter than that with a small α. Therefore, the coalescence process of droplets is inadequate. Combining with the two reasons, the average droplet size becomes small, and the main peaks of the curves shift to the bottom left. Through the experiments, α of π/12 radian is chosen for the rest of experiments. 3.4 Effect of small cone part angle At the optimum Qi of 12.80 L/min and α of π/12 radian, under the other conditions: h = 40 mm; Di = 6 mm; Do = 4 mm, the effect of small cone part angle (β) on the drop size distribution (DSD) is observed in Fig. 7. It is interesting to note that as β increases from π/90 to 13π/300 radian, the profiles of the curves of DSD are maintained constant, of which the ranges are from 0.10 to 79.43 μm, except for flattening slightly. Meanwhile, d43 decreases from 40.92 to 36.94 μm slightly (Fig. 11). Therefore, it is preferable to choose a little β for a bigger d43. When β increases, the centrifugal force, vortex and shear force increase, which aggravates the breakage of droplets. Meanwhile, the residence time of droplets in small cone part decreases as β increases. Therefore, d43u decreases as β increases. The radial velocity, tangential velocity, centrifugal force, vortex and shear force are smaller in the small cone part comparing with those in the large cone part, thus the coales11
cence and breakage phenomenon of droplets are more unobvious in the former. Furthermore, the residence time of droplets in the small cone part is longer than that in the large cone part, hence, the droplet size remains unchanged basically at the bottom of the small cone part and the increase of d43u is not too much. 3.5 Effect of cylindrical part height The effect of cylindrical part height (h) on the drop size distribution (DSD) is investigated (Fig. 8). h is varied in the range of 20 to 60 mm. Qi is 12.80 L/min, α and β are fixed at π/12 and π/90 radian, Di and Do are 6 and 4 mm, respectively. The curves presented here show that DSD becomes more dispersed as h increases, of which the ranges are from 0.10 to 63.10, 3.16 to 79.43 and 7.94 to 100 μm, respectively. For h of 20, 40, 60 mm, d43 are 12.51, 40.92 and 44.18 μm, respectively (Fig. 11). The centrifugal sedimentation in the cylindrical part makes a remarkable contribution to the separation process in the hydrocyclone [31], which means that most of the droplets coalesce in this section. The swirl time of droplets in this section increases as h increases. In short cylindrical part, the residence time of droplets is shorter than that in the long cylindrical part, hence, the coalescence process of droplets in the short one is more inadequate than that in the long one. Therefore, after passing the cylindrical part, the enlargement of d43 in the long cylindrical part is more obvious than that in the short one. The results in Fig. 11 can confirm this phenomenon: d43 increases as h increases. 3.6 Effect of inlet inner diameter Based on the optimum conditions that mentioned above: Qi = 12.80 L/min; α = 12
π/12 radian; β = π/90 radian; h = 60 mm, with Do = 4 mm, as the inner diameter of inlet (Di) increases from 4 to 8 mm, the changes of DSD curve are presented in Fig. 9. Corresponding to Di of 4, 6 and 8 mm, the ranges of these DSD curves are from 0.10 to 79.43, 7.94 to 100 and 7.94 to 100 μm, respectively. DSDs flatten with smaller Di and an evolution towards larger droplets takes place as Di increases, indicating that the droplets have an increase in average size and an increase in d43 (from 27.47 to 44.69 μm, Fig. 11). From the equation: ui 4Qi
Di2
[32], with certain Qi, the feed flow rate (ui) of
emulsion is inverse proportion to Di. At a small Di, the emulsion has a high roation speed in LLHC, hence, the vortex and shear force are greater than those at a big Di. Therefore, the breakage of droplets becomes more intense, and d43u decreases as Di decreases. 3.7 Effect of overflow pipe inner diameter The effect of overflow pipe inner diameter (Do) on DSD was observed by increasing Do from 2 to 6 mm and the results are presented in Fig. 10. Corresponding to Do of 2, 4, 6 mm, the ranges of these DSD curves are from 10 to 100, 7.94 to 100 and 12.59 to 100 μm, respectively, and d43 increases from 41.73 to 50.29 μm (Fig. 11). The possible reason for this is that with an increase of Do, the overflow increases and the number of the small droplets which are taken away by the overflow increases, and the percentage of droplets with a relatively large size which stay in underflow increases. Therefore, both d43u and d43 increases. Fig. 11 shows d43 corresponding to the effects of α, β, h, Di and Do, respectively. 13
On the one hand, according to the degree of influence on d43, the structural factors could be put in order as follows: h > α > Di > Do > β. On the other hand, through these experiments, the optimal conditions of demulsification in LLHC are obtained as follows: Qi: 12.80 L/min, α: π/12 radian, β: π/90 radian, h: 60 mm, Di: 8 mm, Do: 6mm. Under the optimal conditions, d43 is 50.29 μm, and the demulsification efficiency of the emulsion is obvious. 3.8 Changes of droplet size To study the changes of emulsion droplet size, microscopy was used to observe the emulsion pre and post demulsification under the optimum conditions. The emulsion is initially a homogenous system with many spherical droplets dispersed in oil before passing through LLHC (Fig. 12(a)). As demulsification by LLHC under the optimum conditions, small droplets coalesce into bigger ones, and they grow in size. Therefore, the number of small droplet decreases and that of large droplet increases significantly. Some big droplets sedimentate to the bottom of the emulsion and can't be observed in this field of view (Fig. 12(b)). Compare Fig. 12(a) with (b), the diameter of the largest droplet is about 10 times larger of the initial diameter of the droplet.
