Characterisation of Romakon™-PM pervaporation membranes for the separation of dilute aqueous alcohol mixtures

Separation and Purification Technology 240 (2020) 116605

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Characterisation of Romakon™-PM pervaporation membranes for the separation of dilute aqueous alcohol mixtures

T

⁎

D.A. Sapegina,b, , G.N. Gubanovab, S.V. Kononovab, E.V. Kruchininab, N.N. Saprykinab, A.Ya. Volkovb, M.E. Vylegzhaninab a b

S&R Systems LLC, 197022 st. Prof. Popova 38 b.2, Saint-Petersburg, Russia Institute of Macromolecular Compounds Russian Academy of Science, 199004 Bolshoy pr. 31, Saint-Petersburg, Russia

A R T I C LE I N FO

A B S T R A C T

Keywords: Pervaporation Thin-film composite membrane Permeate temperature Polyimides Poly(amide-imides) Beer dehydration

Thin-film composite pervaporation membranes with selective layer from pyromellitic dianhydride -4,4′ oxydianiline polyimide on porous poly(amide-imide) support (Romakon™-PM) for treatment of water/ethanol mixtures with exceeding water content (> 50 wt% water) are characterised by the use of scanning electron microscopy, atomic force microscopy and X-ray diffraction. Membranes showed high values of flux, separation factor and PSI (up to 774) in the selected feed mixture composition range. The influence of the support and structural features of the polyimide in the coating layer on the transport properties is discussed. The membranes are successfully tested in terms of pervaporation dehydration of beer. The influence of the permeate temperature on the separation process performance is discussed.

1. Introduction Membrane technologies during past years have found widespread use in the industry and are considered as economically and energetically beneficial way for the realisation of separation processes. Introduction of membrane-based separations allows implementing a wide range of unique processes and provides less-resource-consuming alternatives for classical separation methods. Among all membrane processes, pervaporation (PV) remains one of the most active areas of scientific research due to its potential industrial applicability. Pervaporation is a membrane separation process of liquid mixtures conjugated with phase change of permeating components from liquid to vapour [1]. The driving force for mass transfer in pervaporation is a gradient of components chemical potential mainly defined by the vapour pressure gradient [2]. For theoretical explanation, estimation of selective-transport properties and modelling of transport through PV membranes solution-diffusion model (SDM) is widely used [3]. To describe the efficiency of the process such numerical characteristics as flux, selectivity and separation factor [4] are commonly used. PV is applied commercially in some processes, mainly in the area of

solvent dehydration [5,6], isolation and concentration of aroma compounds [7,8], and different hybrid separation processes [9,10]. The main challenge which has to be overcome for the introduction of PV into a specific industrial process is to find the membrane material with proper transport characteristics, chemical stability and etc. which could fulfil process requirements and show significant process feasibility. PV separations due to the great process selectivity but relatively low fluxes [11] are usually applied in terms of low values of activity of separated component. Application of PV membranes in terms of high activity of the separated component is usually accompanied with great selectivity loss due to the swelling of the selective layer material [12]. Intense swelling leads to the increase in polymers free volume so as to a decrease in a diffusion selectivity, which is believed to play the most significant role in separation efficiency [1]. A large number of commercially available polymeric PV membranes or membranes prepared from commercially available polymers suffer from selectivity loss due to the intense swelling of the separation layer material [12,13]. The limitation of swelling by chemical [14], or physical crosslinking [15], is mainly used by researchers to obtain effective membranes for separation of mixtures with the exceeding content of target penetrant such as azeotropic mixtures of methanol/toluene [16], aromatic/

Abbreviations: PV, pervaporation; SDM, solution diffusion model; PVA, polyvinyl-alcohol; PMDA, pyromellitic dianhydride; ODA, 4,4′-oxydianiline; N-MP, Nmethyl-2-pyrrolidone; PAA, poly(amic-acid; PAI, poly(amide-imide); NIPS, non-solvent induced phase separation; PMDA-ODA, poly-oxydiphenylene-pyromellitimide; XRD, X-ray diffraction; AFM, atomic force microscopy; NRTL model, non-random two liquid model; TFC membrane, thin-film composite membrane; RMSD, root mean square deviation ⁎ Corresponding author at: S&R Systems LLC, 197022 st. Prof. Popova 38 b.2, Saint-Petersburg, Russia. E-mail address: [email protected] (D.A. Sapegin). https://doi.org/10.1016/j.seppur.2020.116605 Received 22 December 2019; Received in revised form 20 January 2020; Accepted 20 January 2020 Available online 23 January 2020 1383-5866/ © 2020 Published by Elsevier B.V.

