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Phase separation dynamics in Oil- Polyethylene GlycolSulfate- Water based Three-Liquid-Phase Systems Jieyuan Lin, Kun Huang, Zhicheng Suo, Xiaopei Li, Chuanxu Xiao, and Huizhou Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 31 Mar 2015 Downloaded from http://pubs.acs.org on March 31, 2015

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Industrial & Engineering Chemistry Research

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Phase separation dynamics in Oil- Polyethylene Glycol- Sulfate-

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Water based Three-Liquid-Phase Systems

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Jieyuan Lina,b,c, Kun Huang*,a,b, Zhicheng Suod, Xiaopei Lia,b,c, Chuanxu Xiaoa,b,c, Huizhou Liu*,a,b

5 6

a

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Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R.

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China

9

b

State Key Laboratory of Biochemical Engineering, Key Laboratory of Green Process and

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing

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100190, China

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cUniversity

12

d

of Chinese Academy of Sciences, Beijing 100049, P.R. China

School of Science, Tianjin Chengjian University, Tianjin 300384, China

13

*Prof. Dr. Kun HUANG

[email protected]

Tel: (86)-(10)-82544910 Fax: (86)-(10)-62554264

*Prof. Dr. Huizhou LIU

[email protected]

Tel: (86)-(10)-62554264 Fax: (86)-(10)-62554264

14

ACS Paragon Plus Environment

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Phase separation dynamic processes in Three-Liquid-Phase System

1

ABSTRACT

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(TLPS), composed of organic oil (P507 extractant), water-soluble polymer (PEG2000),

3

ammonium sulfate and water, with the change of mass composition of phase-forming

4

components were investigated. It was found that dynamic separation of three-layered

5

liquid phases in TLPS is in fact a course of dispersive polymer and organic oil droplets

6

aggregated and separated out respectively from continuous salt aqueous bottom phase.

7

Formation rate of organic oil phase was controlled mainly by coalescence rate of

8

dispersed oil droplets, however rate-determining process for formation of polymer middle

9

phase may change from drop sedimentation to coalescence or co-determined by both,

10

when mass composition of the TLPS changed along different operation lines. With the

11

formation of organic oil phase, it becomes another continuous phase, from which

12

dispersive polymer droplets separated out and aggregated into a bulk phase gradually.

13

Phase separation equilibrating time of TLPS, tE, depends on formation rate of the

14

polymer middle phase and its equilibrium volume. A quantitative correlation of phase

15

separation rate of TLPS with its physic-chemical properties was given. The present work

16

promotes further understanding about influence from change in mass composition of

17

phase-forming components in TLPS on three-phase separation dynamic processes.

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19


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1. INTRODUCTION

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Three-Liquid-Phase System (TLPS) has attracted extensive interests in past decades,

3

due to its unique advantages to achieve process intensification for extraction and

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separation of multiple targets simultaneously from their complex leach solutions.1 Based

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on difference in physicochemical properties and hydrophilic-hydrophobic environment of

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three coexisting liquid phases, different target compounds have different partitioning

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behaviors in different liquid layers.2 Application of TLPS has been extended towards

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many areas, such as separation and purification of various biochemicals3,4, preparation of

9

natural products5,6, treatment of polyphenol waste water7 and grouping separation of

10

multi-metals8,9. All of those works advance such a unique separation method gradually to

11

its maturity. However, this technology has not been used in industrial scale, partially due

12

to the limited engineering knowledge and understanding of phase separation behavior in

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this system.

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Up till now, most of investigations on TLPS focus only on thermodynamics

15

phase-forming conditions and its stabilization mechanism, without considering its phase

16

separation dynamic process.10-12 The lack of data in the aspect makes a main hinder for

17

further developing three-liquid-phase partition technique toward its future engineering

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applications. Our previous work suggested a mixer-settler-mixer three-chamber

19

integrated extractor to conduct three-liquid-phase partitioning processes.13,14 That work

20

provides a feasible equipment for continuous operation and future industrial scale-up.

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However, design of such a three-liquid-phase extractor needs detailed data about phase


3


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separation dynamic behavior of TLPS. Researches into phase separation rate of

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three-layered liquid phases between each other, and the minimum time needed for

3

completion of phase separation are crucial to obtain optimal design of that extractor and

4

thus its operation cost.

5

Some works have been reported on phase separation kinetics of oil-water system and

6

polymer-salt aqueous biphasic systems. J. Golob and R. Modic suggested a model to

7

calculate phase separation rate of oil and water two phases based on the change of

8

dispersion band thickness over time.15 They found a correlation between the phase

9

separation rate and physic-chemical properties of system, by which the rate-determining

10

process can be obtained. Following their work, A. Kaul et al. derived a modified model to

11

correlate phase separation rate with system properties for polymer-salt aqueous

12

two-phase system.16-18 They pointed out that phase separation rate is highly related to

13

which phase is continuous phase and which phase is dispersive one during the separation

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of two immiscible liquids. Based upon these works, A. V. Narayan et al. determined

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phase separation rate of t-butanol/ammonium sulfate two-phase system and Polyethylene

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glycol (PEG) 4000/ammonium sulfate aqueous biphasic system.19 Results obtained in

17

their works provide theoretical basis for understanding phase-forming dynamic processes

18

and phase separation behaviors. However, report on the dynamic behaviors of phase

19

separation in multi-liquid-phase systems is rare. The main difficulty is due to lack of

20

suitable method to quantitatively describe the phase separation behavior in complex

21

system containing more than two liquids. Besides, acquisition on which phase(s) is/are


4

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continuous one(s) is also an uneasy issue in such a multiple phase separation process.

