Investigation and Comparison of Two Effects of Bubble in Solvent

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Keywords: Solvent Sublation, Adsorption, Volatilization, Butyl Acetate. 1. ... large hydrophobicity, mass transfer of solute depends on the adsorption on the ...

Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

Investigation and Comparison of Two Effects of Bubble in Solvent Sublation under Different Temperature Yinchen Ma1,2, Zhidong Chang1, Bo Xiao1,2, Kang Wang1,2 and Huizhou Liu1 1. Laboratory of Separation Science and Engineering, State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Science, Beijing 100080, China 2. Graduate School of the Chinese Academy of Sciences, Beijing 100049, China Abstract: The surface tension of the aqueous solution of butyl acetate was measured as a function of the total concentration at different temperatures under atmospheric pressure. Equilibrium tension data have been related to the concentration of butyl acetate. The applicability and suitability of the Langmuir and Henry adsorption models to the equilibrium data was investigated. A calculation and comparison is made between the surface adsorption amount of a bubble and the total amount of solute interior the bubble. The proportional of each is determined by theoretical calculation. A comparison was made between experimental results of the air stripping and solvent sublation. Keywords: Solvent Sublation, Adsorption, Volatilization, Butyl Acetate 1. INTRODUCTION Solvent sublation, a non-foaming adsorptive bubble separation process first originated by Sebba[1], has been used in the separation and recover organic solutes from wastewater. In this process a surface-active(or volatile) solute is transported from the aqueous phase by the bubbles rising through the solvent sublation column. It has been indicated that there may exist some mechanisms in the process of solvent sublation depending on the system disposed and many researchers have made investigations on such subject. The overall mechanism of the process was first suggested by Pinfold and coworkers[2, 3] and by Karger and coworkers[4, 5]. It was suggested that the solute transfer from the aqueous phase by air bubbles into the organic solvent phase in sublation is unidirectional, the predominant solute transport mechanism is due to the bubbles carrying solute across the solvent-water interface. It was also found that the air bubbles drag a sheath of water into the solvent layer, which drains as water droplet with equilibrium amounts of solutes through the organic solvent. So the overall mechanism involves piggyback transport of solute by the air bubble(both inside and on the surface), a simultaneous molecular diffusion mass transfer and transport associated with a water sheath dragged by the bubbles[6, 7], among which mass transfer by the air bubbles is of paramount importance. As to the effect of bubble, however, one solute makes a great difference to another that with some distinct characteristics. For organic substances with large hydrophobicity, mass transfer of solute depends on the adsorption on the bubbles surface, while for compounds with fairly volatility it was through the vaporization into the gas phase interior of the bubbles. As for a less volatile compound with certain kind of hydrophobicity, both aspects coexist. Butyl acetate is such a kind of organic substance. Solvent sublation has been successfully applied to separating and recovering butyl acetate from penicillin wastewater, from which butyl acetate is carried into

n-nonane by the bubbles rising through the column[8, 9]. Three possible transport pathways by bubbles were introduced by Sun[7] which were adsorption, attachment on the bubble surface and vaporization into the bubble interior. These concentration mechanisms by bubbles have been already included in the simulation of the whole process and achieved a great success. However, it is not very clear yet that which aspect has the greatest effect on the separation and which one overwhelms when the temperature changes. It is well known that surfactant can easily form unimolecular layers at the air-liquid interface and dramatically change the nature of the interface, and Langmuir isotherm parameter has been used for adsorption on the bubble surface[10-13]. During the simulation process of volatile solute separation by solvent sublation, Henry’s constant was commonly used[14,15], which gives the equilibrium relationship between the solute concentrations in the vapor and aqueous phases. Lu et al.[16] have used Gibbs adsorption isotherm to investigate the thermodynamics of surfactants in solvent sublation and some thermodynamic values were obtained. In this work, the surface tension of the aqueous solution of butyl acetate was measured as a function of the total concentration at two different temperatures under atmospheric pressure. Equilibrium tension data have been related to the Langmuir adsorption equilibrium isotherm and the Gibbs isotherm. A calculation and comparison is made between the surface excess of the bubbles and the total amount of volatile solute interior the bubbles. The total removal amount in the bubble interior by calculation was compared with that of air stripping and solvent sublation experiment. 2. EXPERIMENTAL 2.1. Reagent and procedure Butyl acetate, n-nonane and hexane were purchased from Beijing Chemical Reagent Company, China. All with analytical grade purity. A series of butyl acetate so-

