Apparent and intrinsic properties of commercial PDMS based membranes in pervaporative removal of acetone, butanol and ethanol from binary aqueous mixtures

Journal of Membrane Science 453 (2014) 108–118

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Apparent and intrinsic properties of commercial PDMS based membranes in pervaporative removal of acetone, butanol and ethanol from binary aqueous mixtures Anna Rozicka a, Johanna Niemistö b, Riitta L. Keiski b, Wojciech Kujawski a,n a b

Faculty of Chemistry, Nicolaus Copernicus University, 7 Gagarina Str., Torun (Poland) Street, 87-100 Torun, Poland University of Oulu, Department of Process and Environmental Engineering, Mass and Heat Transfer Process Laboratory, POB 4300, FI-90014, Finland

art ic l e i nf o

a b s t r a c t

Article history: Received 2 September 2013 Received in revised form 27 October 2013 Accepted 28 October 2013 Available online 4 November 2013

The performance of three commercial poly(dimethylsiloxane) based membranes (Pervatech, Pervap 4060 and PolyAn) was studied in pervaporation of removal of acetone, butanol and ethanol from binary aqueous mixtures at 25 1C. Physicochemical properties of solvents and PDMS were applied to describe affinities and interactions between transported mixture components and membrane. It was shown that high vapour pressure of acetone causes the most pronounced transport of this component through all investigated membranes. In terms of apparent properties, all membranes show high separation towards organic component of the binary mixture. Separation factors (βi/w) determined for aqueous mixture containing 0.01 mol fraction of acetone are equal to 30, 62 and 39 for Pervatech, Pervap 4060 and PolyAn, respectively. Comparing these values to βBuOH/w values of water–butanol mixture (9, 31, 9 for Pervatech, Pervap 4060 and PolyAn, respectively), it seems that acetone is the most preferentially transported compound. In contrast, the intrinsic membrane properties discussed in terms of permeances and selectivities reveal that investigated membranes are selective towards butanol (intrinsic membrane selectivity αBuOH/w is equal to 3.6, 11.6 and 3.4 for Pervatech, Pervap 4060 and PolyAn, respectively). However, these membranes are nonselective or water selective (αi/w r1) in contact with water–acetone and water–ethanol mixtures. & 2013 Elsevier B.V. All rights reserved.

Keywords: Pervaporation Acetone Butanol Ethanol PDMS commercial membranes Intrinsic membrane selectivity

1. Introduction Butanol has been recently proposed as an alternative biofuel, replacing ethanol. Butanol possesses higher energy content and its solubility in water is smaller compared to ethanol [1,2]. Butanol can be produced via chemical synthesis, e.g. the Oxo process [3] or by the acetone–butanol–ethanol (ABE) fermentation process by Clostridium bacterium strain [3]. The latter method is nowadays of a great interest due to the possibility of butanol production from renewable resources, such as lignocellulosic ones. ABE fermentation products are obtained in an A:B:E weight ratio equal to 3:6:1, with butanol being the compound of the highest interest. However, ABE fermentation products are toxic for bacteria and their maximum total content in the fermentation broth is usually not higher than 3.0 wt% [4]. In order to produce acetone, butanol and ethanol continuously, additional methods should be applied and combined with the fermentation step for a direct removal of organic solvents [5–23].

n

Corresponding author. Tel.: þ 48 56 611 43 15; fax: þ 48 56 654 24 77. E-mail address: [email protected] (W. Kujawski).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.10.065

Several methods have already been proposed for the ABE fermentation products recovery: distillation [5], adsorption [6–11], freeze crystallization [5], gas stripping [12,13], liquid– liquid extraction [14,15], pertraction [16], membrane distillation [17], thermopervaporation [18] and pervaporation [19–23]. When applying conventional distillation it is difficult to obtain pure solvents, due to the creation of azeotropic mixture of butanol and water at 92.7 1C [5]. The adsorption method requires introduction of an additional substance (adsorbent) to the system, though adsorbent can be poisonous to bacteria present in the fermentation broth. Freeze crystallization may be energetically more favourable than distillation, but the process plant investment costs for solid phase will be significantly higher than that for liquid and vapour phases in distillation [5]. In a gas stripping method, gases (e.g. CO2, H2, N2) are bubbled through the fermentation broth to remove ABE products. Subsequently volatile fermentation products are condensed and collected in a product vessel [13]. In the liquid–liquid extraction butanol is removed from the fermentation broth due to its higher solubility in the organic (extractant) phase than in the aqueous (fermentation broth) phase. Several problems are associated with the liquid–liquid extraction, such as toxicity of extractants to the cells, possible formation of an