4. MODEL ANALYSIS In the early literatures, dimensionless analysis was proposed and popularize by Buckingham [33], Bridgman [34] and Langhaar [35]. Dimensionless analysis provides the basic tool to correlate and interpret data in terms of key dimensionless groups [36]. The theoretical basis of dimensional analysis is Buckingham Pi theorem. 14
The details of the Buckingham Pi theorem can be found from Tan [37] and other literatures. In previous literatures, dimensionless analysis approach has been used on cyclones in some early researches. Rietema [38] formed a correlation between the volumetric flow rate and the applied pressure by this approach. Firth [39] presented a model which was based on this approach and accounted for the issue of transporting coarse particles through the spigot in a physically meaningful manner. As we described previously, six variables (Qi, α, β, h, Di, Do) were expected to affect d43 significantly and were identified as follows: Qi [L3T-1], α [1], β [1] (here α and β are expressed in radians), h [L], Di [L] and Do [L], d43 [L] (here L is length and T is time). And then the Buckingham Pi theorem was applied to suggest a suitable form of correlation for d43 as a function of the above-described variables. In order to obtain the dimensionless variables, the inner diameter of cylindrical part (D) that was fixed at 15 mm was introduced. Using these significant dimensional variables which involve two fundamental dimensions (L and T), five independent dimensionless groups can be formed as shown below according to the Buckingham Pi theorem: * Dimensionless average particle size difference d 43
Dimensionless cylindrical part height = h* Dimensionless inlet inner diameter = Di*
h D
Inlet Reynolds number = Rei
ui e Di
e
4Qi e Di e
(5) (6)
Di D
Dimensionless overflow pipe inner diameter = Do*
d 43 D
(7) Do D
(8) (9)
15
where
the
density
of
emulsion
ρe
[40]
is
calculated
as:
a X a o (1 X a ) 1.26 0.17 0.92 0.83 0.94 g / mL (ρa is density of the aqueous phase, ρo is density of the oil phase, Xa is volume ratio of the aqueous phase in the mixture), the viscosity of emulsion μe is equal to 3.36 cP. Dimensional analysis leads to the following functional form for the model: * = function [ Di* , Do* , h* , α, β, Rei ] d43
(10)
The concrete form of Eq. (10) was obtained by using the experimental data and an index model [39] was used in the actual analysis. The experimental data were divided into two groups by the Reynolds number of 12000. The way of computing the nonlinear regression equation is obtained by mathematical analysis software, and through the way of comparing experimental values with calculated ones, a preferably fitting equation can be selected. After these processes, the final form can be compressed to the expression:
a1Di*b1 Do*c1 h*d1 e1 f1 Reig1 d *b2 *c2 *d 2 e2 f 2 g2 a 2 Di Do h Rei * 43
Rei 12000 Rei 12000
(11)
Wherein the values of these constants are as follows:
a1 1.741102 ; b1 6.173 ; c1 1.656 10 1 ; d1 5.118 10 1 ; e1 3.899 ; f1 1.015 ; g1 5.340 10 1 ;
a 2 8.115 103 ; b 2 4.987 ; c2 3.982 101 ; d2 6.304 10 1 ; e 2 1.002 ; f 2 1.799 102 ; g 2 6.389 . The comparison of d43 between the calculated and experimental data are shown in Fig. 13. A few points of this model have big relative errors, but the rest of calculated data have a good correspondence with the experimental data. It is possible to 16
match and predict d43 for most demulsification conditions of the PA-TBP emulsions system, but it needs more evaluation and potential modification.
5. CONCLUSIONS The effects of the inlet flow rate, the large cone part angle, the small cone part angle, the cylindrical part height, the inlet inner diameter and the overflow pipe inner diameter of the hydrocyclone on the demulsification efficiency have been investigated. The average droplet size of the emulsion pre and post demulsification have also been measured. On the basis of this study, the following conclusions can be summarized. (1) The optimum inlet flow rate and structural parameters are as follows: the inlet flow rate: 12.80 L/min, the angle of large cone part: π/12, the angle of small cone part: π/90, the height of cylindrical part: 60 mm, the inner diameter of inlet: 8 mm, the inner diameter of overflow pipe: 6 mm. Under the optimum conditions, the average droplet size difference is 50.29 μm, and the demulsification efficiency of the emulsion is obvious. (2) According to the degree of influence on the average droplet size difference, the structural factors could be put in order as follows: the height of cylindrical part > the angle of large cone part > the inner diameter of inlet > the inner diameter of overflow pipe > the angle of small cone part. (3) A mathematical model for estimating d43 based on the dimensional analysis is proposed. Most calculated data have a good correspondence with the experimental data. It demonstrates that this model could be used for predicting d43 for most 17
demulsification conditions of the PA-TBP emulsions system. However, this model should be further evaluated and modified.
ACKNOWLEDGEMENTS Project supported by the National Natural Science Foundation of China (No. 21306116) and Phosphorus Key Technology R&D Program of Sichuan University (SCU2015C002).
NOMENCLATURE a
acceleration of the droplets (m/s2)
a, b,···, g
equation constants
d43
average droplet size (μm)
d43i
average droplet size of the emulsion pre demulsification (μm)
d43u
average droplet size of the emulsion post demulsification (μm)
dj
droplet diameter (μm)
Di
inner diameters of the inlet (mm)
Do
inner diameter of the overflow pipe (mm)
Di*
dimensionless inlet inner diameter
Do*
dimensionless overflow pipe inner diameter
fs
stirring frequency (r/min)
FB
centripetal buoyancy (N)
FC
centrifugal force (N) 18
FD
Stokes' drag (N)
FM
magnus force (N)
h
height of the cylindrical part (mm)
h*
dimensionless cylindrical part height
hc
critical thickness of the film (m)
ho
initial thickness of the film (m)
L
length
Q3
function of volume cumulative distribution
Qi
inlet flow rate (L/min)
r
distance from axis of rotation (mm)
Rei
inlet Reynolds number
ts
stirring time (s)
T
time
ui
feed flow rate (m/s)
v
centrifugal sedimentation velocity of the droplets (m/s)
vθ
tangential velocity of the droplets (m/s)
vr
relative velocity between droplets (m/s)
Va
volume of the aqueous phase (mL)
Vk
volume of the kerosene (mL)
Vo
volume of the oil phase (mL)
VP
volume of the phosphoric acid (mL)
Vtot
total volume of the emulsion (mL) 19
VT
volume of TBP (mL)
xi
lower limit of the divided range of DSD (μm)
Xa
volume ratio of the aqueous phase in the mixture
Greek letters α
angle of the large cone part (radian)
β
angle of the small cone part (radian)
d43
average droplet size difference (μm)
* d43
dimensionless average particle size difference
density difference between the oil phase and aqueous phase (g/mL)
