Facilitated transport of CO2 and SO2 through Supported Ionic Liquid Membranes (SILMs)

Desalination 245 (2009) 485–493

Facilitated transport of CO2 and SO2 through Supported Ionic Liquid Membranes (SILMs) P. Luisa,*, L.A. Nevesb, C.A.M. Afonsoc, I.M. Coelhosob, J.G. Crespob, A. Gareaa, A. Irabiena a

Departamento de Ingeniería Química y Química Inorgánica, Universidad de Cantabria, 39005 Santander, Spain Email: [email protected] b REQUIMTE-CQFB, Departamento de Química, Facultade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal c Centro de Química-Física Molecular (CQFM) and Institute of Nanoscience and Nanotechnology (IN), Instituto Superior Técnico, 1040-001 Lisboa, Portugal Received 30 June 2008; revised 09 December 2008; accepted; 09 February 2009

Abstract Sulfur dioxide (SO2) and carbon dioxide (CO2) emissions have to be efficiently controlled due to environmental, economic and social demands. An interesting approach for SO2 and CO2 separation/concentration is the use of supported ionic liquid membranes (SILMs). Ionic liquids (ILs) are very interesting compounds for this application since they have a negligible vapour pressure, leading to high stability and reducing solvent losses by volatilization to the gas stream. pIn this work, the permeabilities of air, CO2 and 10 vol.% SO2–air have been obtained using different SILMs. The permeability of air is one order of magnitude lower than CO2 permeability and it is also lower than the permeability of the mixture of air and 10 vol.% SO2. In addition, the selectivities of CO2 and the SO2 mixture have been obtained. Keywords: Supported ionic liquid membranes; Permeability; Selectivity; Sulfur dioxide and carbon dioxide recovery.

1. Introduction Sulfur dioxide (SO2) and carbon dioxide (CO2) emissions have to be controlled and minimized in order to reduce environmental risks. Separation and concentration of SO2 is required in order to achieve *Corresponding author.

its recovery and reuse in some processes [1]. On the other hand, CO2 is mainly produced in combustion processes. Because CO2 is recognized as the main greenhouse gas leading to global warming, reduction of CO2 emissions is considered a key issue. One of the most attractive approaches for the separation of a target compound from a mixture

Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved. doi: 10.1016/j.desal.2009.02.012

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of gases is the selective absorption into a liquid [2,3]. A large number of gas–liquid absorption processes have been developed and due to interest in achieving regenerative processes to increase the raw materials efficiency, some organic solvents are also widely used as absorption liquids (e.g. aqueous solutions of amines, tetraethyleneglycol dimethyl ether, N,Ndimethylaniline, etc.) [4–7]. As these solvents chemically react with SO2 or CO2 a large amount of heat must be supplied to regenerate these solvents and these processes become unattractive when the target compound concentration in the feed gas stream is high. Membrane-based gas separation processes are known for their modular design and energy efficiency. In previous works [8,9], the solvent N,N-dimethylaniline was combined with a non-dispersive membrane-based technology in order to recover SO2 and decrease solvent losses by drops dragging. Previous works have been also performed using membrane contactors for CO2 absorption [10,11]. Non-dispersive absorption processes are very attractive and a selective and facilitated transport of CO2 or SO2 through a supported liquid membrane (SLM) is also an interesting approach to be studied. The supported liquid membranes (SLMs) are porous materials whose pores are filled with liquids and can be used in several applications [12– 14]. In SLMs, the solute molecule dissolves into the membrane at the feed/membrane interface, it diffuses through the membrane and desorbs at the opposite membrane surface. The stability of the membrane is questioned, but the combination of ionic liquids with SLMs gives many advantages over conventional SLMs due to the high stability and non-volatile character of ionic liquids [15–21]. Thus, supported ionic liquid membranes (SILMs) are an interesting approach for CO2 and SO2 recovery without solvent losses in the gas stream. Ionic liquids (ILs) are organic salts with a melting point lower than 100 ºC and a negligible vapour pressure. Evaluation of SO2 and CO2 sol-

ubility in RTILs (Room Temperature Ionic Liquids) is the first step to use these liquids instead of the conventional solvents and this issue is under study in the recent literature [22–31]. In this work, we report the facilitated transport of SO2, CO2 and air through a supported liquid membrane (SILM) based on a task-specific ionic liquid to separate SO2 or CO2 from air. Two different membranes (one hydrophilic and one hydrophobic) made of polymeric materials and three liquids (two ionic liquids and polyethyleneglycol) were used in the experimental study. The studied ionic liquids were previously developed as a task-specific ionic liquid to capture SO2, considering interesting features: high affinity towards SO2, transport properties, easy synthesis and low cost [28].