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The aim of present work is to investigate phase separation dynamic behavior of the

3

TLPS composed of an oil phase and water-soluble polymer based aqueous two phases

4

with the change of mass composition of phase-forming components. A model TLPS in

5

current investigation is prepared by mixing four components: organic extractant,

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polyethylene glycol (PEG), salt electrolyte (ammonium sulfate), and water. When

7

three-phase separation of such a system is finished, an oil top phase, a polymer-rich

8

middle aqueous phase and a salt-rich bottom aqueous phase can be obtained. The

9

variation in volume of each phase in TLPS over time is employed to describe dynamic

10

separation of the three-layered liquid phases. By changing weight percentage of

11

phase-forming components in systems, the rate-control process in formation of such a

12

kind of TLPS is recognized; and, the influence on phase separation equilibrating time, tE,

13

is discussed. A quantitative correlation of phase separation rate of TLPS with its

14

physic-chemical properties is given. The present work provides insight into the phase

15

separation dynamic processes of TLPS, which is crucial for the design of appropriate

16

extraction separators for future industrial application of three-liquid-phase separation

17

technology.

18 19

2. EXPERIMENTAL SECTION

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2.1. Chemicals and Reagents Polyethylene glycol (average molecular weight of 2000, denoted as PEG2000) and

21


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ammonium sulfate were purchased from Sino-Pharmaceutical Group, China. Organic oil,

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2-ethylhexyl hydrogen-2-ethylhexylphosphonate (denoted as P507), was supplied by

3

Shanghai Laiyashi Chemical Co. Ltd, China. All chemicals were of analytical grade and

4

used as received without further purification.

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2.2. Preparation of oil-polyethylene glycol-sulfate-water based TLPS with three

6

layered coexisting liquid phases

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Certain mass of ammonium sulfate, PEG2000, deionized water and P507 was added

8

into a graduated cylinder (see Fig. s1 in Supporting Information) and then mixed

9

thoroughly by electromagnetic stirring. Once stirring stopped, let the system stand to

10

settle for phase separation, until turning into three layers of volume-stable liquid phase.

11

From up to down, they are organic oil top phase, PEG2000-rich middle phase and

12

ammonium sulfate-rich aqueous bottom phase, respectively.20 This state of system with

13

three layered coexisting liquid phases is called a TLPS in present work.

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The time when stirring just stopped was recorded as zero. Since then, mixed liquid

15

phases started to separate between each other. During three-phase separation and

16

formation of a stable TLPS, volumes of each phase were recorded over time by visual

17

observation. The minimum time required for complete three-phase separation was also

18

recorded. Experiments were conducted in triplicate, and all the experiments were

19

performed under 20 oC.

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2.3. Variation in weight percentage of phase-forming components in TLPS Formation of a TLPS mentioned above requires four phase-forming components:

21


6

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P507 (oil), PEG2000 (polymer), ammonium sulfate (salt) and water. Usually, isothermal

2

phase diagram of a quaternary system is described by a tetrahedron.12, 21-22 When the ratio

3

of any two components is fixed, the phase diagram is described as a sectional view of the

4

tetrahedron. Here, the mass ratio of water and oil is fixed at 1:1. Fig.1 depicts a schematic

5

cross-section view of isothermal phase diagram for polymer (P)-water (W)-oil (O)-salt (S)

6

quaternary system. Firstly, a system point M in the three-phase zone was chosen. Mass

7

composition of M point would result in a stable TLPS after complete phase separation.

8

Then, three straight lines, X1-M-Y1-W/O, X2-M-Y2-P and X3-M-Y3-S, were drawn,

9

which all passed through the point M and ended at the three vertices W/O, P, S

10

respectively. We define these three straight lines, X1-M-Y1-W/O, X2-M-Y2-P and

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X3-M-Y3-S, as the water/oil operation line, polymer operation line and salt operation line,

12

respectively. On the three operation lines in the range of three-phase zone, fifteen

13

experimental points, A1 to E1, A2 to E2, and A3 to E3 were chosen as experimental

14

system points. The compositions of all experimental points are shown in Table. 1.

15

In phase separation experiments, from A1 to E1, the added mass of PEG2000 was

16

the same, and the amount of ammonium sulfate was also the same, while the mass of

17

water and P507 increased simultaneously at a fixed mass ratio of 1:1. From A2 to E2, the

18

mass of ammonium sulfate was equal, so is that of water and P507 (mass ratio 1:1), while

19

the mass of PEG2000 increased. Similarly, from A3 to E3, only the mass of ammonium

20

sulfate raised, while that of other components was unchanged.

21

2.4. Determination of physical and chemical parameters of TLPS


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Density and viscosity

2

After phase separation finished, the three liquid layers in TLPS were isolated out

3

respectively using a separatory funnel. The density of the three isolated phases was

4

calculated by weighing the mass per unit volume. The viscosity of each phase was

5

measured by using an Ostwald viscometer.

6

Surface and Interface tension

7

The surface tension of the polymer phase and the interfacial tension between the

8

polymer middle phase and the organic top phase in TLPS was measured on a KRUSS

9

Tensiometer (K100) by using Wilhelmy platinum plate method. The tensionmeter has a

10

tension range of 1~1000 mM.m-1 with 0.01 solutions. Calibration of the tensionmeter was

11

performed by determining the surface tension of distilled water and comparing with

12

literature data. All above determination of physicochemical properties of the TLPSs were

13 14

performed under 20 oC, and the experiments were conducted in triplicate.

15 16

3. RESULTS and DISCUSSION

17

3.1 Phase separation process during formation of TLPS

18

Fig.2 shows a schematic phase separation process during formation of a TLPS. At

19

t=t0=0, the system is well mixed (Fig.2(I)). After a while (t=t1), an interface (a) appears

20

at the bottom of the system (Fig.2 (II)). At t=t2, another interface (b) above the interface


8

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(a) becomes visible (Fig.2 (III)). Interface (b) rises with the time, while interface (a) has

2

no evident shift. Since time t=t3, the third interface (c) appears at the top of system (Fig.2

3

(IV)), which then moves down towards interface (b). Finally, the two interfaces, (b) and

4

(c), merges and turns into a new interface (d) at the time t=t4 (Fig.2 (V).We believe that

5

the three liquid phases in the system has been separated completely between each other at

6

the time t=t4.