x
length of the divided range of DSD (μm)
μ
dynamic viscosity of the mixture (cP)
e
viscosity of the emulsion (cP)
ρa
density of the aqueous phase (g/mL)
e
density of the emulsion (g/mL)
ρo
density of the oil phase (g/mL)
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[3] H. Ahmed, H. Diamonta, C. Chaker, R. Abdelhamid, Purification of wet process phosphoric acid by solvent extraction with TBP and MIBK mixtures, Sep. Purif. Technol. 55 (2007) 212–216. [4] Y. Takahashi, K. Fukuyasu, T. Horiuchi, Y. Kondo, P. Stroeve, Photoinduced demulsification of emulsions using a photoresponsive gemini surfactant, Langmuir 30 (2014) 41–47. [5] A.M. Al-Sabagh, N.G. Kandile, N.M. Nasser, M.R. Mishrif, A.E. El-Tabey, Demulsification efficiency of some new demulsifiers based on 1,3,5-triethanolhexahydro-1,3,5-triazine, J. Dispersion Sci. Technol. 35 (2014) 1361–1368. [6] W.L. Kang, S.R. Liu, B. Xu, X.Z. Wang, B.T. Zhang, B.J. Bai, Study on demulsification of a demulsifier at low temperature and its field application, Pet. Sci. Technol. 31 (2013) 572–579. [7] E.R. Binner, J.P. Robinson, S.W. Kingman, E.H. Lester, B.J. Azzopardi, G. Dimitrakis, J. Briggs, Separation of oil/water emulsions in continuous flow using microwave heating, Energy Fuels 27 (2013) 3173−3178. [8] X.W. Long, G.L. Zhang, C. Shen, G.S. Sun, R.X. Wang, L.J. Yin, Q. Meng, Application of rhamnolipid as a novel biodemulsifier for destabilizing waste crude oil, Bioresour. Technol. 131 (2013) 1–5. [9] S.Sh. Amirabadi, A. Jahanmiri, M.R. Rahimpour, B. Rafienia, P. Darvishib, A. Niazic, Investigation of Paenibacillus alvei ARN63 ability for biodemulsifier production: Medium optimization to break heavy crude oil emulsion, Colloids Surf., B 109 (2013) 244–252. [10] X. Li, A. Li, C. Liu, J.X. Yang, F. Ma, N. Hou, Y. Xu, N.Q. Ren, Characterization of the extracellular biodemulsifier of Bacillus mojavensis XH1 and the enhancement of demulsifying efficiency by optimization of the production medium composition, Process Biochem. 47 (2012) 626–634. 21
[11] E.R. Binner, J.P. Robinson, S.W. Kingman, Ed H. Lester, B.J. Azzopardi, G. Dimitrakis, J. Briggs, Separation of oil/water emulsions in continuous flow using microwave heating, Energy Fuels 27 (2013) 3173–3178. [12] R. Martínez-Palou, R. Cerón-Camacho, B. Chávez, A.A. Vallejo, D. Villanueva-Negrete, J. Castellanos, J. Karamath, J. Reyes, J. Aburto, Demulsification of heavy crude oil-in-water emulsions: A comparative study between microwave and thermal heating, Fuel 113 (2013) 407–414. [13] X.G. Yang, W. Tan, X.F. Tan, Demulsification of crude oil emulsion via ultrasonic chemical method, Pet. Sci. Technol. 27 (2009) 2010–2020. [14] N.M. Kocherginsky, C.L. Tan, W.F. Lu, Demulsification of water-in-oil emulsions via filtration through a hydrophilic polymer membrane, J. Membr. Sci. 220 (2003) 117–128. [15] D.Z. Sun, X.D. Duan, W.X. Li, D. Zhou, Demulsification of water-in-oil emulsion by using porous glass membrane, J. Membr. Sci. 146 (1998) 65–72. [16] A.H. Nour, F.S. Mohammed, R.M. Yunus, A. Arman, Demulsification of virgin coconut oil by centrifugation method: A feasibility study, Int. J. Chem. Technol. 1 (2009) 59–64. [17] S. Amini, D. Mowla, M. Golkar, Developing a new approach for evaluating a de-oiling hydrocyclone efficiency, Desalination 285 (2012) 131–137. [18] N. Kharoua, L. Khezzar, Z. Nemouchi, Hydrocyclones for de-oiling applications-a review, Pet. Sci. Technol. 28 (2010) 738–755. [19] S. Noroozi, S.H. Hashemabadi, A.J. Chamkha, Numerical analysis of drops coalescence and breakage effects on de-oiling hydrocyclone performance, Sep. Sci. Technol. 48 (2013) 991–1002. [20] G.D. Guo, S.S Deng, Research on dispersed oil droplets breakage and emulsification in the dynamic oil and water hydrocyclone, Adv. J. Food Sci. Technol. 5 (2013) 1110–1116. [21] T. Husveg, O. Rambeau, T. Drengstig, T. Bilstad, Performance of a deoiling hydrocyclone during variable flow rates, Miner. Eng. 20 (2007) 368–379. [22] X.B. Li, H.X. Yuan, Study to the gathering mechanism of the drip in the cyclone, Min. Process. Eqpt. 34 (2006) 67–69. (in Chinese) [23] M.J. Prince, H.W. Blanch, Bubble coalescence and break-up in air-sparged bubble columns, 22
AIChE J. 36 (1990) 1485–1499. [24] Y.X. Liao, D. Lucas, A literature review on mechanisms and models for the coalescence process of fluid particles, Chem. Eng. Sci. 65 (2010) 2851–2864. [25] S. Noroozi , S.H. Hashemabadi, A.J. Chamkha, Numerical analysis of drops coalescence and breakage effects on de-oiling hydrocyclone performance, Sep. Sci. Technol. 48 (2013) 991–1002. [26] Q. Yao, J Li, Y. Zhang, X. Xie, Application of emulsification extraction in WPA purification, Phosphate Compound Fertilizer 25 (2010) 14–16. (in Chinese) [27] A. Koocheki, R. Kadkhodaee, Effect of Alyssum homolocarpum seed gum, Tween 80 and NaCl on droplets characteristics, flow properties and physical stability of ultrasonically prepared corn oil-in-water emulsions, Food Hydrocolloids 25 (2011) 1149–1157. [28] J. Dueck, The sedimentation velocity of a particle in a wide range of Reynolds numbers in the application to the analysis of the separation curve, Adv. Powder Technol. 24 (2013) 150–153. [29] C.M. Scheid, L.A. Calçada, R.S.A. Gonçalves, G. Massarani, An investigation of the behavior of a classifying hydrocyclone with pseudoplastic fluids, Braz. J. Chem. Eng. 30 (2013) 781–791. [30] Z.S. Bai, H.L. Wang, Crude oil desalting using hydrocyclones, Chem. Eng. Res. Des. 85 (2007) 1586–1590. [31] L.Y. Chu, W.M. Chen, X.Z. Lee, Effect of structural modification on hydrocyclone performance, Sep. Purif. Technol. 21 (2000) 71–86. [32] C.H. Kim, J.W. Lee, A new collection effciency model for small cyclones considering the boundary-layer effect, Aerosol Sci. 32 (2001) 251–269. [33] E. Buckingham, On physically similar systems: Illustrations of the use of dimensional equations, Phys. Rev. 4 (1914) 345–376. [34] P.W. Bridgman, Dimensional analysis, Yale University Press, New Haven, 1922. [35] H.L. Langhaar, Dimensional analysis and theory of models, John Wiley & Sons, Inc., New York, 1951. [36] B. Mukherjee, B.A. Wrenn, P. Ramachandran, Relationship between size of oil droplet generated during chemical dispersion of crude oil and energy dissipation rate: Dimensionless, scaling, and experimental analysis, Chem. Eng. Sci. 68 (2012) 432–442. 23
[37] Q.M. Tan, Dimensional analysis with case studies in mechanics, The Press of University of Science and Technology of China, Hefei, 2005. [38] K. Rietema, Performance and design of hydrocyclones, Parts I to IV, Chem. Eng. Sci. 15 (1961) 298–325. [39] B. Firth, Hydrocyclones in dewatering circuits, Miner. Eng. 16 (2003) 115–120. [40] Y.M. Harshe, R.P. Utikar, V.V. Ranade, A computational model for predicting particle size distribution and performance of fluidized bed polypropylene reactor, Chem. Eng. Sci. 59 (2004) 5145–5156.