2. Theory The permeability of a gas through the membrane is calculated from the pressure data from the feed and permeate compartments according to [32]: 1 ⎛ Δp0 ⎞ t ⋅ ln ⎜ ⎟ = P⋅ b ⎝ Δp ⎠ δ

(1)

Δp0 = p feed ,t = 0 − p permeate,t = 0

(2)

Δp = p feed ,t − p permeate,t

(3)

where pfeed and ppermeate are the pressures in the feed and permeate compartments respectively; P is the permeability; t is the time; and δ is the membrane thickness. The geometric parameter b is experimentally calculated from ⎛ 1 1 b = Am ⎜ + ⎜ V feed V permeate ⎝

⎞ ⎟⎟ ⎠

(4)

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Table 1 Studied liquids and their characteristics [28]

Compound

Abbreviation

Molecular structure

Molecular weight (g mol–1)

SO2 SO2 absorption desorption (wt.%) (wt.%)

N

1Methylimidazolium acetate

[MIM][ace]

O +

HN

142.15

45.3

99.5

184.23

41.7

99.2

350

33.8

99.6

O–

NH+ N

1Butylimidazolium acetate

O

[BIM][ace]

O– O O

Polyethylene glycol

PEG

and it takes a value of 116.3 m–1 when pure gases are used and 107.0 m–1 for the mixture of SO2 and air. Vfeed and Vpermeate are the volumes of the feed and permeate compartments, respectively, and Am is the membrane area. The data can be plotted as 1/b·ln(Δp0/Δp) versus t/δ and the permeability is obtained from the slope. The Selectivity (SA/B) can be calculated from the relationship between the permeabilities: S A/ B =

PA PB

(5)

3. Experimental The ionic liquids 1-methylimidazolium acetate and 1-butylimidazolium acetate were synthesized according to an experimental procedure based on a neutralization reaction. Basically, the cation donor reagent is mixed with the anion donor reagent inside a balloon with diethyl ether, which is used as solvent to dilute the reagents.

After some minutes, the ionic liquid is observed as a miscible liquid in diethyl ether. Then, the solvent is removed by means of evaporation and the ionic liquid is finally obtained. Table 1 shows the molecular structure and abbreviations of these ionic liquids. It is worth mentioning that these room temperature ionic liquids have a very low viscosity, which is of great importance when a high diffusivity of gas through liquid is desired. Absorption and desorption experiments of SO2 in several ILs were carried out elsewhere [28] and the ILs in Table 1 were selected due to their high affinity towards SO2 as it can be seen in Table 1. A reversible absorption can be achieved because almost a complete desorption was observed [28]. In addition, polyethylene glycol has been also used to prepare the SILM due to its affinity towards SO2 and low viscosity [28]. Its molecular structure is also shown in Table 1. The SILMs were prepared using porous membranes made of PVDF with different materials: one is hydrophobic (from Millipore Corporation)

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Table 2 Commercial membranes used as the support of SLM Membrane

Material

VERICEL (Pall Corporation) MILLIPORE (Millipore Corporation)

Pore size (μm)

Thickness (μm)

0.20

129

0.22

125

Hydrophilic PVDF Hydrophobic PVDF

these membranes are shown in Table 2. To prepare the SILMs, the membrane was introduced into a vacuum chamber in order to facilitate the wetting. After 1 h of vacuum, the membrane was soaked in the ionic liquids or in the PEG for 1 h. Then the liquid excess on the membrane surface was wiped up softly. The experimental setup used for the gas separation method is shown in Fig. 1. The permeation

and the other one hydrophilic (from Pall Corporation). These membranes with different nature, but similar pore size were selected and after some previous experimental tests it was observed that these membranes immobilized with ionic liquids are stable at the pressures tested (up to 0.2 bar) [33,34]. In addition, they are characterized by their high chemical resistance and usefulness for a wide range of applications. Some features of

I/O card TC PT

PT

Of

Op

If

Ip

Water bath Twater = 30°C

Feed compartment

Membrane

Permeate compartment

If: Inlet of the feed compartment Ip: Inlet of the permeate compartment Of: Outlet of the feed compartment Op: Outlet of the permeate compartment

Fig. 1. Experimental setup for gas permeation tests.