7

In Fig.2, H is the height of total system. Ha, Hb and Hc, Hd are the heights of

8

interface (a), (b), (c) and (d), respectively. The cross-sectional area of cylinder is known

9

as A. At time t, the volume of salt bottom phase (Vst), polymer middle phase (Vpt) and oil

10

top phase (Vot) , can be expressed as follows:

11

Vst  H at  A

(1)

12

V pt   H bt  H at   A

(2)

13

Vot   H  H ct   A

(3)

14

When time arrived at t4, the volumes of the three phases become constant. Here, the

15

time t4 is defined as the phase separation equilibrium time of TLPS, denoted as tE. The

16

final volume of each phase is defined as their equilibrium volume, denoted as VsE, VpE

17

and VoE:

18

VsE  H a E  A

(4)

19

Vp E   H d E  H a E   A

(5)

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Vo E   H  H d E   A

(6)

For system point M, increase in volumes of three liquid phases over time is shown in

21


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Fig.3. Clearly, the volume of salt-rich bottom phase becomes readable at 0.5 minute,

2

changing little with time and reaching its plateau early. The volumes of polymer-rich

3

middle phase and that of oil top phase become readable at 1 minute and 2.5 minute,

4

respectively, both of which increase significantly with time until reaching their

5

equilibrium value at 6.5 minute (tE), when the TLPS forms completely.

6

3.2 Effect of weight percentage of phase-forming components on variation of

7

volumes of three liquid phases with time during formation of TLPS

8

In order to compare the phase formation rate of three-layered liquid phases among

9

the systems which has different weight percentage of phase-forming components, the

10

following variables are defined:

t' 

11

t tE

12

Vot V  E Vo

13

V 

14

Vs' 

' o

' p

V pt V pE

Vst VsE

(7)

(8)

(9)

(10)

15

Here, t’ is defined as the relative separation time and V’ is the relative phase volume

16

corresponding to t’. Subscripts o, p, s denote oil top phase, polymer middle phase and

17

salt aqueous bottom phase, respectively. The curves of Vo’ to t’ , Vp’ to t’, Vs’ to t’ are

18

defined as the formation curves of oil top phase, polymer middle phase and salt

19

aqueous bottom phase, respectively. 10


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1

In Fig. 4 (I), formation curves of oil phase in systems A1 to E1 is similar in

2

appearance. A similar phenomenon was also observed in literatures which focused on

3

phase separation kinetics of polymer-salt aqueous two-phase system16, where the

4

curves for relative thickness of dispersion band versus relative separation time were

5

similar for a small-scale (5grams) and a large-scale (1300 grams) system. This

6

phenomenon was attributed to phase separation mechanisms of those two systems are

7

the same. The rate-determining process during formation of the two systems is reported

8

identical (eg. coalescence of dispersed liquid droplets). Therefore, it can be inferred

9

that formation mechanism of the oil phase in TLPSs of present work is also the same,

10

when the system composition varies along the water/oil operation line. Studies on

11

oil-water systems15 and polymer aqueous two phase systems16 indicated that, if the

12

curve of relative thickness of dispersion band versus the relative separation time was a

13

sigmoidal curve, the phase separation process was controlled by coalescence rate of

14

dispersed droplets; while if that curve was an exponential curve, the process was

15

controlled by sedimentation rate of droplets. Analogously, the formation curves of oil

16

phase in Fig. 4 (I) is a sigmoid curve, implying that coalescence of oil droplets was the

17

rate-determining process for formation of oil top phase.

18

Different from oil top phase, PEG-rich polymer middle phase exhibits obvious

19

difference in phase formation curves from system points A1 to E1 (Fig. 4 (II)). This is

20

probably because that the formation mechanism of PEG-rich polymer phase changes

21

with the change of mass composition in those systems. From A1 to D1, the 11



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Page 12 of 41

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rate-determining process is believed to be drop coalescence. However, for system E1,

2

the phase formation curves of polymer phase is neither sigmoid nor exponential,

3

implying the formation process of the polymer phase might be determined by both of

4

drop coalescence rate and sedimentation rate.

5

Fig. 4 (III) shows the variations of relative volume of salt bottom phase, Vs’, versus

6

relative separation time, t’. Here, Vs’ of system points A1 to E1 all reaches 1 within a

7

short time and remains unchanged afterwards. Therefore, it is believed that the

8

ammonium sulfate-rich bottom phase in all of the five TLPSs is a continuous phase

9

during the course of three phase separation. In fact, our previous experiments on

10

determination of surface tension of the three isolated liquid phases in those TLPSs

11

confirmed above designation. It is found that the surface tension of salt-rich bottom

12

phase is larger than that of organic oil top phase and polymer-rich middle phase.

13

Therefore, coalescence rate of salt aqueous droplets is also believed larger than that of

14

oil and polymer droplets. That designation was confirmed by most of literatures about

15

discussion the separation behavior in oil-water two phase systems and polymer-salt

16

based aqueous biphasic systems.

17

In system points A2 to E2 whose composition varies along the polymer operation

18

line, plots of Vo’ versus t’ could be fitted by the same sigmoidal curve (Fig. 5 (I)).

19

Therefore, phase separation of the five systems are also believed having similar

20

mechanism and rate-control is coalescence of oil droplets. In Fig. 5 (III), Vs’ of the five

21

systems all reaches 1 rather quickly, implying that the salt aqueous phase is still a 12


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1


continuous phase in the process of phase separation.

2

In Fig. 5 (II), the formation curves of polymer phase (Vp’ vs t’) can be classified

3

into two types. An exponential curve is observed for system A2 where the weight

4

percentage of PEG2000 in the system is very small (wt.5.7%), implying that the

5

formation of polymer phase is controlled by the rate of drop sedimentation. On the

6

other hand, as PEG2000 percentage increased up to a certain threshold, the coalescence

7

rate of dispersed polymer droplets becomes dominant in controlling the formation

8

process of the polymer phase, so a sigmoidal curve is observed. When the PEG2000

9

percentage increases further, from B2 to E2, the curve of Vp’ vs t’ does not shift or

10

change in appearance. It tells that formation mechanism of polymer phase during phase

11

separation remains unchanged in those four systems.