A. Rozicka et al. / Journal of Membrane Science 453 (2014) 108–118

emulsion, loss of extraction solvent, and the accumulation of microbial cells at the extractant and fermentation broth interphase [13]. In general, membrane based techniques are considered to be the most suitable one for fermentation products recovery [13,17– 23]. During the pertraction process, the fermentation broth and the extractant are separated by a membrane, which provides surface area where the two immiscible phases can exchange organic solvents [13]. In membrane distillation, the separation takes place at temperature below the normal boiling point, however to avoid wetting of the membrane pores, highly hydrophobic membranes are required in this process [17]. Pervaporation seems to be one of the most promising methods which can be used for the recovery of ABE fermentation products [18–23]. This process could be applied to separate ABE fermentation products directly from the fermentation broth because the separation process has no negative impact on the microorganisms in the broth [22]. Pervaporation (PV) is a separation process allowing a separation of binary or multicomponent liquid mixtures by a partial vaporization through a dense non-porous polymeric membrane [24]. In the PV process the feed mixture is in a direct contact with one side of the membrane. Transport of components from feed to permeate side of the membrane occurs due to a difference in chemical potentials, created by vacuum (vacuum pervaporation – VPV), sweep gas (sweeping gas pervaporation – SGPV) [25] or temperature difference (thermopervaporation – TPV) [18]. According to a solution–diffusion model, permeation through the membrane is regarded as a combination of three consecutive steps: liquid sorption from the feed into the membrane, diffusion of the vaporous components through the membrane and desorption at the permeate side [24]. Permeate is received in a vapour phase and subsequently condensed outside the membrane module [24,25]. Hydrophobic membranes must be used to recover organic compounds from aqueous solutions by pervaporation. There are various polymeric membranes available for this application: poly (dimethylsiloxane) (PDMS), PDMS filled with hydrophobic zeolites, ethylene propylene diene rubber (EPDR), styrene butadiene rubber (SBR), poly(methoxy siloxane) (PMS), poly(octylmethyl siloxane) (POMS), poly(ether block amide) (PEBA), ionic liquid–poly dimethylsiloxane (IL–PDMS) and poly[-1-(trimethylsilyl)-1propyne] (PTMSP) [18–23,26,27]. Recently, PV has been intensively investigated for the ABE products removal from the fermentation broth [19,21,23,26,28–31]. Fouad et al. [19] investigated pervaporative separation of n-butanol from dilute aqueous solutions at concentration below 0.5 wt% using a silicalite-filled poly(dimethyl siloxane) composite membrane at different temperatures. It was shown that at a given temperature, the total flux increased with an increase in butanol feed concentration over the low feed concentration range studied. It was also observed that at elevated temperature the permeation flux increased as well. Liu and co-workers [21] tested the properties of poly(ether block amide) (PEBA 2533) membranes of different thicknesses in contact with binary as well as quaternary aqueous mixtures at 23 1C. The authors found the membrane separation factor being in the following order: n-butanol4acetone4ethanol. At the concentration of 2 wt% of organic solvent in the feed, the membrane exhibited separation factors of 9, 5 and 3 for butanol, acetone and ethanol, respectively. Fadeev et al. [23] assessed the fouling and structural changes of PTMSP membrane during pervaporative recovery of n-butanol from aqueous solutions and/or ABE fermentation broth. Authors reported that during pervaporation of aqueous solution of ABE the PTMSP membrane changes its geometry becoming thicker and denser. Moreover, a strong lipids adsorption from the fermentation broth occurs on the membrane surface. The latter phenomenon also causes the important decline of permeate flux [23]. Izák et al. [26] tested properties of poly(dimethylsiloxane) (PDMS) and supported

109

ionic liquid–polydimethylsiloxane membranes by pervaporation in contact with ternary mixture (butanol–acetone–water) at 23 1C. Authors used ultrafiltration ceramic membranes impregnated by two types of ionic liquids 1-ethenyl-3-ethyl-imidazolium hexafluorophosphate (IL-1), tetrapropylammonium tetracyano-borate (IL-2) and poly(dimethylsiloxane) (PDMS). Application of PDMS membranes allowed to obtain higher selectivity (enrichment factor of butanol recovery was changed from 2.2 for pure PDMS membrane to 3.1 for IL-1 PDMS and the highest enrichment factor was obtained for IL-2 PDMS membrane – 10.9). It has to be pointed out that butanol fluxes obtained during PV experiments with pure PDMS membrane were higher comparing to results found out for ionic liquid modified membranes. Properties of (tetrapropylammonium tetracyano-borate) ionic liquid membrane on PDMS support were also tested by pervaporation in contact with fermentation broth [26]. Flux of butanol was equal to 13.64 g m  2 h  1 in contact with PDMS-IL at 5.71 g dm  3 content of butanol in the fermentation broth at 37 1C. Qureshi and Blaschek [28] applied pervaporation for the direct removal of products from fed-batch reactors containing C. beijerinckii BA101 bacteria. In an integrated pervaporation-fermentation system production of 51.5 g dm  3 total solvents was achieved, whereas in a non-integrated fermentor only 24.2 g dm  3 total solvents were obtained. Moreover, the C. beijerinckii BA101 strain was not negatively affected by the pervaporative conditions [28]. Liu et al. [29] tested the properties of PDMS/ceramic composite membrane in a pervaporation process coupled with the ABE fermentation system. During the 36 h of a continuous PV experiment, the performance of PDMS/ceramic composite membrane was constant, with small deviations resulting from the fluctuations of the ABE feed composition. The average total flux was 951 g m  2 h  1 with an average separation factor for acetone, n-butanol and ethanol equal to 21, 16 and 7, respectively. The brief review of the literature reveals that majority of papers published on pervaporation discuss only the apparent membrane properties. The aim of this work was to examine intrinsic properties of three commercially available poly(dimethylsiloxane) (PDMS) based membranes: Pervatech, Pervap 4060 and PolyAn were applied for the pervaporative separation of acetone, butanol and ethanol (ABE) from binary aqueous mixtures. The obtained results were presented and discussed in terms of permeances and selectivities, as proposed recently by Baker et al. [32]. Additionally, physicochemical properties of solvents as well as interactions between solvents and the polymer membranes were considered in the description of transport in the PV process. Additional experiments were performed to determine the affinity of solvents to swell the commercial membranes and the pure PDMS membranes.