24
Table 1 The experimental ranges of the structure parameters. Structural parameters
Ranges
α (radian)
π/12
π/9
π/6
β (radian)
π/90
π/45
13π/300
h (mm)
20
40
60
Di (mm)
4
6
8
Do (mm)
2
4
6
25
Fig. 1. The schematic diagram of LLHC.
26
(a)
(b)
Fig. 2. Schematic diagram of demulsification mechanism of W/O emulsions by using hydrocyclone. (a) trajectories of the droplets (planform); (b) coalescence process of the droplets.
27
Fig. 3. Drop size distributions (by density distribution) of the prepared emulsions (Vtot = 3000 mL, VP/VT/Vk = 1:3:2, fs = 3000 r/min, ts = 30 s; ■ 1st run, ● 2nd run, ▲ 3rd run, ▼ 4th run).
28
Fig. 4. The effect of inlet flow rate on the drop size distribution: (A) group A, (B) group B (group A: α = π/12 radian, β = π/90 radian, h = 40 mm, Di = 6 mm, Do = 4 mm; group B: α = π/9 radian, β = 13π/300 radian, h = 60 mm, Di = 8 mm, Do = 4 mm; ■ 9.52 L/min, ● 11.01 L/min, ▲ 12.80 L/min, □ 14.27 L/min, ○ 15.16 L/min, ∆ 16.81 L/min). 29
Fig. 5. The effect of inlet flow rate on the average droplet size difference (■ group A: α = π/12 radian, β = π/90 radian, h = 40 mm, Di = 6 mm, Do = 4 mm; ● group B: α = π/9 radian, β = 13π/300 radian, h = 60 mm, Di = 8 mm, Do = 4 mm).
30
Fig. 6. The effect of large cone part angle on the drop size distribution (■ π/12 radian, ● π/9 radian, ▲ π/6 radian) (Qi = 12.80 L/min; β = π/90 radian; h = 40 mm; Di = 6 mm; Do = 4 mm).
31
Fig. 7. The effect of small cone part angle on the drop size distribution (■ π/90 radian, ● π/45 radian, ▲ 13π/300 radian) (Qi = 12.80 L/min; α = π/12 radian; h = 40 mm; Di = 6 mm; Do = 4 mm).
32
Fig. 8. The effect of cylindrical part height on the drop size distribution (■ 20 mm, ● 40 mm, ▲ 60 mm) (Qi = 12.80 L/min; α = π/12 radian; β = π/90 radian; Di = 6 mm; Do = 4 mm).
33
Fig. 9. The effect of inlet inner diameter on the drop size distribution (■ 4 mm, ● 6 mm, ▲ 8 mm) (Qi = 12.80 L/min; α = π/12 radian; β = π/90 radian; h = 60 mm; Do = 4 mm).
34
Fig. 10. The effect of overflow pipe inner diameter on the drop size distribution (■ 2 mm, ● 4 mm, ▲ 6 mm) (Qi = 12.80 L/min; α = π/12 radian; β = π/90 radian; h = 60 mm; Di = 8 mm).
35
Fig. 11. The average droplet size difference of different structural parameters.
36
(a)
(b)
Fig. 12. Micrographs of the (W/O) emulsion: (a) at the inlet before demulsification, (b) at the underflow outlet after demulsification under the optimum conditions. Scale bar is 10 m
37
Fig. 13. Comparison between experimental and calculated data. (A) Rei 12000 , (B) Rei 12000 .
38
• The phosphoric acid‒tributyl phosphate (W/O) emulsion is demulsified by a hydrocyclone. • The effects of the inlet flow rate and the structure parameters of LLHC on the demulsification performance are investigated. • The demulsification performance of the emulsion by using a hydrocyclone is obvious • A model that is used to predict the demulsification performance is proposed.
39
S1383-5866(15)30407-X http://dx.doi.org/10.1016/j.seppur.2015.12.038 SEPPUR 12763
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
9 May 2015 17 December 2015 21 December 2015
Please cite this article as: Y. Cao, Y. Jin, J. Li, D. Zou, X. Chen, Demulsification of the phosphoric acid−tributyl phosphate (W/O) emulsion by hydrocyclone, Separation and Purification Technology (2015), doi: http://dx.doi.org/ 10.1016/j.seppur.2015.12.038
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Demulsification of the phosphoric acid‒tributyl phosphate (W/O) emulsion by hydrocyclone
Yuqing Cao, Yang Jin*, Jun Li, Da Zou, Xi Chen Department of Chemical Engineering, Sichuan University, Chengdu 610065, PR China
*Corresponding author. Tel/Fax: +0086−028−85468288 E-mail address: [email protected] (Y. Jin). 1
Abstract: A demulsification process of phosphoric acid-tributyl phosphate emulsion by using a liquid-liquid hydrocyclone was investigated. In this study, the droplet size distributions and average droplet sizes of the emulsions pre and post demulsification were measured and were used to evaluate the demulsification performance. The effects of the inlet flow rate and the structural parameters such as large cone part angle, small cone part angle, cylindrical part height, inlet inner diameter and overflow pipe inner diameter of the hydrocyclone on the demulsification performance were investigated. The optimum inlet flow rate and structural parameters were obtained. Under the optimum conditions, average droplet size of the emulsion increased from 4.78 to 55.07 μm, the difference was 50.29 μm. The average droplet size differences were associated with these above-mentioned parameters through a dimensionless model. The comparison of experimental and calculated data showed that this model was reliable to predict the average droplet size difference in most demulsification conditions by hydrocyclone. Keywords:
Hydrocyclone; Demulsification; Phosphoric acid; Tributyl phosphate;
Emulsion
1. INTRODUCTION Solvent extraction is an effective and economical method for purifying phosphoric acid (PA), in which tributyl phosphate (TBP) can serve as an effective extractant because of its immiscibility with the aqueous solutions and good selectivity to phosphoric acid [1−3]. Reciprocating sieve plate column and mixer settler are usu2
ally applied in the extraction process. This process suffers from the fact that the extraction rate is slow and the residence time of material is long, which makes the extraction equipment and extractant circulation volume large. In order to greatly improve the extraction rate and shorten the extraction equilibrium time, we emulsify the mixture of PA and TBP using high-speed shearing to prepare a W/O emulsion to obtain a large contact interfacial area between the two phases. However, the follow-up demulsification process is needed to separate the two phases. In the demulsification process, chemical methods [4−6] have been widely applied. They have several disadvantages, such as significant cost and environmental impact [7]. Some researchers also reported about the application of biological methods [8−10] for the demulsification. Because the growth requirements for demulsifying microorganisms are very strict, the biological methods can not be applied broadly. Physical methods for demulsification include heating [11,12], ultrafiltration [13], precoat filtration [14], membrane process [15] and centrifugation [16]. Centrifugation is especially preferred in some cases. As inexpensive, convenient, efficient and simple in designing devices, liquid-liquid hydrocyclones (LLHC) are used in various industries to separate two immiscible liquids of different densities with the aid of the strong centrifugal force created by the swirling flow. Fig. 1 shows a schematic diagram of LLHC. The emulsion is injected tangentially with a pump through the inlet into the cyclonic body. The fast movement of liquid causes an intense whirling motion and generates centrifugal force within the device. There is a forced vortex in the region close to LLHC axis and a free-like vortex in the outer region [17]. 3