12

In system points A3 to E3 along salt operation line, inference obtained for both oil

13

phase and salt phase is the same as that from A1 to E1 and A2 to E2. However, the

14

polymer phase formation curves, as shown in Fig. 6 (II), are different from A3 to E3,

15

which implies that formation mechanism of polymer phase in those five systems is

16

different. Formation process of the polymer phase in system A3 is controlled by drop

17

coalescence rate only, where the ammonium sulfate concentration is quite small

18

(6.68%). However in the other four systems the curves are not typical sigmoidal or

19

exponential, implying that formation process of the polymer phase is co-determined by

20

the rate of the drop coalescence and its sedimentation. Here, an interesting phenomenon should be addressed. Top, middle and bottom

21

13



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Page 14 of 41

1

phase in TLPS emerge at different time respectively, but salt bottom phase reaches its

2

equilibrium early without further change in volume, as shown in Figures 4, 5 and 6. As

3

above mentioned, the salt bottom phase is a continuous phase, from which dispersive

4

polymer and organic oil droplets aggregated and respectively separated out. Therefore,

5

the salt bottom phase appears firstly. In addition, the departure rate of polymer and oil

6

droplets from salt continuous phase would be fast according to the Stokes Law, due to the

7

viscosity of salt aqueous phase was very low (which is approximate to that of water).

8

Therefore, salt aqueous phase reached its equilibrium volume very early. In comparison

9

with salt aqueous phase, sedimentation and coalescence of dispersive polymer and oil

10

droplets require a period of time, so the readable volume of polymer and oil phases

11

appeared late. The data in Figures 4, 5 and 6 also indicate that the oil phase and polymer

12

phase arrive at their equilibrium volume at same time. That means the separation between

13

oil and polymer phase may be continue after they separated out from the salt aqueous

14

bottom phase. When relative separation time, t’, is around 0.4, the salt aqueous phase

15

already reached its equilibrium completely without further change in volume. Therefore,

16

variations of relative volume of those two phase, Vo’ and Vp’ , versus relative separation

17

time, t’, result mainly from the separation between oil and polymer phase.

18

3.3 Effect of weight percentage of phase-forming components on phase separation

19

equilibrium time of TLPS

20

It can be seen from Fig. 7, tE decreases with increasing weight percentage of water

21

and oil from A1 to E1 (Fig. 7(I)). In contrast, tE increases with increasing weight 14


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1


percentage of PEG (Fig. 7(II)), or that of ammonium sulfate (Fig. 7(III)).

2

As mentioned previously, salt phase reaches its equilibrium volume within a short

3

time, while polymer phase and oil phase reach their equilibrium volumes much later.

4

For this reason, tE for formation of a stable TLPS is equal to that time required for

5

complete formation of polymer middle phase or oil top phase. From previous

6

discussions, it is known that formation mechanism of oil phase remains the same, when

7

phase-forming components percentage varies along the three operation lines. If the

8

equilibrium volume of oil top phase was similar, such as from A2 to E2 and from A3 to

9

E3 (see Fig. s2 in Supporting Information), the time required for formation of their oil

10

phases should be similar, since formation rates of those oil phases are similar. However,

11

tE changes a lot in those systems. So it is believed that tE could be the time for

12

formation of polymer phase rather than that for oil phase.

13

There are two factors in deciding tE: the formation rate and the equilibrium

14

volume of the PEG-rich polymer phase. If the former was the same, the larger the

15

phase volume, the longer tE will be; if the latter was the same, the greater the

16

formation rate, the shorter tE will be. Fig. 8 shows the equilibrium volume of polymer

17

middle phase of each system points along three operation lines. By comparing Fig. 7

18

with Fig. 8, we can find out which factor is dominating the change of tE. On the

19

water/oil operation line, tE shortens gradually from A1 to E1 (Fig. 7(I)), although the

20

corresponding polymer phase equilibrium volume (VpE) becomes larger consistently

21

(Fig. 8(I)). Hence, it is reasonable to believe that the decrease in tE is a result of faster 15



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Page 16 of 41

1

formation rate of polymer phase. From A1 to E1, the increase in water percentage

2

decreases concentration of PEG in polymer aqueous phase, so as to decrease the

3

hydrophobicity of polymer phase. Drops of those dispersive polymer phase will have

4

greater ability for coalescence if the difference of hydrophobicity between it and the

5

oil is larger. Along the polymer operation line from A2 to E2, the equilibrium volume

6

of polymer phase (VpE) increases (Fig. 8(II)). Accordingly, tE was getting longer in

7

those systems (Fig. 7(II)). So it is hard to tell whether the phase formation rate or

8

volume of the phase is the dominant factor in prolonging tE. However, from the

9

previous discussions, it is known that formation mechanism of polymer phase in those

10

systems along the polymer operation line is unchanged from B2 to E2. This is highly

11

possible to result in a similar formation rate of polymer phase in those four systems.

12

Therefore, it is speculated that increase of tE from B2 to E2 might be a result of the

13

increase of VpE. Along the salt operation line, VpE is getting smaller from A3 to E3 (Fig.

14

8(III)). tE, on the contrary, increases gradually (Fig. 7(III)). So it can be assured that

15

the decrease of phase formation rate is the dominant factor in extending tE from

16

systems A3 to E3.

17

Actually, along the oil/water operation line from A1 to E1, weight percentage of water

18

and oil mixture increases, which result in an increase of water content in polymer middle

19

phase of final obtained TLPSs, and therefore an increase in equilibrium volume of

20

polymer phase. The increase of water content in polymer middle phase will result in

21

decrease in viscosity of polymer phase and increase in the density difference between 16


Page 17 of 41

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1

polymer phase and oil phase. According to the Stokes Law, the increase in density

2

difference will increase the moving rate of dispersed droplets, and the decrease in

3

viscosity of polymer phase is also favor of promoting the movement of oil drops.