2. Experimental 2.1. Membranes Three poly(dimethylsiloxane) based commercially available flat sheet membranes: Pervatech 030705 delivered by Pervatech (The Netherlands), Pervap 4060 purchased from Sulzer Chemtech (Switzerland), and PolyAn bought from PolyAn GmbH (Germany) were applied in this research. Moreover, commercial membranes as well as laboratory made PDMS membranes were used in sorption experiments. The PDMS membranes were prepared using poly(dimethylsiloxane) solution, prepared from EL.LR 7660 A elastomer (component A – vinyl-methylpolysiloxane) and EL.LR 7660B curing agent (component B – hydrogen functional crosslinker). The 20 wt% solution of component A in n-hexane was prepared and subsequently component B was added to adjust the A:B ratio to 20:1 (PDMS 20:1 membrane) and 5:1

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(PDMS 5:1 membrane). The solution of PDMS A and B components in hexane was poured onto a mould and kept for 24 hours at ambient temperature. Finally, membranes were crosslinked for 2 h at 70 1C and 90 1C for PDMS 20:1 and PDMS 5:1 membrane, respectively.

2.2. Solvents 1-Butanol (denoted as butanol throughout this work), acetone and ethanol of analytical grade were purchased from POCH (Poland). Deionized water (RO) was used for the preparation of binary water–organic mixtures. All chemicals were used as received.

2.3. Gas chromatography The composition of feed and permeate mixtures was analysed using gas chromatography (GC). Varian 3300 gas chromatograph was equipped with a TCD detector working at the following parameters: injection port temperature: – 200 1C, column temperature: – 180 1C and detector temperature: – 220 1C. Analyses were carried out on a PorapakQ packed column. To analyse the water–butanol permeate, which formed a two-phase system, acetone was added to homogenize the sample. Borwin software (JMBS, France) was used for data acquisition and processing.

2.4. Swelling Swelling experiments of Pervatech, Pervap, PolyAn commercial membranes and PDMS 20:1 and PDMS 5:1 membrane were performed. Dry samples of membranes were immersed in pure solvents (acetone, butanol and ethanol). After a given period of time membranes were removed from the solvents, wiped from the excess solvent with a tissue-paper and weighed. The degree of swelling (SW) was calculated according to [33] SW ¼

Ws Wd 100% Wd

ð1Þ

where Wd and Ws are masses [g] of a dry and a swollen membrane, respectively.

2.5. Pervaporation experiments Pervaporation experiments were performed using a standard pervaporation laboratory rig schematically presented in Fig. 1 [25]. The sample of the PDMS based membrane was placed on a porous stainless steel support and sealed using an O-ring in a stainless steel membrane module (3). The thermostated feed was circulated between the module (3) and the feed tank (1) by a circulating feed pump (2). A vacuum pump (6) ensured low pressure on the permeate side (below 1 mbar). Permeate was collected into two parallel permeate traps (4), cooled with liquid nitrogen, what allowed a continuous work of the system. Binary water–organic solvent (butanol, acetone or ethanol) mixtures were used as a feed mixture during the pervaporation measurements. Concentration of the feed solutions was in the range of 0–5 wt% of organics. Pervaporation experiments were performed at 25 70.5 1C for all systems. Additionally, pervaporation experiments were performed at 40 1C and 60 1C for the water–butanol mixture and the Pervap 4060 membrane.

Fig. 1. Scheme of the pervaporation rig: (1) thermostated feed tank, (2) feed pump, (3) membrane module, (4) permeate traps cooled with liquid nitrogen, (5) safety trap, and (6) vacuum pump.

3. Results and discussion 3.1. Swelling properties of membranes Acetone, butanol, ethanol and water are solvents possessing different physicochemical properties (Table 1). Butanol is the biggest molecule among all tested solvents, whereas water is the smallest. Small size can facilitate penetration of water molecules between polymer chains in the membrane structure. Acetone shows the highest saturation vapour pressure of the investigated species, creating the highest transport driving force. Due to the partial pressure values, the driving force of species transport in the pervaporation process decreases in the following order: acetone4ethanol 4water4butanol (Table 1). Significant differences between the saturation vapour pressures at 25 1C and 40 1C can be seen, especially for butanol. At higher temperature the driving force of the transport through a membrane will obviously increase. Additional information on interactions between solvents and the membrane material can be obtained by discussing Hansen's solubility parameters (δ) [37,38]. The concept proposed by Hildebrand and Scott [38] relates energy of mixing with the energies of vaporization of the pure components. This assumption was investigated and further developed by Hansen [37]. According to the model proposed by Hansen [37,39], the solubility parameter δ refers to the density of cohesive energy, which can be divided into three types of interactions: δh – hydrogen bonding interactions, δp – polar interactions and δd – dispersion interactions (Table 2). The partial components of solubility parameter (δi) are used to calculate the distance parameter Δ – Eq. (2), determining the extent of affinity of two substances [37]. The greater the affinity between the two components the smaller the value of the distance parameter (Δ) [37,41]. Δ ¼ ½ðδd;i  δd;j Þ2 þ ðδp;i  δp;j Þ2 þ ðδh;i  δh;j Þ2 0:5

ð2Þ

Values of components δ as well as Δ values of PDMS and the investigated solvents are presented in Table 2. The dispersion cohesion parameter (δd) is similar for all the tested substances, which means that δd does not have a significant influence on the interactions between substances. Only polar cohesion (δp) and hydrogen bonding (δh) components control the affinity of substances. The highest distance parameter was found for water and PDMS which suggests that water is the least compatible solvent with PDMS.

A. Rozicka et al. / Journal of Membrane Science 453 (2014) 108–118

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Table 1 Chosen physicochemical properties of water, acetone, butanol and ethanol [34–36]. Component

Molecular weight [g mol  1]

Kinetic diameter [nm]

Dielectric constant at 20 1C

Molar volume [cm3/mol]

psat at 25 1C [kPa]

psat at 40 1C [kPa]

Boiling point [1C]

Water Acetone Butanol Ethanol

18.01 58.08 74.12 46.07

0.30 0.47 0.50 0.43

80.1 21.4 17.9 25.1

18.1 73.0 91.5 58.4

3.2 30.8 0.9 7.9

7.4 56.6 2.4 17.9

100.0 56.1 117.8 78.3

psat – saturated vapour pressure.