Previous studies have shown that LLHC can be used for two-phase separation of the emulsion systems. N. Kharoua et al. [18] reviewed the performance parameters affecting the de-oiling hydrocyclone operation. Noroozi et al. [19] investigated the effects of drops breakage and coalescence on de-oiling hydrocyclone performance. Guo and Deng [20] researched the dispersed oil droplets breakage and emulsification in the dynamic oil and water hydrocyclone. Husveg et al. [21] described the performance of a de-oiling hydrocyclone during variable flow rates. However, these studies focused only on the oil-in-water emulsions, wherein oil is the dispersed phase and water is the continuous phase. Few people studied the application of LLHC in water-in-oil emulsion system. It is exactly the same as PA-TBP emulsion system. When this emulsion passes LLHC, oil migrates towards the centre as the heavier water droplets are forced towards the wall. Therefore, most of the water droplets concentrate in the pseudo free vorticity region. In this region, the processes of the water droplets movement and drop‒drop coalescence are effected by some forces. These forces are centrifugal force (FC), centripetal buoyancy (FB), Stokes' drag (FD), Magnus force (FM). The changes of fluid tangential velocity and the Magnus force are very small in the pseudo free vorticity region. Therefore, this force can be neglected. Meanwhile, the breakage effect is neglected. Droplet with different diameter has different resultant force's magnitude of the above-mentioned forces (FB, FC and FD, neglect FM) and different centrifugal sedimentation velocity. A big droplet has a higher centrifugal sedimentation velocity than a small droplet, which means there is a relative radial velocity between the droplets. In the axial direction, the water droplets that are simultaneously 4
injected into the cyclonic body move to the wall in a spiral-like trajectory. Because of the relative radial velocity, there must be a coincidence point on the trajectories. Then collision occurs on this point (Fig. 2a), which provides the chance of coalescence [22]. Fig. 2b shows the coalescence process of the water droplets in the hydrocyclone. Generally, the coalescence process has been assumed to divided into three manageable subprocesses: (1) two droplets collide under the action of the resultant force of the above-mentioned forces, trapping a small amount of continuous phase between them; (2) droplets keep in contact till the liquid film drains out to a critical thickness; (3) the critical film thickness between two droplets ruptures resulting in coalescence, i.e. the coagulated droplets combine and become bigger droplets [23–25]. The increase of droplet size is propitious to accelerate the sedimentation and separation rate of the two phases. However, few people studied the change of droplet size in LLHC or demulsification performance of emulsion by using LLHC. In this research, a demulsification process of PA-TBP emulsion by using LLHC was investigated. This research focused on the effects of the inlet flow rate (Qi), large cone part angle (α), small cone part angle (β), cylindrical part height (h), inlet inner diameter (Di) and overflow pipe inner diameter (Do) of LLHC on the demulsification performance of this emulsion system. The drop size distribution (DSD) and average droplet size (d43) of emulsion pre and post demulsification were measured to evaluate the demulsification performance. A model was proposed to correlate the average droplet size difference (d43) and the above-mentioned parameters (Qi, α, β, h, Di, Do) by dimensional analysis. 5
2. EXPERIMENTAL SECTION 2.1 Materials Phosphoric acid (analytically pure, 85%) was supplied by Du Jiangyan Jiahong Phosphoric Acid Plant (Sichuan, China). TBP (purity ≥ 98.5%) and kerosene were provided by Sichuan Zhongcui Chemical Co. (Sichuan, China). TBP was used without any further purification. Kerosene was washed with concentrated sulfuric acid, then neutralized with 5% Na2CO3 solution, and finally washed with water until the pH was neutral and distilled at 185−255 °C. Deionized water was used to prepare the aqueous phase. 2.2 The structure of hydrocyclone In this experiment, when the emulsion passed through LLHC, the structure parameters of LLHC had a great influence on its DSD and d43. Five important structure parameters were selected namely, the angles of large cone part (α) and small cone part (β), the height of cylindrical part (h), and the inner diameters of inlet (Di) and overflow pipe (Do). The structure parameter ranges were given in Table 1. 2.3 Emulsion preparation Experiments were carried out at 50 ± 0.2 ℃. A known amount of TBP was diluted in kerosene with the volume ratio of 3:2 (VT/Vk) to prepare the oil phase while 85% PA was diluted to 45% by changing the mass percentage of H3PO4 with deionized water to prepare the aqueous phase. Appropriate amount of aqueous phase and oil phase were mixed with the phase ratio of 1:5 (Va/Vo) in a beaker. The mixture was stirred vigorously with a high shear emulsification machine (Youyi Instruments Co., 6
Ltd., Fluko JRJ-300-I, Shanghai, China) to obtain a homogenization emulsion. In order to reduce the experimental error, the total volume of the emulsion (Vtot) was fixed at 3000 mL, the stirring frequency (fs) and time (ts) were kept constant at 3000 r/min and 30 s, respectively. Previous work [26] demonstrated that the two liquids can form a water-in-oil emulsion, wherein PA was the dispersed phase and TBP was the continuous phase. 2.4 Procedure Measurements of DSD and the average droplet size (the average droplet size of emulsion pre and post demulsification, d43i and d43u, respectively) of the emulsion were
performed
in
a
particle
size
analyzer
(Sympatec
GmbH,
OPUS,
Clausthal-Zellerfeld, Germany), which had a measuring method based on the principle of ultrasonic attenuation. The average particle size d43 was calculated from the equation [27]: n
d 43 d 4j j 1
n
d j 1
3 j
(1)
The density distribution of drop size (q3 ln(x)) was calculated by the equation: q 3 ln( x)
Q3 ( xi x) Q3 ( xi ) 2.3 ln
( xi x) xi
(2)
where Q3 is the function of volume cumulative distribution, xi is the lower limit of the divided range of DSD, x is the length of the divided range of DSD. Before demulsification, the average droplet sizes of four runs emulsion prepared with the same homogenization and physical conditions were measured to obtain d43i. The emulsion was pumped into LLHC by a gear pump (Shenyang Pumps Manufac7
turing Co., Ltd., 20CQB-15, Wenzhou, China). We adjusted the gear rotational speed of the pump to change Qi. TBP with a small amount of PA flowed out from the overflow, and separated into two layers obviously. PA with a large amount of TBP flowed out from the underflow, and existed in an emulsion form in a long time. Meanwhile, compared with the overflow amount, the underflow amount was large. Therefore, after passing through LLHC, about 150 mL underflow was charged in a beaker and transferred into the particle size analyzer
to measure d43u immediately.