Corresponding author: Huizhou Liu, [email protected]; Yinchen Ma, [email protected]

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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

lutions from 7.58×10-4molL-1 to 3.79×10-2molL-1 (0.01% to 0.5% by volume correspondingly) were prepared with bidistilled water before measurement to prevent hydrolysis of butyl acetate, very low concentration solutions were prepared by dilution. Surface tension of the aqueous butyl acetate solutions were measured by the Pt-Wilhelmy plate method, with a commercial Krüss K9 digital tensiometer under atmospheric pressure. The Pt plate was thoroughly cleaned and flame dried before each measurement. The experimental accuracy of the measurements was ±0.1mN/m, and the reproducibility of the results was better than ±0.2mN/m. Measurements of the surface tension of pure water at 298.15K were performed to calibrate the tensiometer and to check the cleanliness of the glassware. The sample solutions were airproofed in the conical flask and kept in constant temperature shaking gas bath with a accuracy of ±0.1K for about two hours before measurement. All the surface tension measurements were made when the room temperature was the same to our samples. In all cases three successive measurements were carried out quickly. 2.2. Apparatus Air stripping and solvent sublation experiments were carried out in a glass cylindrical column with sampling ports along the column. The column is 200cm tall with an inner diameter of 5cm. Experimental procedure and the analysis of sample method is similar to that described by Sun[8].

1

1

=

Γ

Γ∞

Γ=−

Cw



2 RT dC w

A=

σ =

1 A0 + A1C w

Rearrangement of Eq.2, we get 1

σ

= A0 + A1C w

(3)

The value of A0 and A1 at different temperature can be obtained from the 1/σ-c profile by application of Eq.3 between the measure value of surface tension and concentration of butyl acetate, which are listed in Tab.1. The adsorption of solute at the air-liquid interface can be written as the formation of Langmuir adsorption isotherm Γ = Γ∞

KC w 1 + KC w

(5)

Γ ∞ K Cw

1

(6)

Γ∞ N A

H cc , T = H cc , 20

⎡ ⎢10 ⎣

−B

( )⎤ 1



T

1

293

⎥ ⎦

(7)

Where Hcc,20 is the Henry’s constant at 20℃ and B is a constant. For butyl acetate, Hcc,20 and B are 8.29×10-3 and 2486 respectively. Mass transfer rate of solute carried in the bubble interior is given by =−

dt

QA Vw

H cc , T C w

(8)

Where QA is the airflow rate, Vw is the volume of the column’s aqueous phase, Cw is the concentration of aqueous phase. Integration of Eq.8 gives ln

(2)

1

where NA is the Avogadro number. Henry’s constant, which represents the air-water equilibrium partition coefficient for a particular chemical compound present in a dilute aqueous solution, can be expressed in a variety of ways. Valsaraj et al.[17] have defined the Henry’s constant using the combination of the vapor pressure of the solute, aqueous solubility, molecular weight of the solute and absolute temperature. In our case we use the dimensionless air to water concentration ratio Hcc, defined by Staudinger[18]

(1)

Where R is the universal gas constant, T is the absolute temperature. The relationship between surface tension and solute can be written as

1

Γ ∞ can be obtained by the plot of 1/Γ to 1/c, which is also tabulated in Tab.1. The molecular area is defined by the expression

dC w , o

3. THRORY The thermodynamic behavior of the adsorption of butyl acetate can be described by surface excess concentration, which can be determined from the surface tension by Gibbs adsorption isotherm