Table 2 Hansen solubility parameters of PDMS and solvents [40,41] used in pervaporation experiments and calculated values of distance parameter Δi,j. Component

δd [MPa1/2]

δp [MPa1/2]

δh [MPa1/2]

Δi,PDMS [MPa1/2]

Δi;H2 O [MPa1/2]

PDMS Water Acetone (AcO) Butanol (BuOH) Ethanol (EtOH)

15.9 15.5 15.5 16.0 15.8

0.0 16.0 10.4 5.7 8.8

4.1 42.3 7.0 15.8 19.4

– 41.4 10.8 13.0 17.7

41.4 – 35.7 28.4 24.0

Table 3 Degree of swelling (SW) of PDMS based membranes and PDMS membranes in pure acetone, butanol and ethanol solvents. Membrane

Pervatech Pervap PolyAn PDMS 20:1 PDMS 5:1

3.2. Membrane efficiency in pervaporation of water–ABE binary mixtures Results obtained during pervaporation are usually presented in terms of flux (Ji) – Eq. (3), pervaporation separation index (PSI) – Eq. (6) and separation factor (βi/w) – Eq. (7) [40,42,43]. Ji ¼

ni tA

ð3Þ

where Ji is a partial flux of component i [mole m  2 h  1], ni is the number of moles of component i [mol], t is the permeation time [h], and A is the membrane area [m2]. If a binary aqueous mixture is processed, the total flux (Jt) of the permeate is equal to the sum of water flux (Jw) and organic component flux (Jo): Jt ¼ Jw þ Jo

SW [wt%] Acetone

Butanol

Ethanol

62.5 41.7 32.6 25.4 18.8

73.2 54.7 42.8 28.7 18.7

63.5 50.5 40.1 7.2 4.4

On the other hand, the Δ value calculated for acetone–PDMS interactions, points out that this solvent is the most compatible with PDMS among all the tested liquids. Butanol is slightly less compatible with PDMS, compared to acetone. Such observation can be used in predicting the swelling behaviour of membranes in a given solvent. From Table 2 it can also be seen that the affinity of acetone, butanol and ethanol to PDMS is significantly higher than that to water. The smaller distance parameter values between ABE and PDMS are much smaller compared to ABE and water. Table 3 presents the swelling behaviour of the commercial PDMS based membranes as well as dense, homogenous PDMS ones in contact with investigated pure organic solvents. Swelling of PDMS based composite commercial membranes is similar for various solvents used in the experiments. Composite membranes consist of hydrophobic skin layer and one or more intermediate layers with properties evidently different from those of a skin layer. Differences between swelling properties of various commercial membranes can be caused by a different structure of these support layers. Because of that, it is much more suitable to compare sorption into pure PDMS membranes. Homogenous PDMS membranes used in sorption experiments differ in a crosslinking degree. The PDMS 5:1 membrane was much more cross-linked than the PDMS 20:1 one. As it is seen (Table 3), sorption of solvents into PDMS 20:1 is higher comparing with PDMS 5:1. It can be concluded that the more PDMS membrane is crosslinked the less solvent is sorbed into the membrane. Sorption of acetone to a higher crosslinked PDMS is easier due to a smaller molecular size of acetone and slightly lower distance parameter (Table 2). These results are also in accordance with the sorption result presented recently by Niemistӧ et al. [22].

ð4Þ

On the other hand, the flux of a component i (Ji) can be calculated using the following formula: Ji ¼ Jt Y i

ð5Þ

where Yi is the molar fraction of organic compound in permeate. Pervaporation Separation Index (PSI) – Eq. (6) is frequently used to compare the PV performance of various membranes [42]. The pervaporation separation index (PSI) allows to compare separation effectiveness of membranes possessing different selectivities and permeabilities [43]. PSI ¼ J t ðβi=w  1Þ

ð6Þ 2

1

where PSI is the Pervaporation Separation Index [mol m h ] and βi/w is the separation factor. A given membrane showing higher PSI value should be more efficient in the pervaporation separation. Separation factor (βi/w) [32] is calculated according to βi=w ¼

yi =yw xi =xw

ð7Þ

where yi, yw are molar or weight fractions of component i and water (w) in permeate, respectively. xi, xw are molar or weight fractions of component i and water in feed, respectively. It should be underlined that permeate flux (Ji), separation factor (βi/w) and Pervaporation Separation Index (PSI) depend strongly on the operating conditions, like feed composition, pressure on the permeate side and feed temperature. Changes of permeate composition as a function of acetone, butanol and ethanol content in feed in contact with tested membranes are presented in Fig. 2. Comparing all tested membranes it can be seen that for Pervap 4060 membrane the highest concentration of organics in permeate can be obtained regardless of the binary system investigated. Results obtained for Pervatech and PolyAn in contact with butanol aqueous mixture differ only slightly, the significant difference of membrane properties can be seen for these membranes in contact with water–acetone system. As it can be observed, acetone is

A. Rozicka et al. / Journal of Membrane Science 453 (2014) 108–118

Molar fraction of acetone in permeate [mol/mol]

112

Table 4 Separation factors characteristic for investigated water–organic mixtures in contact with Pervatech, Pervap and PolyAn, 0.01 M fraction of organic compound in feed, 25 1C.