4

Therefore, the separation rate between oil phase and polymer phase is enhanced.

5

Along the polymer operation line from A2 to E2, increasing polymer weight

6

percentage result in an increase of mass fraction of polymer in middle phase. Therefore,

7

viscosity of polymer phase increases and density difference between polymer and oil

8

phases decreases. The separation rate between oil phase and polymer phase decreases.

9

However, the increase of mass fraction of polymer in middle phase will result in an

10

obvious increase of the equilibrium volume. Therefore, the time for polymer phase

11

achieve its equilibrium volume prolongs.

12

Along the salt operation line from A3 to E3, increasing salt weight percentage result

13

in decrease of equilibrium volume of polymer middle phase due to the salting-out

14

dehydration of polymers. The decrease in water content in polymer phase result in

15

increase in its viscosity and decrease in density difference with oil phases. Therefore, the

16

separation rate between oil phase and polymer phase decreases.

17

Generally speaking, when the weight percentage of components changes along the

18

water/oil or the salt operation line, variation in the formation rate of the polymer phase

19

decides the tendency of tE; whilst, when the components percentage changes along the

20

polymer operation line, variation of polymer phase equilibrium volume might become a

21

dominant factor in tE trend. The change in total volumes of those experimental TLPSs 17



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Page 18 of 41

1

were given in Supporting Information (see Fig. s3 in Supporting Information), but that

2

was obvious only for those systems from A1 to E1. As for systems from A2 to E2 and

3

from A3 to E3, the change in equilibrium volume of polymer middle phases was main

4

concern during discussion on the change of tE.

5

3.4 Correlation of formation rate of TLPS with physic-chemical properties of system

6

There are three separation processes during formation of TLPS: the separation

7

between oil phase and salt aqueous phase, polymer phase and salt aqueous phase,

8

polymer phase and oil phase. It is necessary to determine which separation is the

9

slowest process, whose rate can be on behalf of the formation rate of TLPS. It is known

10

that the separation rate between two liquid phases closely relates to the differentiation

11

between the continuous phase and the dispersed phase16. The phase separation rate is

12

usually described by the change rate of thickness of dispersion band (h) between two

13

immiscible liquid phases. A widely accepted expression for the separation rate in

14

oil-aqueous bi-phasic system is as follows15 : N2

N3

       C  dh  N1          dt  D   C   W 

15

N4

(11)

16

where Δρ is the density difference between the oil and the aqueous phase, ρC is the

17

density of the continuous phase, μC and μD are the viscosity of the continuous phase and

18

the dispersion phase respectively, σ is the interface tension between the two liquid

19

phases, and σw is the surface tension of the aqueous phase. N1, N2, N3, N4 are constants.

20

For PEG-sulfate aqueous bi-phasic systems the expression is also available23: 18


Page 19 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


   dh C  N1'    dt   C 

1

N 2'

   C   D 

N 3'

    '   W 

N 4'

(12)

2

where σw’ is the surface tension of pure water, N1’, N2’, N3’, N4’ are constants and other

3

symbols’ meanings are the same as in Eq. (11).

4

In present work, ammonium sulfate-rich salt aqueous phase is a continuous phase.

5

This means that polymer phase is a dispersive phase when it is separated from the salt

6

aqueous phase. Similarly, the oil phase is also a dispersive phase when it is separated

7

from the salt aqueous phase. Therefore, Eq. (11) and Eq. (12) might be applicable to

8

describe the rates of oil phase and PEG-rich polymer phase separating out from salt

9

aqueous phase. Maybe there is some inconsistency for the two separation rates in TLPS,

10

due to the coexistence of the third phase. But it is noticed that the salt phase reaches its

11

equilibrium volume very early (Fig. 3), which means that both of oil phase and

12

polymer phase separating out from the salt aqueous phase very fast.

13

Comparatively, the dispersion band between polymer phase and the oil phase takes

14

a long time to disappear (Fig. 2(IV)). The separation rate between polymer and oil

15

phases is slow enough to be on behalf of the formation rate of TLPS. Because PEG is

16

water-soluble polymer, PEG-rich polymer middle phase is an aqueous liquid phase.

17

Therefore, the separation between polymer phase and oil phase can be considered as

18

separation between oil and aqueous phase. Eq. (11), which describes the correlation

19

between phase separation rate and physic-chemical properties in oil-water biphasic

20

system, can be used for reference in description of the correlation of the formation rate 19



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Page 20 of 41

1

of TLPS and the system properties. However, to find out which properties are related,

2

it is necessary to determine which one is the continuous phase. Here, the oil phase is

3

assumed to be a continuous phase, from which the polymer phase drops coalescence

4

and separate out. Different from the phase separation in oil-water bi-phasic system, the

5

coexistence of the salt phase in the system should be taken into consideration. So, a

6

correlation between formation rate of TLPS and system properties is given by:

  p  o  d [( H c  H b ) A]  N1"    dt  o 

7

8

N 2"

  o   p

  

N 3"

  p o     p

  

N 4"

(13)

where

( H c  H b ) A  Vtotal  Vs  Vp  Vo

9

(14)

10

Hc and Hb are the heights of interface (c) and interface (b), respectively. A is the

11

cross-sectional area of cylinder. Δρp-o is the density difference between polymer middle

12

phase and oil top phase. ρo is the density of oil phase. μo and μp are the viscosity of oil

13

phase and polymer phase, respectively. σp-o is the interface tension between oil and

14

polymer phases. σp is the surface tension of polymer phase. N1’’, N2’’, N3’’, N4’’ are

15

constants.

16

There are three variables in Eq. (13): Δρp-o/ρo, μo /μp , σp-o/σp. The exact values of

17

those variables depend on the phase composition of TLPS in equilibrium. The detailed

18

equilibrium composition of each phase in those experimental TLPSs is given in

19

Supporting Information (see Table. s1 to Table. s3 in Supporting Information). Fig. 9, 10 and 11 give the variation trends of those variables when the weight

20

20


Page 21 of 41

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1


percentage of the phase-forming components changes along the three operation lines.