+

Cw , o 0

Cw

=−

QA Vw

H cc , T t

(9)

4. RESULTS AND DISCUSSION 4.1.Adsorption on the bubble surface and volatilization of bubble interior The surface tension γ vs molar concentration of butyl acetate Cw curves at 298.15K and 303.15K are shown in Fig.1. It is seen that γ decreases with increasing of Cw. An increase of temperature led to a decrease in the surface tension. Combination of Eq.1 and Eq.2, the value of Γ are also gained. Results from calculations for different temperatures are shown in Fig.2. It is seen from Fig.2 that the amount of adsorbed butyl acetate increases with concentration and saturation was not observed even when the solution reaches saturation. The surface excess concentration of the solution decreases with increasing temperature obviously. The calculation results of A from Eq.6 and that of Henry’s constant at different temperature from Eq.7 are listed in Tab.1.

(4)

Rearrangement of Eq.4 yields

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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

Eq.11 m ( vapor ) =

Surface Tension, mN.m

-1

70

Temperature,K 303.15 298.15

65

4 3

π r H cc ,T C w

m ( surface ) = 4π r Γ

55

50

0

5

10

15

20

25

30

35

40

-1

Concentration of Butyl Acetate, mmol.L

Fig.1 Surface tension profile against molar concentration of butyl acetate at 298.15K and 303.15K 3.5

The calculated results of m(vapor) and m(surface) at 298.15K and 303.15K and their ratios are listed in Tab.2. It can been seen from Tab.2 that the adsorption amount of a bubble is two orders of magnitude bigger than the amount of volatilization into a bubble although the ratio of them is smaller when the temperature becomes higher. So we can come to a conclusion safely that the adsorption of butyl acetate on the bubble surface is more important in the solvent sublation than vaporization into the bubble interior. This calculation is very useful in continuous operation when the aqueous phase were almost saturation. Our thought of adsorption will be overwhelmed by vaporization when temperature increases turns out to be uncorrected.

Temperature,K 298.15 303.15

3.0

50

simulation result air stripping solvent sublation

2.5

40 2.0

30

1.5

6

Efficiency, %

-2

(11)

2

60

45

Γ, 10 molm

(10)

3

1.0

0.5

20

10 0.0 0

5

10

15

20

25

30

35

40

0

-1

Concentration of Butyl Acetate, mmol.L

0

Fig.2 Surface excess concentration vs molar concentration curves at different temperature 4.2. Comparison of adsorption and vaporization Each run of solvent sublation was operated for 2 hours and the airflow rate is 400ml.min-1. The average diameter of the bubbles has been measured by putting a scaler into the bubble column as reference, which is shown to be 0.3mm[9]. Assuming that solute adsorption on the bubbles rising through the aqueous saturation solution reach the saturation, then Γ can be substituted by Γ∞. Then we can use the values of Γ∞ and Hcc,T to calculate the total amount of butyl acetate that would be carried in the vapor phase and on the surface of a bubble at equilibrium in a saturated solution of butyl acetate in water by Eq.10 and

20

40

60

80

100

120

Operationg time, min

Fig.3 Comparison of simulating results of vaporization to experimental results of air stripping and solvent sublation 4.3. Comparison of air stripping and solvent sublation Air stripping is commonly used to remove volatile organics from aqueous solution. Solvent sublation outbalances air stripping due to its reducing of solute backmixing into the aqueous phase from the organic layer when the air bubbles break. The comparison of simulating results to experimental results of air stripping and solvent sublation is demonstrated in Fig.3.