AcO -H2O / 25°C

0.6

Pervatech Pervap

0.5

PolyAn

System

0.4

Acetone–water Butanol–water Ethanol–water

0.3

Pervap

PolyAn

30 9 7

62 31 10

39 9 7

0.1

0.005

0.010

0.015

Molar fraction of acetone in feed [mol/mol]

Molar fraction of butanol in permeate [mol/mol]

Pervatech

0.2

0.0 0.000

BuOH -H2O / 25°C

0.6

Pervap

0.5

Table 5 Selectivity coefficients of transport of tested membranes. 0.01 M fraction of organics in feed. System

Acetone–water Butanol–water Ethanol–water

Pervatech

αi/w [dimensionless] Pervatech

Pervap

PolyAn

0.6 3.6 0.6

1.2 11.6 0.9

0.7 3.4 0.6

PolyAn

0.4 0.3 0.2 0.1 0.0 0.000

0.005

0.010

0.015

Molar fraction of butanol in feed [mol/mol]

Molar fraction of ethanol in permeate [mol/mol]

βi/w [dimensionless]

EtOH -H2O / 25°C

0.6

Pervatech Pervap

0.5

PolyAn

0.4 0.3

Table 4 presents the values of separation factor (βi/w) calculated for various membranes in contact with aqueous binary mixtures according to Eq. (6). Regardless of the binary mixture tested, the highest βi/w was found for Pervap membrane (Table 4). The value of βi/w for butanol separation with Pervap 4060 was equal to 31. Claes et al. [44] obtained slightly higher separation factor (39) of butanol recovery; however the experiments were done at 50 1C and the feed concentration was 5 wt%. Separation factor is a parameter which depends on experimental conditions, therefore with the change of temperature, separation factor value is also changed. Another approach to present pervaporation results was proposed by Baker et al. [32]. Authors suggested to use permeability coefficients (Pi), and selectivity (αi/j) coefficients. Pi and αi/j are related to the intrinsic properties of the membrane and allowed to compare properties of various membranes without taking into account experimental conditions or physicochemical properties of solvents [45]. The membrane permeance is defined by [46]   Pi Ji ¼ ð8Þ p l X i γ i psat i  Y ip

0.2

Permeate pressure (pp) during pervaporation experiments is usually very low, therefore the term (Yipp) in Eq. (8) can be neglected and the latter equation can be rewritten as follows:

0.1 0.0 0.000

0.005

0.010

0.015

Molar fraction of ethanol in feed [mol/mol]

Pi Ji ¼ l X i γ i psat i

ð9Þ

Fig. 2. Permeate composition of organic compounds, acetone (A), butanol (B) and ethanol (C) as a function of feed composition; feed temperature: 25 1C.

the most efficiently transported solvent regardless the applied membrane. With an increase of organic solvent concentration in feed, a nonlinear increase of organic content in permeate is observed (Fig. 3A). Acetone was the most preferentially transported solvent, what can be explained by the highest vapour pressure of this component at 25 1C (Table 1), causing the highest driving force comparing with other solvents. Moreover, the smallest distance parameter between acetone and PDMS was determined (Table 2), and that is in agreement with the obtained results.

Membrane selectivity coefficient (αi/w) is defined as a ratio of the permeability coefficient of components i and j through a given membrane [45]: αi=j ¼

P i =l P i ¼ P j =l P j

ð10Þ

To calculate the values of selectivity the activity coefficients were calculated using the process simulation software Aspen Plus (version 2006.5) and The Non-Random Two Liquid (NRTL) model was applied [22].

A. Rozicka et al. / Journal of Membrane Science 453 (2014) 108–118

Selectivity of ethanol recovery in contact with all of tested membranes is smaller than unity, which indicates that in such a system membranes are intrinsically water selective (Table 5), which is in a good accordance with data presented by Baker et al. [32]. Authors described silicone membrane properties in separation of water–ethanol mixture and found that tested membrane possesses αi/j selectivity below unity which implies that this membrane is water selective. All membranes tested in this work are selective

Molar flux of organic component [mol m-2 h-1]

Pervatech / 25°C 25

acetone ethanol

15

10

5

0 0.000

0.005

0.010

0.015

Molar fracton of organic component in feed [mol/mol]

Molar flux of organic component [mol m-2 h-1]

Pervatech / 25°C 25

acetone butanol ethanol

20

15

10

5

0 0.000

0.005

toward butanol (αi/w selectivity is higher than 1) and evidently it can be seen that selectivity of butanol transport determined in contact with Pervap 4060 is the highest (Table 5). Moreover, Pervap 4060 is selective towards acetone, whereas Pervatech and PolyAn membranes are water selective in contact with water–acetone system. It has to be also pointed that selectivities of Pervatech and PolyAn membranes are similar in each of tested aqueous mixture. Pervap is the most selective membrane towards organic which pointed out that this membrane will be the most suitable for acetone, butanol and ethanol recovery. 3.3. Transport of organics in pervaporation

butanol 20

0.010

0.015

Molar fracton of organic component in feed [mol/mol]

Transport of acetone, butanol and ethanol through investigated PDMS based membranes is presented in Fig. 3. Molar acetone flux is the highest for all tested membranes. It can be seen in Fig. 3 that molar fluxes increase linearly with an increase of organic solvent content in feed. It is worth to consider transport of butanol and ethanol through PDMS based membranes. Molar fluxes of these compounds remain comparable in contact with Pervatech (Fig. 3 (A)) and PolyAn (Fig. 3(C)) membranes, whereas in contact with Pervap 4060 (Fig. 3(B)) butanol flux is visibly higher than ethanol flux. Comparing transport properties of investigated membranes it can be stated that the highest fluxes were obtained for PolyAn, what can suggest that a selective layer of PolyAn membrane is the thinnest and/or less crosslinked than other tested membranes. There are no significant differences between butanol and ethanol fluxes for PolyAn and Pervatech membranes. On the other hand butanol flux through PolyAn membrane is slightly lower than transport of this compound through Pervap one. The smallest organic fluxes are obtained for Pervatech membrane, with molar flux of butanol only slightly higher than that for ethanol. Comparing results obtained during pervaporation in contact with various membranes it can be stated that the highest fluxes of organic components were obtained for PolyAn (Fig. 3(C)). According to Niemistö et al. [22] poor separation of ethanol is caused by the aggregation of ethanol and water molecules, which in result hinders ethanol transport through the membrane. Authors obtained the highest partial flux of acetone, comparing with other solvents. Butanol flux, at 3.5 wt% content of solvent in feed, was equal to 12.7 mol m  2 h  1. This value is higher than that obtained in this work for the same membrane and at the same organic compound concentration in feed (2.7 mol m  2 h  1), what results 100