2

From previous discussions, it is known that for the systems changing along the

3

oil/water operation and the salt operation line, the trend of their tE is determined by the

4

formation rate of the polymer phase, namely, the formation rate of TLPS. Along the

5

water/oil operation line, tE decreases from A1 to E1 (Fig. 7(I)). It reflects the

6

corresponding formation rate of TLPS increasing from A1 to E1. Fig. 9(I) shows the

7

value of Δρp-o/ρo going down from A1 to E1. The trend of Δρp-o/ρo is opposite to that of

8

formation rate of TLPS, implying it is possible that the exponential constant for the

9

Δρp-o/ρo term (N2’’) is a negative value in Eq. (13). Different from the term of Δρp-o/ρo,

10

the value of μo /μp increases from system A1 to E1 (Fig. 10(I)). So a possibility is that

11

exponential constant for the μo /μp term (N3’’) is a positive value in Eq. (13). In Fig.

12

11(I), the value of σp-o/σp shows an upside trend from A1 to E1, too. Possibly, the

13

exponential constant for this term, N4’’, is also a positive value.

14

On the salt operation line, the value of tE becomes greater from systems A3 to E3

15

(Fig. 7(III)), which means the formation rate of TLPS decreasing gradually.

16

Correspondingly, the value of Δρp-o/ρo rises up in those systems (Fig. 9 (III)). The

17

tendency of Δρp-o/ρo is opposite to that of the formation rate of TLPS. So it is possible

18

that the exponential constant N2’’ is a negative value. The term of μo /μp exhibits a

19

downtrend from A3 to E3 (Fig. 10 (III)), giving a possibility that exponential constant

20

N3’’ is a positive value. The exponential constant N4’’ is possibly a positive value too, as

21

a result of the downside tendency of σp-o/σp from A3 to E3. From both of the oil/water 21



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Page 22 of 41

1

and salt operation lines, the speculation on the sign of the three exponential constants is

2

identical. It is believed that Eq. (13) has its rationality to some extent. Therefore, the

3

estimation that during the separation between polymer and oil phases, the former is the

4

dispersive phase, and oil phase becomes a continuous phase is credible. For the system points on the polymer operation line, it shows that the value of

5 6

Δρp-o/ρo varies a little from B2 to E2 (Fig. 9 (II)), so does the value of μo /μp (Fig. 10 (II))

7

and that of σp-o/σp (Fig. 11 (II)). It implies the formation rate of TLPS obtained by Eq.

8

(13) maybe quite similar in those systems. This inferred result confirms that the

9

increase of tE from B2 to E2 is mainly due to the increase in the polymer phase volume. Eq. (13) is correlated with the experimental data using multiple regressions by

10 11

Matlab 7.0. The following equation is obtained:

d (Vtotal  Vs  V p  Vo )

12

dt

    2.7028   p o   o 

2.0096

  o  p 

  

0.1363

    p o     p 

3.3843

(15)

13

According to Eq. (15), the separation rate between polymer phase and oil phase

14

could be calculated if we know the exact value of Δρp-o/ρo, μo /μp , σp-o/σp , when phase

15

separation reach equilibrium. Eq. (15) gives a good fit to the experimental data. In

16

addition, it is also indicated that the interfacial tension term is one of the most influential

17

factor in determining phase separation rate.

18

Overall, during the separation of polymer middle phase and oil top phase, the former

19

probably has a slower coalescence rate, because it is the dispersive phase while the latter

20

can be seen as a continuous phase. Therefore, the formation rate of TLPS is actually the 22


Page 23 of 41

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1

separation rate of the polymer middle phase from oil phase. For specific equilibrium

2

volume of polymer middle phase, tE of the system depends on the formation rate of

3

polymer phase.

4 5

4. CONCLUSION

6

Experimental results indicate that phase separation dynamic process has a close

7

relationship with the change in mass composition of phase-forming components in

8

P507-PEG2000-ammonium sulfate-water based TLPS. Coalescence of oil droplets is a

9

rate-determining process for oil top phase formation, while formation rate of polymer

10

middle phase depends on mass composition of TLPS along different operation lines. The

11

rate-determining process for formation of polymer middle phase may change from drop

12

sedimentation to coalescence or co-determined by both. Phase separation equilibrium

13

time, tE, depends on the formation rate and equilibrium volume of PEG-rich polymer

14

middle phase. When weight percentage of phase-forming components changes along the

15

water/oil or the salt operation line, formation rate of polymer phase deciding the tendency

16

of tE; whilst, when components percentage changes along the polymer operation line, the

17

variation in polymer phase volume might become a dominant factor. A quantitative

18

correlation of phase separation rate of TLPS with its physic-chemical properties was

19

given, from which we can concluded that formation of three-layered liquid phases in

20

TLPS is in fact a course of dispersive polymer and organic oil droplets aggregated and

21

separated out respectively from continuous salt aqueous bottom phase. With the 23



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Page 24 of 41

1

formation of organic oil phase, it becomes another continuous phase, from which

2

dispersive polymer droplets separated out and aggregated into a bulk phase gradually.

3

Research into phase separation dynamics is essential to understand the influence

4

from phase-forming processes on partitioning behavior of targets in TLPS. It is hopeful

5

the present work could provide necessary data which is needed for design of appropriate

6

extraction separators and formulation of operation rules for future industrial application

7

of three-liquid-phase separation technology.

8 9

ACKNOWLEDGEMENTS

10

This work was financially supported by Key Project of Chinese National Programs

11

for Fundamental Research and Development (973 Programs No. 2013CB632602,

12

2012CBA01203), National Natural Science Foundation of China (No. 51074150,

13

No.21027004) and Innovative Research Group Science Fund (No. 20221603).

14 15

SUPPORTING INFORMATION AVAILABLE

16

Sketch on experimental graduated cylinder in given in Fig. s1. Detailed description

17

about measurement of the volume of each phase in TLPS with time were discussed.