Table 1. The parameters obtained from the calculation of adsorption A0

A1

Γ ∞ , molm-2

A, m2

Hcc,T

298.15K

14.8789

0.1668

6.1410×10-6

2.70×10-19

1.15×10-2

303.15K

14.70907

0.2128

5.9471×10-6

2.79×10-19

1.58×10-2

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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

Table 2. The calculated values of adsorption amounts and vaporization amounts

m(vapor), mol

m(surface), mol

m(vapor):m(surface)

298.15K

4.929×10-14

6.945×10-12

1:140.9

303.15K

6.772×10-14

6.726×10-12

1:99.3

It is seen from Fig.3 that the simulation results of vaporization into the bubble interior was lower than that of experiment of air stripping. This maybe attribute to the neglect of the effect of the bubble size in the simulation. Comparison also shows that the separation efficiency of air stripping is much lower than that of solvent sublation. This has been proved by many researchers and the reason has been indicated as above. 5. CONCLUSIONS The adsorption of butyl acetate on the bubble surface in solvent sublation was simulated by Gibbs and Langmuir adsorption isotherm, the surface excess concentration and the molecular area were obtained. Vaporization of butyl acetate into the bubble interior has been modeled by Henry’s law, but it was not well described for the ignorance of the effect of bubble diameter. It has demonstrated by comparison that the former is about a thousand times larger than the latter one, which shows that adsorption is more important than vaporization in solvent sublation process. The simulating result of vaporization was lower than that of air stripping test which was much lower than solvent sublation. REFERENCES 1. F. Sebba, Ion flotation, Amsterdam, Chap. X. 1962, Elsevier. 2. P.E. Spargo and T.A. Pinfold, Sep. Sci., 5(1970), pp.619 - 635. 3. I. Sheiham and T.A. Pinfold, Sep. Sci., 7(1972), pp.43 50. 4. B.L. Karger, T.A. Pinfold and S.E. Palmer, Sep. Sci., 5(1970), pp.603 - 617. 5. B.L. Karger, Solvent sublation. In Lemlich R., ed. Ad-

sorptive Bubble Separation Techniques. New York: Academic Press, 1972, p.145 6. K.T. Valsaraj, G.J. Thoma, L.J. Thibodeaux and D.J. Wilson, Nonfoaming adsorptive bubble separation processes. Separations Technology, 1991. 1(5), pp.234 - 244. 7. X.-H. Sun, Z.-D. Chang and H.-Z. Liu, Sep. Sci. Technol., 40(2005), pp.941 - 957. 8. X.-H. Sun, Z.-D. Chang and H.-Z. Liu, Sep. Sci. Technol., 40(2005), pp.927 - 940. 9. X.-H. Sun, Bubble Separation and Air-assisted Solvent Extraction for Recovering Organics from Wastewater of Penicillin Plant, Doctoral Dissertation, Institute of Process Engineering, Chinese Academy of Science, 2006. 10. J.L. Womack, J.C. Lichter and D.J. Wilson, Sep. Sci. Technol., 17(1982), pp.897 - 924. 11. D.J. Wilson and K.T. Valsaraj, Sep. Sci. Technol., 17(1982), pp.1387 - 1396. 12. K.T. Valsaraj and L.J. Thibodeaux, Sep. Sci. Technol., 26(1991), pp.37 - 58. 13. K.T. Valsaraj and L.J. Thibodeaux, Sep. Sci. Technol., 26(1991), pp.367 - 380. 14. T. Lionel, D.J. Wilson, and D.E. Pearson, Sep. Sci. Technol., 16(1981), pp.907 - 935 15. S.-D. Huang, K.T. Valsaraj and D.J. Wilson, Sep. Sci. Technol., 18(1983), pp.941 - 968. 16. Y.-J. Lu, Y.-S. Wang, Y. Xiong and X.-H. Zhu, Fresenius J. Anal. Chem., 370(2001), pp.1071 - 1076 17. K.T. Valsaraj, J.L. Porter, E.K. Lihenfeldt and C. Springer, Water Res., 20(1986), pp.1161 - 1175 18. J. Staudinger and P.V. Roberts, Chemosphere, 44(2001), pp.561 - 567

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