Pervatech, AcO-H2O PolyAn, AcO-H2O Pervap, BuOH-H2O Pervatech, EtOH-H2O PolyAn, EtOH-H2O Pervap, H2O

acetone butanol

Molar flux of water [mol m-2 h-1]

Molar flux of organic component [mol m-2 h-1]

Pervatech / 25°C 25

ethanol

20

15

10

5

0 0.000

0.005

113

0.010

0.015

Molar fracton of organic component in feed [mol/mol]

Fig. 3. Molar flux of organic component vs. feed composition obtained during pervaporation in contact with Pervatech (A), Pervap 4060 (B) and PolyAn (C) membranes at feed temperature equal to 25 1C.

80

Pervap, AcO-H2O Pervatech, BuOH-H2O PolyAn, BuOH-H2O Pervap, EtOH-H2O Pervatech, H2O PolyAn, H2O

60

40

20

0

0.000

0.005

0.010

0.015

Molar fraction of organics in feed [mol/mol] Fig. 4. Comparison of molar water fluxes for Pervatech, Pervap 4060 and PolyAn membranes in aqueous binary mixtures of acetone, butanol and ethanol at 25 1C.

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from different temperatures of the systems: 42 1C in [22] and 25 1C in this work. 3.4. Transport of water Transport of water through Pervatech, Pervap 4060 and PolyAn membranes in pervaporation of ABE aqueous binary mixtures is presented in Fig. 4. The highest water flux was obtained during pervaporation in contact with PolyAn membrane, whereas the lowest water flux was reached for Pervap 4060. Membranes were prepared according to different preparation conditions, AcO -H2O 10000

Pervatech Pervap

1000

100 0.000

0.005

0.010

0.015

Molar fraction of acetone in feed [mo/mol] BuOH -H2O 10000

Pervatech Pervap

PSI [mol m-2 h-1]

PolyAn

30

1000

25

100 0.000

0.005

0.010

Butanol flux [mol m-2 h-1]

PSI [mol m-2 h-1]

PolyAn

influencing membrane properties in the pervaporation process. Transport of water through Pervap 4060 and Pervatech remains constant, regardless of the type and concentration of organic compound present in feed. An average water flux was equal to 18.3 mol m  2 h  1 and 30.3 mol m  2 h  1 for Pervap 4060 and Pervatech, respectively. On the other hand it has to be noticed that transport of water through PolyAn increased slightly with an increase of organic component content in feed and differed with a kind of organic compound present in aqueous feed mixture. Such behaviour of PolyAn membrane can be caused by a low degree of crosslinking of polymer in PDMS selective layer. Moreover, it can be assumed that selective layer thickness in PolyAn is the smallest. Water fluxes in each of investigated water–organic compound systems were much higher than fluxes of organic solvents. Water has the smallest molar volume (Table 1) among all tested solvents and this fact contributes to high water transport. Additionally, in these studies water–organic mixtures with concentration of organics below 5 wt% were used, which means that water concentration was equal to 95 wt% and was much higher than the organic content in feed. Such a significant difference also causes higher driving force for water transport. Transport of water through PDMS based membrane in contact with binary aqueous mixtures of acetone, butanol and ethanol was described by Niemistö et al. [22]. Authors described water transport through Pervatech membrane at 42 1C as constant during pervaporation in contact with the investigated mixtures. Water flux in all experiments was in range of 70–75.6 mol m  2 h  1 [22] and is higher than water flux obtained in this work for Pervatech membrane at 25 1C (30.3 mol m  2 h  1). Authors also concluded that water transport through the Pervatech membrane is basically independent of the feed composition in the studied concentration range [22], which is in a good agreement with constant water flux found in this work for Pervatech as well as for Pervap 4060 membranes.

0.015

Molar fraction of butanol in feed [mol/mol] EtOH -H2O 10000

25°C 40°C 60°C

20 15 10 5

Pervatech Pervap

PSI [mol m-2 h-1]

PolyAn

0 0.000

0.002

0.004

0.006

0.008

Feed concentration [mol/mol] Fig. 6. Comparison of butanol transport across Pervap 4060 membrane at three temperatures investigated.

1000

Table 6 Comparison of acetone, butanol and ethanol fluxes (molar ratio 2.4:3.7:1.0). Component 100 0.000

0.005

0.010

Xfeed [mol/mol]

cfeed [wt%]

0.015

Molar fraction of ethanol in feed [mol/mol] Fig. 5. Comparison of membranes performance in contact with water–acetone (A), water–butanol (B) and water–ethanol (C) systems; feed temperature: 25 1C.