18

Equilibrium volume of the oil top phase and the change in the total volume of

19

experimental TLPSs with time is given in Fig. s2 and Fig. s3 respectively. Weight

20

percentage of each component in top, middle and bottom phase of TLPSs after

21

equilibrium is given in Table s1, s2 and s3 respectively. This information is available free 24


Page 25 of 41

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1


of charge via the Internet at http: //pubs.acs.org.

25



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Page 26 of 41

1

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2

(1) Mojski, M.; Gluch, I. Characteristics and Applications of Three-Phase Extraction

3

Systems. J. Anal. Chem. 1996, 51, 329.

4

(2) Dennison, C.; Lovrien, R. Three Phase Partitioning: Concentration and Purification

5

of Proteins. Protein Expression Purif. 1997, 11, 149.

6

(3) Johansson, G.; Hartman, A. Quantitative Model for Partition in Aqueous Multiphase

7

Systems. Eur. J. Biochem. 1976, 63, 1.

8

(4) Chen, J.; Liu, H. Z.; Wang, B.; An, Z. T.; Liu, Q. F. Study on the Three-Phase

9

Extraction of Penicillin G with a Single-step Method. Proceedings of the International

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Solvent Extraction Conference (ISEC 2002), Cape Town. Sole K. C.; Cole P. M.; Preston

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J. S.; Robinson D. J. Eds.; Chris van Rensburg Publications, 602

12

(5) Liu, L.; Dong, Y. S.; Xiu, Z. L. Three-Liquid-Phase Extraction of Diosgenin and

13

Steroidal Saponins from Fermentation of Dioscorea Zingibernsis C. H. Wright. Process

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Biochem. 2010, 45, 752.

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(6) Dang, Y. Y.; Zhang, H.; Xiu, Z. L. Three-Liquid-Phase Extraction and Separation of

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Capsanthin and Capsaicin from Capsicum annum L. Czech. J. Food Sci. 2014, 32, 109.

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(7) Shen, S. F.; Chang, Z. D.; Liu, H. Z. Three-Liquid-Phase Extraction Systems for

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Separation of Phenol and p-Nitrophenol from Wastewater. Sep. Purif. Technol. 2006, 49,

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217.

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(8) Xie, K.; Huang, K.; Xu, L.; Yu, P.; Yang, L.; Liu, H. Three-Liquid-Phase Extraction

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and Separation of Ti(IV), Fe(III), and Mg(II). Ind. Eng. Chem. Res. 2011, 50, 6362. 26


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(9) Sui , N.; Huang, K.; Zhang, C.; Wang, N.; Wang, F.; Liu, H. Light, Middle, and

2

Heavy Rare-Earth Group Separation: A New Approach via a Liquid-Liquid-Liquid

3

Three-Phase System. Ind. Eng. Chem. Res. 2013, 52, 5997.

4

(10) Ruiz, F.; Marcilla, A.; Ancheta, A. M.; Rico, C. Liquid-Liquid Equilibrium of the 3

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Liquid-Phasesat Equilibrium System Water-Phosphoric acid-di-isopropyl Ether at

6

25-degrees-C and 40-degree-C. Solvent Extr. Ion Exch. 1986, 4, 789.

7

(11) Hartwig, G. M.; Hood, G. C.; Maycock, R. L. Quaternary Liquid Systems with Three

8

Liquid Phases. J. Phys. Chem. 1955, 59, 52.

9

(12) Baik, K. J.; Kim, J. Y.; Lee, H. K.; Kim, S. C. Liquid-Liquid Phase Separation in

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Polysulfone/Polyethersulfone/N-methyl-2-pyrrolidone/Water Quaternary System. J Appl.

11

Polym. Sci. 1999, 74, 2113.

12

(13) He, X.; Huang, K.; Yu, P.; Zhang, C.; Xie, K.; Li, P.; Wang, J.; An, Z.; Liu, H.

13

Liquid-Liquid-Liquid Three Phase Extraction Apparatus: Operation Strategy and

14

Influences on Mass Transfer Efficiency. Chinese J. Chem. Eng. 2012, 20, 27.

15

(14) Sui, N.; Huang, K.; Zheng, H.; Lin, J.; Wang, X.; Xiao, C.; Liu, H.

16

Three-Liquid-Phase Extraction and Separation of Rare Earths and Fe, Al, and Si by a

17

Novel Mixer-Settler-Mixer Three-Chamber Integrated Extractor. Ind. Eng. Chem. Res.

18

2014, 53, 16033.

19

(15) Golob, J.; Modic, R. Coalescence of Liquid-Liquid Dispersions in Gravity Settlers.

20

Trans. Inst. Chem. Eng. 1977, 55, 207.

21

(16) Kaul, A.; Pereira, R. A. M.; Asenjo, J. A.; Merchuk, J. C. Kinetics of Phase 27



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Separation for Polyethylene Glycol–Phosphate Two-Phase Systems. Biotechnol. and

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Bioeng. 1995, 48, 246.

3

(17) Mistry, S. L.; Kaul, A.; Merchuk, J. C.; Asenjo, J. A. Mathematical Modelling and

4

Computer Simulation of Aqueous Two-Phase Continuous Protein Extraction. J.

5

Chromatogr. A 1996, 741, 151.

6

(18) Asenjo, J. A.; Andrews, B. A. Aqueous Two-Phase Systems for Protein Separation:

7

Phase Separation and Applications. J. Chromatogr. A 2012, 1238, 1.

8

(19) Narayan, A. V.; Madhusudhan, M. C.; Raghavarao, K. Demixing Kinetics of Phase

9

Systems Employed for Liquid-Liquid Extraction and Correlation with System Properties.

10

Food Bioprod. Process 2011, 89, 251.

11

(20) Xie, K.; Huang, K.; Yang, L.; Liu, H. Enhancing Separation of Titanium and Iron by

12

Three-Liquid-Phase Extraction with 1,10-phenanthroline as Additive. J. Chem. Technol.

13

Biot. 2012, 87, 955.