Demulsification performance was evaluated by comparing with the values of d43 which was calculated by the equation: Δd 43 = d 43u d 43i
(3)
Microscopic images of the emulsion at the inlet and underflow outlet of LLHC were captured using an inverted microscope (Chongqing photoelectric instrument Co., Ltd., XDS-1B, Chongqing, China).
3. RESULTS AND DISCUSSION 3.1 Emulsion properties DSDs of four runs emulsion prepared with the above-mentioned homogenization and physical conditions were measured, and yielded opaque emulsions with DSDs in the range of 0.10 to 31.62 μm (Fig. 3) and with d43 in the range of 4.53 to 5.51 μm. It is observed that all of these profiles exhibit almost coincident unimodal distributions. It means that the emulsions obtained with the same procedures have almost the same homogenization degree. It also indicates that the measurement errors of the instrument 8
are in a permitted range. The average value 4.78 μm of d43 of the four runs emulsion is taken as d43i for calculating d43 of the rest experiments. 3.2 Effect of inlet flow rate Based on the two groups of different experimental conditions, the effects of inlet flow rate (Qi) on DSD and d43 are presented in Figs. 4 and 5. The experimental conditions of group A: h, Di and Do were 40, 6, and 4 mm, while α and β were π/12 and π/90 radian, respectively. The experimental conditions of group B: h, Di and Do were 60, 8 and 4 mm, while α and β were π/9 and 13π/300 radian, respectively. Fig. 4 shows that DSD varied greatly with the increase of Qi. Under the experimental conditions of group A, it is observed that the DSDs are in the range of 2.51 to 63.10 μm (9.52 L/min, 12.80 L/min, 14.27 L/min), 3.16 to 79.43 μm (11.01 L/min), and 0.10 to 79.43 μm (15.16 L/min, 16.81 L/min). With Qi increasing from 9.52 to 14.27 L/min, the main peaks of the curves offset upward, and an evolution towards larger droplets takes place, which results in a higher value of average droplet size. From 14.27 to 16.81 L/min, the main peaks move downward and DSDs are more uniform. Under the experimental conditions of group B, the main peaks of the curves offset upward as Qi increases from 9.52 to 12.80 L/min, and then move downward as Qi increases from 12.80 to 16.81 L/min obviously. Fig. 5 shows that in the group B, d43 at Qi of 12.80 L/min is significantly larger than those at other Qi. While for the group A, although d43 at Qi of 14.27 L/min is bigger than the one at Qi of 12.80 L/min, in order to reduce the energy and the wear of pump, Qi of 12.80 L/min is the best selection. 9
The results show that Qi has a significant impact on DSDs of emulsions and d43 of droplets. In LLHC, the centrifugal sedimentation velocity of the droplets can be described by the Stokes equation [28,29]:
va
d 2j 18
v2 d 2j 18 r
Qi2
(4)
where, v: centrifugal sedimentation velocity of the droplets, : density difference between the oil phase and aqueous phase, a: acceleration of the droplets. From Eq. (4), we know that v increases with the increase of Qi, which increases the chance and force of collision between the droplets and then strengthens the coalescence process. However, while Qi exceeds a value, the droplet size decreases. The cause of this phenomenon is that the vortex and the shear force which cause size reduction of the liquid droplets [30] increase as Qi increases. The breakage of the droplets occurs when the shearing crushing effect exceeds the bear ability of interfacial tension and interfacial film strength, and then weakens the coalescence process of the droplets. Therefore, d43 increases to the maximum and then decreases with the increase of Qi. 3.3 Effect of large cone part angle The effect of large cone part angle (α) on DSD under the conditions: Qi = 12.80 L/min; β = π/90 radian; h = 40 mm; Di = 6 mm; Do = 4 mm was studied. The results are presented in Fig. 6. DSDs range from 3.16 to 79.43 μm (π/12 and π/9 radian) and 0.10 to 50.12 μm (π/6 radian). It is observed that there is a little difference at α of π/12 and π/9 radian. An evolution towards larger droplets takes place as α decreases. We observe that at α of π/6 radian, the emulsion has a flattened DSD with diameters ranging from 0.10 to 50.12 μm, which means d43 is smaller than those of the π/12 and 10
π/9 radian. Corresponding to Fig. 11, we can see that d43 at α of π/12, π/9 and π/6 radian are 40.92, 39.56 and 12.42 μm, respectively. As α increases, the vortex and shear force increase, and this results in the weakening of coalescence process and the aggravation of droplets breakage. Furthermore, the residence time of droplets in large cone part with a large α is shorter than that with a small α. Therefore, the coalescence process of droplets is inadequate. Combining with the two reasons, the average droplet size becomes small, and the main peaks of the curves shift to the bottom left. Through the experiments, α of π/12 radian is chosen for the rest of experiments. 3.4 Effect of small cone part angle At the optimum Qi of 12.80 L/min and α of π/12 radian, under the other conditions: h = 40 mm; Di = 6 mm; Do = 4 mm, the effect of small cone part angle (β) on the drop size distribution (DSD) is observed in Fig. 7. It is interesting to note that as β increases from π/90 to 13π/300 radian, the profiles of the curves of DSD are maintained constant, of which the ranges are from 0.10 to 79.43 μm, except for flattening slightly. Meanwhile, d43 decreases from 40.92 to 36.94 μm slightly (Fig. 11). Therefore, it is preferable to choose a little β for a bigger d43. When β increases, the centrifugal force, vortex and shear force increase, which aggravates the breakage of droplets. Meanwhile, the residence time of droplets in small cone part decreases as β increases. Therefore, d43u decreases as β increases. The radial velocity, tangential velocity, centrifugal force, vortex and shear force are smaller in the small cone part comparing with those in the large cone part, thus the coales11
cence and breakage phenomenon of droplets are more unobvious in the former. Furthermore, the residence time of droplets in the small cone part is longer than that in the large cone part, hence, the droplet size remains unchanged basically at the bottom of the small cone part and the increase of d43u is not too much. 3.5 Effect of cylindrical part height The effect of cylindrical part height (h) on the drop size distribution (DSD) is investigated (Fig. 8). h is varied in the range of 20 to 60 mm. Qi is 12.80 L/min, α and β are fixed at π/12 and π/90 radian, Di and Do are 6 and 4 mm, respectively. The curves presented here show that DSD becomes more dispersed as h increases, of which the ranges are from 0.10 to 63.10, 3.16 to 79.43 and 7.94 to 100 μm, respectively. For h of 20, 40, 60 mm, d43 are 12.51, 40.92 and 44.18 μm, respectively (Fig. 11). The centrifugal sedimentation in the cylindrical part makes a remarkable contribution to the separation process in the hydrocyclone [31], which means that most of the droplets coalesce in this section. The swirl time of droplets in this section increases as h increases. In short cylindrical part, the residence time of droplets is shorter than that in the long cylindrical part, hence, the coalescence process of droplets in the short one is more inadequate than that in the long one. Therefore, after passing the cylindrical part, the enlargement of d43 in the long cylindrical part is more obvious than that in the short one. The results in Fig. 11 can confirm this phenomenon: d43 increases as h increases. 3.6 Effect of inlet inner diameter Based on the optimum conditions that mentioned above: Qi = 12.80 L/min; α = 12