AcO BuOH EtOH

0.016 0.024 0.007

0.9 1.8 0.3

J [mol m  2 h  1] Pervatech

Pervap

PolyAn

14.4 6.5 1.4

18.2 14.7 1.3

33.2 9.4 2.8

A. Rozicka et al. / Journal of Membrane Science 453 (2014) 108–118

115

for ethanol recovery was the lowest. On the other hand, it was found that for all membranes transport of acetone is the most efficient. Obtained results allow choosing the most selective membrane for ABE recovery. If the most desired compound to recover from fermentation broth is butanol, the best membrane for this purpose would be the Pervap 4060 one. This membrane transports butanol most preferentially and the transports ethanol least preferentially transports ethanol. preferentially. To prove the choice of Pervap 4060 membrane as the most efficient one for the removal of butanol from water, the impact of temperature on the transport and selective properties was assessed. The additional pervaporation experiments were performed at 40 1C and 60 1C in contact with water–butanol system. Results are presented in Fig. 6. At 40 1C butanol flux was 8.5 mol m  2 h  1 with separation factor equal to 40 (at 0.005 M fraction of organic compound in feed), whereas at 60 1C flux of butanol was equal to 24.0 mol m  2 h  1 and the separation factor value was equal to 42. With increasing temperature flux of butanol increases, because of the higher driving force (Table 1). The apparent activation energy (Eapp) was calculated using [48,49].   Eapp ð11Þ J ¼ J o exp RT

Guarnieri et al. [47] recently presented transport properties of PDMS membranes of different thickness in contact with pure water and proving that water flux is inversely proportional to the PDMS membrane thickness. Based on results presented in this work and assuming that membranes were prepared at the comparable conditions, an estimation of selective layer thickness of commercial PDMS based membranes was performed. Estimated selective layer thickness was equal to around 2 mm, 4 mm and 6 mm for PolyAn, Pervatech and Pervap 4060, respectively. The smallest water flux was obtained for the Pervap 4060 membrane which corresponds to the thickest selective layer for this membrane. 3.5. Efficiency of pervaporation Pervaporation separation index (PSI) – Eq. (6) is a commonly used parameter which can be employed to compare efficiency of various membranes in the removal of chosen component from the feed mixture [41]. This parameter depends strongly on experiment conditions, and due to this all experiments were performed at constant temperature equal to 25 1C, allowing to compare obtained results in various systems and different membranes. The values of PSI calculated for the investigated membranes in contact with binary ABE aqueous mixtures are presented in Fig. 5. Pervap 4060 seems to be the most efficient membrane for butanol recovery due to the highest PSI value obtained in contact with water–butanol system among all tested membranes. It is also worth mentioning that in contact with Pervap 4060, the PSI value

Calculated Eapp of butanol transport in pervaporation in contact with Pervap is equal to 58.6 kJ mol  1, whereas apparent activation energy of water transport is equal to 38 kJ mol  1, which is slightly

Table 7 Comparison of various membranes performances in the pervaporation process. Membrane

Organic flux [mol m  2 h  1]

Organic solvent content in feed [wt%]

T [1C]

Acetone–water Silica membrane PEBA 2533 Pervatech Pervap 4060 PolyAn

5 5 5 5 5

30 23 25 25 25

20.0 0.4 14.9 18.9 28.5

26 3 28 58 37

[51] [21] This work This work This work

Butanol–water PEBA 2533 PTMSP filled with 25wt% of silica Pervap 4060 Pervap 4060 Pervatech Pervatech PolyAn

5 5 5 5 3.5 5 5

23 50 50 25 42 25 25

0.6 128.2 45.9 7.6 13.5 3.7 6.4

6 104 39 35 22 9 8

[21] [44] [44] This work [22] This work This work

Ethanol–water PEBA 2533 PTMSP filled with 25wt% of silica Pervap 4060 Pervap 4060 Pervatech Pervatech Silica membrane PolyAn

5 5 5 5 5 5 5 5

23 50 50 25 50 25 30 25

0.3 206.2 28.2 3.0 56.4 4.0 19.8 8.0

3 18 7 7 7 6 8 6

[21] [44] [44] This work [44] This work [51] This work

ABE fermentation broth IL-PDMS

(βi/j) [dimensionless]

Reference

0.6 BuOH

37

0.2

–

[26]

PDMS – laboratory made

0.8 BuOH 0.05 EtOH

37

0.8 0.03

10 3

[52]

Pervatech

0.2 AcO 0.8 BuOH 0.1 EtOH

37

0.6 Totala

33 20 10

[53]

a Total flux of acetone, butanol and ethanol in units [kg m  2 h  1], PTMSP – poly[1-(trimethylsilyl)-1-propyne], PEBA – poly(ether block amide), and IL–PDMS – ionic liquid and PDMS.

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higher than value of Eapp presented in the work of Wu et al. [50]. Apparent activation energy for butanol transport is significantly higher than Eapp calculated for transport of water, what implies that with temperature increase the both flux and separation factor will increase. According to Jin et al. [4] the maximum efficiency of organic solvents obtained during the ABE fermentation is 30 g dm  3. Table 6 compares fluxes of organic compounds during pervaporation through investigated membranes for binary mixtures of the composition corresponding to the composition of fermentation broth (0.9 wt% AcO, 1.8 wt% BuOH, 0.3 wt% EtOH). Although during ABE fermentation acetone, butanol and ethanol are produced simultaneously, butanol is the compound of main focus. Comparison of various membranes' properties in pervaporative recovery of acetone, butanol and ethanol from binary aqueous mixtures is presented in Table 7. Membrane selectivity of poly(ether block amide) (PEBA2533) in pervaporation in contact with water–organic solvent (acetone, butanol and ethanol) systems at 23 1C follow the order of n-butanol4acetone4 ethanol. Fluxes equal to 0.57 mol m  2 h  1, 0.37 mol m  2 h  1 and 0.28 mol m  2 h  1 of butanol, acetone and ethanol, respectively, were obtained for PEBA2533 [21]. These results are considerably lower comparing with data obtained in this work at 25 1C. Claes et al. [44] tested properties of poly[1-(trimethylsilyl)-1propyne] (PTMSP) layered on PVDF support membrane at 50 1C and determined βi/w of ethanol recovery equal to 18.3 and for butanol recovery 104 at 5 wt% content of organics in water. Authors also compared properties of the prepared PTMSP-PVDF membrane with properties of commercially available PDMS based membranes in the water–ethanol system at 50 1C. The PSI values for Pervatech and Pervap 4060 were 326 and 174 [mol m  2 h  1] [44], respectively. Values presented by Claes et al. [44] are higher compared to results obtained in this work at 25 1C (189 and 145 [mol m  2 h  1] for Pervatech and Pervap 4060, respectively), clearly due to a higher feed temperature (50 1C) used during pervaporation experiments performed by Claes and co-workers [44]. Jin et al. [51] applied a surface modified mesoporous silica membrane and obtained the separation factor of ethanol equal to 8.7, while that of acetone was 28.4 during pervaporation experiments in contact with aqueous binary mixtures of ethanol and acetone at 30 1C. At 5 wt% organic content in feed the partial permeate fluxes of organic solvents were 20.0 and 19.8 mol m  2 h  1 for ethanol/water and acetone/water mixtures, respectively [51]. Comparison of obtained results in model system with the recently performed pervaporation experiments coupled with fermentation system is worth considering. Application of ionic liquid supported on PDMS membrane resulted in achieving quite low flux of butanol, but separation factor was equal to 11 [26]. Higher fluxes of organic compound were found out for laboratory made PDMS membrane [52], whereas the highest selectivity of butanol recovery was obtained for Pervatech membrane by Van Hecke and co-workers [53] during hybrid design pervaporation coupled with two-stage continuous ABE fermentation (separation factor of butanol recovery was equal to 20).