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(21) Kahlweit, M.; Lessner, E.; Strey, R. Phase-Behavior of Quarternary Systems of the

15

Type H2O-Oil-Nonionic Surfactant Inorganic Electrolyte. 2. J. Phys. Chem. 1984, 88,

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1937.

17

(22) Strey, R.; Jonstromer, M. Role of Medium-Chain Alcohols in Interfacial Films of

18

Nonionic Microemulsions. J. Phys. Chem. 1992, 96, 4537.

19

(23) Asenjo, J. A.; Mistry, S. L.; Andrews, B. A.; Merchuk, J. C. Phase Separation Rates

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1

FIGURE CAPTIONS

2

Figure

3

Oil/Water-Polymer-Salt system at water/oil ratio of 1:1

4

Figure 2. Sketch on phase separation process of TLPS

5

Figure 3. Variation in volume of salt-rich bottom phase (Vs), polymer-rich middle phase

6

(Vp) and oil top phase (Vo) over time in the system M

7

Figure 4. Influence of increasing weight percentage of water and oil mixture (mass

8

ratio of water to oil is 1) on the formation curve of each phase

9

Figure 5. Influence of increasing weight percentage of polymer on the formation curve

1.

Schematic

cross-section

view

of

tetrahedron

phase

diagram

of

10

of each phase

11

Figure 6. Influence of increasing weight percentage of salt on the formation curve of

12

each phase

13

Figure 7. Phase separation equilibrium time for TLPS

14

Figure 8. Equilibrium volume of the polymer middle phase in TLPS

15

Figure 9. Value of Δρp-o/ρo in each TLPS

16

Figure 10. Value of μo /μp in each TLPS

17

Figure 11. Value of σp-o/σp in each TLPS

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Schematic cross-section view of tetrahedron phase diagram of Oil/Water-Polymer-Salt system at water/oil ratio of 1:1 (2, 3 and 4 represents the number of phases in systems. M is a random system point in the three-phase zone. The dashed lines connecting M and the three vertices are the oil/water operation line, the polymer operation line and the salt operation line, respectively. X1, Y1, X2, Y2, X3, Y3 are the intersection points of those operation lines with the boundaries of the three-phase zone. A1 to E1, A2 to E2, and A3 to E3 are selected system points located on those operation lines between the two intersection points. )


Page 30 of 41

Page 31 of 41

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Figure 2. Sketch on phase separation process of TLPS (The starting point of timing t0=0 is the time when stirring stops. t1，t2，t3 each denotes the time point at which interface a, b and c appears. t4 is the time point when interface b and c combine to form interface d. H represents the total height of the system, and Ha, Hb, Hc, Hd is the height of interface a, b, c and d respectively.)



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Figure 3. Variation in volume of salt-rich bottom phase (Vs), polymer-rich middle phase (Vp) and oil top phase (Vo) over time in the system M (The composition of system M is 7% salt, 8.11% polymer and 84.89% mixture of oil and water in which mass percentage of oil and water is identical.)


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Figure 4. Influence of increasing weight percentage of water and oil mixture (mass ratio of water to oil is 1) on the formation curve of each phase (Subscripts o, p, s denote the oil, polymer and salt phases respectively. The curve in Fig.4 (I) is a fit curve of all the systems, R2=0.98. The curves in Fig.4(II) are fit curves of corresponding color points, R2=0.985, 0.989, 0.997, 0.992 and 0.977 for A1 to E1, respectively. All the curves are third order polynomial fitting curves.)



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Figure 5. Influence of increasing weight percentage of polymer on the formation curve of each phase (Subscripts o, p, s denote the oil, polymer and salt phases respectively. The curve in Fig.5 (I) is a fit curve of all the systems, R2=0.958. In Fig.5 (II) the black curve is a fit curve of system A2, R2=0.967. The red curve is a fit curve of system B2, C2, D2 and E2, R2=0.97. All the curves are third order polynomial fitting curves.)


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Figure 6. Influence of increasing weight percentage of salt on the formation curve of each phase (Subscripts o, p, s denote the oil, polymer and salt phases respectively. The curve in Fig.6 (I) is a fit curve of all the systems, R2=0.993. The curves in Fig.6 (II) are fit curves of corresponding color points, R2=0.996, 0.98, 0.993, 0.985 and 0.975 for A3 to E3, respectively. All the curves are third order polynomial fitting curves.)



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Figure 7. The phase separation equilibrium time for TLPS


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Figure 8. The equilibrium volume of the polymer middle phase in TLPS



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Figure 9. Value of Δρp-o/ρo in each TLPS (  p  o denotes the density difference between the polymer phase and the oil

phase.  p denotes the density of the polymer phase.)


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Figure 10. Value of μo /μp in each TLPS (  o and  p denotes the viscosity of the oil and the polymer phases, respectively.)



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Figure 11. Value of σp-o/σp in each TLPS (  p  o denotes the interfacial tension between the oil and the polymer phases.

 p denotes the surface tension of the polymer phase.)


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Table.1 The mass composition of components in TLPS Weight percentage (w%) water and oil point

salt

polymer （wwater%= woil%）

M

7%

8.11%

84.89%

A1

11.75%

13.62%

74.63%

Water/oil

B1

9.9%

11.48%

78.62%

operation

C1

8.56%

9.92%

81.52%

line

D1

7.53%

8.73%

83.74%

E1

6.73%

7.80%

85.47%

A2

7.18%

5.70%

87.12%

Polymer

B2

7%

8.11%

84.89%

operation

C2

6.8%

10.78%

82.42%

line

D2

6.61%

13.13%

80.26%

E2

6.45%

15.35%

78.20%

A3

6.68%

8.14%

85.18%

Salt

B3

9.22%

7.92%

82.86%

operation

C3

11.61%

7.71%

80.68%

line

D3

13.89%

7.51%

78.60%

E3

16.05%

7.32%

76.62%

Salt, polymer, water and oil denotes (NH4)2SO4, PEG2000, deionized water and P507, respectively.


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