π/12 radian; β = π/90 radian; h = 60 mm, with Do = 4 mm, as the inner diameter of inlet (Di) increases from 4 to 8 mm, the changes of DSD curve are presented in Fig. 9. Corresponding to Di of 4, 6 and 8 mm, the ranges of these DSD curves are from 0.10 to 79.43, 7.94 to 100 and 7.94 to 100 μm, respectively. DSDs flatten with smaller Di and an evolution towards larger droplets takes place as Di increases, indicating that the droplets have an increase in average size and an increase in d43 (from 27.47 to 44.69 μm, Fig. 11). From the equation: ui 4Qi
Di2
[32], with certain Qi, the feed flow rate (ui) of
emulsion is inverse proportion to Di. At a small Di, the emulsion has a high roation speed in LLHC, hence, the vortex and shear force are greater than those at a big Di. Therefore, the breakage of droplets becomes more intense, and d43u decreases as Di decreases. 3.7 Effect of overflow pipe inner diameter The effect of overflow pipe inner diameter (Do) on DSD was observed by increasing Do from 2 to 6 mm and the results are presented in Fig. 10. Corresponding to Do of 2, 4, 6 mm, the ranges of these DSD curves are from 10 to 100, 7.94 to 100 and 12.59 to 100 μm, respectively, and d43 increases from 41.73 to 50.29 μm (Fig. 11). The possible reason for this is that with an increase of Do, the overflow increases and the number of the small droplets which are taken away by the overflow increases, and the percentage of droplets with a relatively large size which stay in underflow increases. Therefore, both d43u and d43 increases. Fig. 11 shows d43 corresponding to the effects of α, β, h, Di and Do, respectively. 13
On the one hand, according to the degree of influence on d43, the structural factors could be put in order as follows: h > α > Di > Do > β. On the other hand, through these experiments, the optimal conditions of demulsification in LLHC are obtained as follows: Qi: 12.80 L/min, α: π/12 radian, β: π/90 radian, h: 60 mm, Di: 8 mm, Do: 6mm. Under the optimal conditions, d43 is 50.29 μm, and the demulsification efficiency of the emulsion is obvious. 3.8 Changes of droplet size To study the changes of emulsion droplet size, microscopy was used to observe the emulsion pre and post demulsification under the optimum conditions. The emulsion is initially a homogenous system with many spherical droplets dispersed in oil before passing through LLHC (Fig. 12(a)). As demulsification by LLHC under the optimum conditions, small droplets coalesce into bigger ones, and they grow in size. Therefore, the number of small droplet decreases and that of large droplet increases significantly. Some big droplets sedimentate to the bottom of the emulsion and can't be observed in this field of view (Fig. 12(b)). Compare Fig. 12(a) with (b), the diameter of the largest droplet is about 10 times larger of the initial diameter of the droplet.
4. MODEL ANALYSIS In the early literatures, dimensionless analysis was proposed and popularize by Buckingham [33], Bridgman [34] and Langhaar [35]. Dimensionless analysis provides the basic tool to correlate and interpret data in terms of key dimensionless groups [36]. The theoretical basis of dimensional analysis is Buckingham Pi theorem. 14
The details of the Buckingham Pi theorem can be found from Tan [37] and other literatures. In previous literatures, dimensionless analysis approach has been used on cyclones in some early researches. Rietema [38] formed a correlation between the volumetric flow rate and the applied pressure by this approach. Firth [39] presented a model which was based on this approach and accounted for the issue of transporting coarse particles through the spigot in a physically meaningful manner. As we described previously, six variables (Qi, α, β, h, Di, Do) were expected to affect d43 significantly and were identified as follows: Qi [L3T-1], α [1], β [1] (here α and β are expressed in radians), h [L], Di [L] and Do [L], d43 [L] (here L is length and T is time). And then the Buckingham Pi theorem was applied to suggest a suitable form of correlation for d43 as a function of the above-described variables. In order to obtain the dimensionless variables, the inner diameter of cylindrical part (D) that was fixed at 15 mm was introduced. Using these significant dimensional variables which involve two fundamental dimensions (L and T), five independent dimensionless groups can be formed as shown below according to the Buckingham Pi theorem: * Dimensionless average particle size difference d 43
Dimensionless cylindrical part height = h* Dimensionless inlet inner diameter = Di*
h D
Inlet Reynolds number = Rei
ui e Di
e
4Qi e Di e
(5) (6)
Di D
Dimensionless overflow pipe inner diameter = Do*
d 43 D
(7) Do D
(8) (9)
15
where
the
density
of
emulsion
ρe
[40]
is
calculated
as:
a X a o (1 X a ) 1.26 0.17 0.92 0.83 0.94 g / mL (ρa is density of the aqueous phase, ρo is density of the oil phase, Xa is volume ratio of the aqueous phase in the mixture), the viscosity of emulsion μe is equal to 3.36 cP. Dimensional analysis leads to the following functional form for the model: * = function [ Di* , Do* , h* , α, β, Rei ] d43
(10)
The concrete form of Eq. (10) was obtained by using the experimental data and an index model [39] was used in the actual analysis. The experimental data were divided into two groups by the Reynolds number of 12000. The way of computing the nonlinear regression equation is obtained by mathematical analysis software, and through the way of comparing experimental values with calculated ones, a preferably fitting equation can be selected. After these processes, the final form can be compressed to the expression:
a1Di*b1 Do*c1 h*d1 e1 f1 Reig1 d *b2 *c2 *d 2 e2 f 2 g2 a 2 Di Do h Rei * 43
Rei 12000 Rei 12000
(11)
Wherein the values of these constants are as follows:
a1 1.741102 ; b1 6.173 ; c1 1.656 10 1 ; d1 5.118 10 1 ; e1 3.899 ; f1 1.015 ; g1 5.340 10 1 ;
a 2 8.115 103 ; b 2 4.987 ; c2 3.982 101 ; d2 6.304 10 1 ; e 2 1.002 ; f 2 1.799 102 ; g 2 6.389 . The comparison of d43 between the calculated and experimental data are shown in Fig. 13. A few points of this model have big relative errors, but the rest of calculated data have a good correspondence with the experimental data. It is possible to 16
match and predict d43 for most demulsification conditions of the PA-TBP emulsions system, but it needs more evaluation and potential modification.