Gas source

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489

Dimensionless permeability (–)

1.0 0.9 0.8 0.7

Air SO2(10%v)-Air CO2

0.6 0.5 0.4 0.3 0.2 0.1 0.0 PEG + hydrophobic PVDF

PEG + hydrophilic PVDF

[MIM][ace] + [MIM][ace] + hydrophobic hydrophilic PVDF PVDF

[BIM][ace] + hydrophobic PVDF

[BIM][ace] + hydrophilic PVDF

Fig. 2. Dimensionless permeability (maximum air permeability = 6.29•10–11 m2•s–1; maximum 10 vol.% SO2/air permeability = 33.6•10–11 m2•s–1; maximum CO2 permeability = 204.1•10–11 m2•s–1;).

cell consists of two compartments (feed and permeate) and the SILM is fixed between them. The effective membrane area was about 12.6 cm2. The feed gas (air, CO2 or a mixture of 10% SO2/air v/v)) is introduced in the compartments and after opening the permeate outlet, a driving force of around 0.45 bar between the feed and the permeate is established. This pressure difference leads to a flux across the membrane. Two pressure transducers measure the pressure in both compartments during the experiment. By measuring the pressure difference as a function of time, the permeability can be inferred from the slope of an appropriate semi-log plot, according to Eq. (1). The temperature is maintained at 30 ºC. The end point of the experiment is considered when the pressure difference is 90% of the initial difference in order to maintain the linearity of the permeation data. 4. Results and discussion After soaking the membrane, it increases the weight and thickness, depending on the hydrophilic or hydrophobic character of the liquid

and membrane pairs. Table 3 shows that the increase is higher when the hydrophilic PVDF membrane is used, which was expected due to the hydrophilicity of the studied liquids. The gas permeation results are shown in Table 3 within a 95% confidence range. The permeabilities for air, CO2 and the mixture SO2 (10 vol.%)/air through the membranes indicate that all the SILM act as a barrier for air. Permeability of air is one order of magnitude lower that CO2 permeability. In addition, it is worth mentioning that with only 10 vol.% of SO2 in the gas feed, the total permeability through the membrane increases around 5–6.5 times comparing with the air permeability. Fig. 2 shows a comparison between the permeability of air, CO2 and the mixture SO2/air in order to highlight the facilitated flux of CO2 and SO2. Permeabilities are shown in a dimensionless form referred to the maximum permeability. It can be observed that the ionic liquid [BIM][ace] shows the highest permeability of CO2 and SO2 in both membranes, being higher in the membrane made of hydrophilic PVDF. Selectivities of the membranes towards CO2 and the mixture 10 vol.%SO2-air have been also evaluated according to Eq. (5). Fig. 3 shows that

[BIM] [ace]

[MIM] [ace]

PEG

Liquid

Hydrophilic PVDF

Hydrophobic PVDF

Hydrophilic PVDF

Hydrophobic PVDF

Hydrophilic PVDF

Hydrophobic PVDF

Membrane Air CO2 SO2 (10 vol.%)— air Air CO2 SO2 (10 vol.%)— air Air CO2 SO2 (10 vol.%)— Air Air CO2 SO2 (10vol.%)— Air Air CO2 SO2 (10 vol.%)— air Air CO2 SO2 (10 vol.%)— air

Gas

13.55 13.55 11.87

3.95 3.95 1.37

123.3 123.3 125.8 210.6 210.6 209.2

13.20 13.20 6.41

1.38 1.38 1.67

134.9 134.9 136.5 229.5 229.5 211.6

11.63 11.63 14.07

1.65 1.65 4.52

% thickness increase

228.8 228.8 219.0

131.7 131.7 130.6

% weight increase

Membrane immobilization

Table 3 Weight increase and thickness after immobilization, permeability and selectivity