4. Conclusions The pervaporative performance of commercial PDMS based membranes (Pervatech, Pervap 4060 and PolyAn) was investigated for separation in contact with model binary aqueous solutions of acetone, butanol and ethanol. Physiochemical properties of investigated solvents as well as membrane material properties were used to describe transport in pervaporation. From analysis of HSP values it can be seen that only polar cohesion (δp) and hydrogen bonding (δh) parameters impact the affinity of tested substances.

Affinity of acetone, butanol and ethanol to PDMS is significantly higher than to water (smaller distance parameter values between ABE and PDMS comparing with ABE and water). Water flux for Pervatech and Pervap 4060 membranes did not change with change of concentration or type of organic component present in tested aqueous mixtures. Water flux through PolyAn increased slightly with an increase of organic content in feed. Various water fluxes for different membranes were assumed to be caused by different membrane preparation conditions, and as well as by the degree of crosslinking of the tested membranes. According to observed fluxes of organic compounds, acetone was the most efficiently transported compound (at 0.01 M fraction of acetone, solvent flux was equal to 11.5 mol m  2 h  1, 9.3 mol m  2 h  1 and 17.6 mol m  2 h  1 for Pervatech, Pervap 4060 and PolyAn membrane, respectively) due to the highest vapour pressure. In contrast, the intrinsic membrane properties discussed in terms of permeances and selectivities reveal that investigated membranes are selective towards butanol only (selectivity coefficient αBuOH/w is equal to 3.6, 11.6 and 3.4 for Pervatech, Pervap 4060 and PolyAn, respectively). Whereas in contact with water– acetone and water–ethanol mixtures membranes are nonselective or water selective (αi/w r 1). Acknowledgements This work was financially supported by the grant N N209 761240 founded by the Polish Ministry of Science and Higher Education. The authors (J.N. & R.L.K.) would also like to thank the Doctoral Programme in Energy Efficiency and Systems (EES) financed by the Ministry of Education and Culture in Finland. Special thanks are due to M.Sc. Karolina Jarzynka and Dr. Maciej Kujawski for their kind assistance with the text editing.

Nomenclature A AcO BuOH EtOH Eapp EPDR HSP Ji Jo Jt Jw l mi PDMS PEBA Pi pi,f pi,p Pi/l pisat pp PMS POMS

membrane area [m2] acetone butanol ethanol apparent activation energy [kJ mol  1] ethylene propylene diene rubber Hansen solubility parameter partial flux of component i [g m  2 h  1] flux of organic compound [g m  2 h  1] total flux of the permeate [g m  2 h  1] water flux [g m  2 h  1] membrane thickness [m] mass of component i [g] poly(dimethylsiloxane) poly(ether block amide) permeability coefficient of component i [mol m  1 Pa  1 s  1] partial pressure of component i in feed (f) [kPa] partial pressure of component i in permeate (p) [kPa] permeance [mol m  2 Pa  1 s  1] saturated vapour pressure of the pure component i at given temperature [Pa] permeate pressure [Pa] poly(methoxy siloxane) poly(octylmethyl siloxane)

A. Rozicka et al. / Journal of Membrane Science 453 (2014) 108–118

PSI PTMSP PV SBR SGPV SW TPV T t Wd Ws xi Xi xw Xw yi Yi yw Yw αi/w βi/w γi δ δd δh δp Δi,j Δp

Pervaporation Separation Index poly[1-(trimethylsilyl)-1-propyne] pervaporation styrene butadiene rubber sweeping gas pervaporation degree of swelling [%] thermopervaporation temperature [1C] permeation time [h] weight of dry membrane [g] weight of swollen membrane [g] weight fraction of component i in the feed [g/g] molar fraction of the component i in the feed [mol/ mol] weight fraction of water in the feed [g/g] molar fraction of water in the feed [mol/mol] weight fraction of component i in the permeate [g/ g] molar fraction of compound i in the permeate [mol/ mol] weight fraction of water in the permeate [g/g] molar fraction of water in the permeate [mol/mol] selectivity coefficient [dimensionless] separation factor [dimensionless] activity coefficient [dimensionless] Hansen's solubility coefficient [MPa1/2] dispersion interactions [MPa1/2] hydrogen bonding interactions [MPa1/2] polar interactions [MPa1/2] distance parameter [MPa1/2] pressure difference between opposite sides of the membrane [Pa]

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