5. CONCLUSIONS The effects of the inlet flow rate, the large cone part angle, the small cone part angle, the cylindrical part height, the inlet inner diameter and the overflow pipe inner diameter of the hydrocyclone on the demulsification efficiency have been investigated. The average droplet size of the emulsion pre and post demulsification have also been measured. On the basis of this study, the following conclusions can be summarized. (1) The optimum inlet flow rate and structural parameters are as follows: the inlet flow rate: 12.80 L/min, the angle of large cone part: π/12, the angle of small cone part: π/90, the height of cylindrical part: 60 mm, the inner diameter of inlet: 8 mm, the inner diameter of overflow pipe: 6 mm. Under the optimum conditions, the average droplet size difference is 50.29 μm, and the demulsification efficiency of the emulsion is obvious. (2) According to the degree of influence on the average droplet size difference, the structural factors could be put in order as follows: the height of cylindrical part > the angle of large cone part > the inner diameter of inlet > the inner diameter of overflow pipe > the angle of small cone part. (3) A mathematical model for estimating d43 based on the dimensional analysis is proposed. Most calculated data have a good correspondence with the experimental data. It demonstrates that this model could be used for predicting d43 for most 17
demulsification conditions of the PA-TBP emulsions system. However, this model should be further evaluated and modified.
ACKNOWLEDGEMENTS Project supported by the National Natural Science Foundation of China (No. 21306116) and Phosphorus Key Technology R&D Program of Sichuan University (SCU2015C002).
NOMENCLATURE a
acceleration of the droplets (m/s2)
a, b,···, g
equation constants
d43
average droplet size (μm)
d43i
average droplet size of the emulsion pre demulsification (μm)
d43u
average droplet size of the emulsion post demulsification (μm)
dj
droplet diameter (μm)
Di
inner diameters of the inlet (mm)
Do
inner diameter of the overflow pipe (mm)
Di*
dimensionless inlet inner diameter
Do*
dimensionless overflow pipe inner diameter
fs
stirring frequency (r/min)
FB
centripetal buoyancy (N)
FC
centrifugal force (N) 18
FD
Stokes' drag (N)
FM
magnus force (N)
h
height of the cylindrical part (mm)
h*
dimensionless cylindrical part height
hc
critical thickness of the film (m)
ho
initial thickness of the film (m)
L
length
Q3
function of volume cumulative distribution
Qi
inlet flow rate (L/min)
r
distance from axis of rotation (mm)
Rei
inlet Reynolds number
ts
stirring time (s)
T
time
ui
feed flow rate (m/s)
v
centrifugal sedimentation velocity of the droplets (m/s)
vθ
tangential velocity of the droplets (m/s)
vr
relative velocity between droplets (m/s)
Va
volume of the aqueous phase (mL)
Vk
volume of the kerosene (mL)
Vo
volume of the oil phase (mL)
VP
volume of the phosphoric acid (mL)
Vtot
total volume of the emulsion (mL) 19
VT
volume of TBP (mL)
xi
lower limit of the divided range of DSD (μm)
Xa
volume ratio of the aqueous phase in the mixture
Greek letters α
angle of the large cone part (radian)
β
angle of the small cone part (radian)
d43
average droplet size difference (μm)
* d43
dimensionless average particle size difference
density difference between the oil phase and aqueous phase (g/mL)
x
length of the divided range of DSD (μm)
μ
dynamic viscosity of the mixture (cP)
e
viscosity of the emulsion (cP)
ρa
density of the aqueous phase (g/mL)
e
density of the emulsion (g/mL)
ρo
density of the oil phase (g/mL)
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24
Table 1 The experimental ranges of the structure parameters. Structural parameters
Ranges
α (radian)
π/12
π/9
π/6
β (radian)
π/90
π/45
13π/300
h (mm)
20
40
60
Di (mm)
4
6
8
Do (mm)
2
4
6
25
Fig. 1. The schematic diagram of LLHC.
26
(a)
(b)
Fig. 2. Schematic diagram of demulsification mechanism of W/O emulsions by using hydrocyclone. (a) trajectories of the droplets (planform); (b) coalescence process of the droplets.
27
Fig. 3. Drop size distributions (by density distribution) of the prepared emulsions (Vtot = 3000 mL, VP/VT/Vk = 1:3:2, fs = 3000 r/min, ts = 30 s; ■ 1st run, ● 2nd run, ▲ 3rd run, ▼ 4th run).
28
Fig. 4. The effect of inlet flow rate on the drop size distribution: (A) group A, (B) group B (group A: α = π/12 radian, β = π/90 radian, h = 40 mm, Di = 6 mm, Do = 4 mm; group B: α = π/9 radian, β = 13π/300 radian, h = 60 mm, Di = 8 mm, Do = 4 mm; ■ 9.52 L/min, ● 11.01 L/min, ▲ 12.80 L/min, □ 14.27 L/min, ○ 15.16 L/min, ∆ 16.81 L/min). 29
Fig. 5. The effect of inlet flow rate on the average droplet size difference (■ group A: α = π/12 radian, β = π/90 radian, h = 40 mm, Di = 6 mm, Do = 4 mm; ● group B: α = π/9 radian, β = 13π/300 radian, h = 60 mm, Di = 8 mm, Do = 4 mm).
30
Fig. 6. The effect of large cone part angle on the drop size distribution (■ π/12 radian, ● π/9 radian, ▲ π/6 radian) (Qi = 12.80 L/min; β = π/90 radian; h = 40 mm; Di = 6 mm; Do = 4 mm).
31
Fig. 7. The effect of small cone part angle on the drop size distribution (■ π/90 radian, ● π/45 radian, ▲ 13π/300 radian) (Qi = 12.80 L/min; α = π/12 radian; h = 40 mm; Di = 6 mm; Do = 4 mm).
32
Fig. 8. The effect of cylindrical part height on the drop size distribution (■ 20 mm, ● 40 mm, ▲ 60 mm) (Qi = 12.80 L/min; α = π/12 radian; β = π/90 radian; Di = 6 mm; Do = 4 mm).
33
Fig. 9. The effect of inlet inner diameter on the drop size distribution (■ 4 mm, ● 6 mm, ▲ 8 mm) (Qi = 12.80 L/min; α = π/12 radian; β = π/90 radian; h = 60 mm; Do = 4 mm).
34
Fig. 10. The effect of overflow pipe inner diameter on the drop size distribution (■ 2 mm, ● 4 mm, ▲ 6 mm) (Qi = 12.80 L/min; α = π/12 radian; β = π/90 radian; h = 60 mm; Di = 8 mm).
35
Fig. 11. The average droplet size difference of different structural parameters.
36
(a)
(b)
Fig. 12. Micrographs of the (W/O) emulsion: (a) at the inlet before demulsification, (b) at the underflow outlet after demulsification under the optimum conditions. Scale bar is 10 m
37
Fig. 13. Comparison between experimental and calculated data. (A) Rei 12000 , (B) Rei 12000 .
38
• The phosphoric acid‒tributyl phosphate (W/O) emulsion is demulsified by a hydrocyclone. • The effects of the inlet flow rate and the structure parameters of LLHC on the demulsification performance are investigated. • The demulsification performance of the emulsion by using a hydrocyclone is obvious • A model that is used to predict the demulsification performance is proposed.
39