0.46 0.37 0.44

0.43 0.44 0.44

0.43 0.43 0.36

0.45 0.41 0.39

0.44 0.42 0.43

0.46 0.44 0.45

Δp0 (bar)

6.29 204.1 33.6

3.91 108.1 20.9

1.187 43.92 12.83

1.313 36.13 11.20

0.975 22.56 6.46

1.346 33.76 6.866

± 0.02 ± 0.5 ± 0.1

± 0.01 ± 0.2 ± 0.1

± 0.002 ± 0.07 ± 0.05

± 0.001 ± 0.04 ± 0.06

± 0.002 ± 0.03 ± 0.02

± 0.002 ± 0.05 ± 0.008

Permeability × 1011 (m2/s)

32.4

27.7

37.0

27.5

23.1

25.1

Selectivity CO2/Air

Gas permeation test

5.34

5.34

10.8

8.53

6.63

5.10

Selectivity 10 vol.%SO2– air/air

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1.0 0.9

Selectivity (–)

0.8

CO2/air 10vol.%SO2-air/air

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 PEG + hydrophobic PVDF

PEG + hydrophilic PVDF

[MIM][ace] + hydrophobic PVDF

[MIM][ace] + hydrophilic PVDF

[BIM][ace] + hydrophobic PVDF

[BIM][ace] + hydrophilic PVDF

Fig. 3. Dimensionless selectivity (maximum selectivity SCO /air = 37.0; maximum selectivity 10 vol.% SO2–air/air = 10.8). 2

Table 4 Permeabilities of SO2 and CO2 through several SILMs Permeability × 1011

SILM Gas

Membrane material

SO2

Polyethersulfone

CO2

Polyethersulfone

Polysulfone a

Ten percent humidity. Eighty-five percent humidity.

b

Ionic liquid [emim][BF4] [bmim][BF4] [bmim][PF6] [hmim][BF4] [bmim][Tf2N] [emim][BF4] [bmim][BF4] [bmim][PF6] [hmim][BF4] [bmim][Tf2N] [emim][Tf2N]1 [emim][Tf2N]2 [emim][CF3SO3] [emim][dca] [thtdp][Cl] [hmim][Tf2N]

(m2 • s–1) 776.1 669.8 431.6 604.2 710.5 39.84 38.18 29.88 43.16 81.34 79.68 87.15 76.36 50.63 29.05 71.38

Reference ±19.1 ±21.6 ±9.7 ±12.5 ±9.3 ±1.25 ±1.00 ±1.25 ±0.91 ±1.91 – – ±2.49 ±1.66 ±1.66 –

[35] [35] [35] [35] [35] [35] [35] [35] [35] [35] [36] [36] [36] [36] [36] [37]

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the highest selectivity is reached with the SILM composed of [MIM][ace] and hydrophilic PVDF. The SILM made of [BIM][ace] and hydrophilic PVDF shows the most promising combination in the researched system. Permeabilities of SO2 and CO2 through several SILMs from the literature are shown in Table 4. It can be observed that by using [BIM][ace], the highest permeability towards CO2 is reached. The same behaviour would be expected for pure SO2.

[4]

[5]

[6]

[7]

Conclusions The permeabilities of air, CO2 and 10 vol.%SO2–air have been obtained using different SILMs. The maximum permeabilities in the studied systems are: air, 6.29·10–11 m2·s–1; 10 vol.% SO2–air, 33.6·10–11 m2·s–1; CO2, 204.1·10–11 m2·s–1. Permeability of air is one order of magnitude lower that CO2 permeability and it is also lower than the permeability of the mixture of air and 10 vol.% SO2. The membrane made of hydrophilic PVDF and [BIM][ace] shows the highest permeability which is important in order to develop an efficient separation process.

[8]

[9]

[10]

[11]

Acknowledgements This research was performed thanks to the collaboration between the University Nova in Lisbon and the University of Cantabria in Spain. Spanish side thanks to the Spanish Ministry of Education and Science (Project No. CTM2006-00317). P. Luis thanks the Spanish Ministry of Education and Science (Grant No. AP-2004-6222).

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