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2. Materials and methods

aliphatic separations [17] or concentration of dilute water-organic mixtures [18]. However, such processes are common and important to the food and pharmaceutical industry. The concentration of heat-sensitive compounds solutions or selective removal of solvents from their dilute mixtures with organic compounds is still poorly investigated as potential PV applicability areas, mainly because of low fluxes and selectivity loss characteristic for a wide variety of polymeric PV membranes due to the mentioned above reasons. Such challenges as increasing ethanol and aroma content in intermediate products in the processes of alcoholic beverages production, the concentration of active components in the production of water-alcohol extracts from medicinal plants may be considered as prospective application areas for PV process. For providing the feasibility of such processes membranes should show great values of fluxes and selectivity. However, for operation in terms of the requirements for the processes in the pharmaceutical and food industry which are mainly guided by Good Practice group of standards such as GMP [19], membranes should also provide proper resistance to biological contamination, be inert to non-target components of the separated mixture and withstand sterilisation as they are suggested to contact directly with the product. Commercially produced hydrophilic PV membranes which may be considered to be applicable in that types of processes such as polyvinyl alcohol (PVA)-based membranes (AzeoSep™ [20] and Pervap™ [12]) are most likely to be not resistant to microbiological contamination [21–23], non-effective in terms of separation of dilute mixtures [12,24] and could lose selective transport properties during certified sterilisation procedures [25], by which their application in terms of treatment of the aforementioned dilute mixtures is limited and may not be considered as suitable. Moreover, their behaviour in terms of separation of complex multicomponent feed mixtures is poorly studied. Many researchers have shown that, unlike PVA based materials, membranes with polyimide separation layers provide great selectivity in the whole concentration range in terms of alcohol and other solvents dehydration [11]. This is caused mainly by physical crosslinking provided by the high values of intermolecular interaction energies and dense structure. The moderately hydrophilic character of polyimide materials interferes their excess swelling in water [5] even in terms of its exceeding activity in the feed mixture. This, accompanied with good microbiological contamination resistance [26], great chemical and thermal stability [11] (providing availability of dry heat sterilisation) makes them perspective in terms of the PV treatment of dilute solutions of the biological origin. Such solutions include intermediate products in the technology of alcoholic beverages, extracts of medicinal plants, feed mixtures in bioethanol production and etc. Although polyimide materials posses great perspectives they often provide poor values of fluxes [11] and are susceptible to hydrolysis at high temperatures in aqueous media [27] which should be taken into account in the development of PV process. One of the major used techniques to improve the productivity of the membrane material due to the diffusion nature of PV is thinning a separation layer of the membrane. Although that preparation of thin-film composite and asymmetric membranes with defectless diffusion layer [11] from various polyimides is a well-studied field, the particular features of polyimide structure in thin layers and its affection on the membrane performance is not often discussed by researchers. However, the difference in polymer chain packaging in terms of the formation of a thin separation layer on the porous support affects greatly on transport properties [28]. Due to the absence of high performance cost-effective membranes for the treatment of aforementioned mixtures and the absence of data on their PV separation, the aim of the presented research is to characterise thin-film composite polyimide-based membranes that could provide necessary properties for treatment of dilute water-alcohol based complex mixtures to extend the field of PV separation application.

2.1. Reagents Pyromellitic dianhydride (PMDA) (97%), 4,4′-oxydianiline (ODA) (98%), triethylamine, N-methyl-2-pyrrolidone (N-MP), and all other chemicals were of reagent grade, supplied by Sigma-Aldrich USA, and were used without further purification. For the synthesis of poly-amideimide ODA was preliminarily dried in a vacuum oven at 50 °C for 72 h. 2.2. Polymer synthesis Poly(amic-acid) (PAA) powder was obtained by polycondensation of pyromellitic dianhydride and 4,4′-oxydianiline followed by the precipitation of reaction solution in methanol and drying of obtained PAA powder to constant weight in a vacuum oven at 50 °C. The synthetic procedure was performed as described elsewhere [29]. Poly(diphenyl oxide amido)-N-phenylphthalimide (PAI) was obtained through polycondensation of 4-chloroformyl-N-(p-chloroformylphenyl)-phthalimide and ODA in N-MP as described elsewhere [30]. 2.3. Membrane preparation Thin-film composite membranes (Romakon™-PM) were produced at S&R Systems LLC (Russia) by coating porous PAI support, obtained from 10 wt% solution of PAI in N-MP through a non-solvent induced phase separation (NIPS) with water as a non-solvent, with 1 wt% (Romakon™-PM 101), 2 wt% (Romakon™-PM 102) solution of PAA triethylammonium salt in water and subsequent imidization of PAA at 200 °C for 3 h to form a PMDA-ODA polyimide layer. 2.4. Dense films preparation Dense films of poly-oxydiphenylene-pyromellitimide (PMDA-ODA) were prepared by casting 13 wt% solution of PAA triethylammonium salt in N-MP onto a glass plate followed by drying at 80 °C for 12 h followed by imidization at 200 °C for 4 h. Obtained films were removed from glass by immersing in 60 °C water for 30 min and then dried in a vacuum oven at 50 °C overnight. 2.5. X-ray diffraction X-ray diffraction (XRD) studies were performed at room temperature on a SEIFERT XRD 3003 TT (GE, Germany) diffractometer equipped with a primary monochromator (U = 40 kV, I = 40 mA). Cu Kα-radiation with a wavelength λ = 1.5406 Å was used. X-ray diffraction patterns were obtained with a step of 0.05° and a scanning time of 10 sec at each point of the scattering angle region of 2θ from 2° to 40°. The penetration depth of the X-ray radiation is estimated to be between 100 and 200 μm. 2.6. Atomic force microscopy (AFM) AFM studies of the morphology were performed on a Nanotop NT206 atomic-force microscope (ODO “Microtestmachines”, Belarus) in the contact mode under atmospheric conditions using silicon cantilevers FMG01 with a force constant 1–5 N/m and a tip curvature radius of 10 nm. The experimental data were processed using the Surface Explorer program. 2.7. Beverage parameters Ethanol content, extract, extracted sugars and density values of the studied beverages were determined using ultrasonic beverage test system Kolos-2 (Biomer, Russia). Prior to the measurement beverage samples were degassed by thoroughly mixing at 40 °C for 30 min in an 2

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Fig. 1. Illustration of SEQUOIA autonomous continuous flow system; Ti- temperature indicator; Pi- pressure indicator.

open flask and then cooled to room temperature.

sat Pi = Ji ·(pifeed − piperm )−1

2.8. Pervaporation experiments

Pi (T2) =

1. An autonomous non-continuous flow system with an operating membrane area of 6.6⋅10−4 m2 (PC-1). Feed temperature was maintained by submerging the cell in the heated water bath. Permeate was removed by vacuum degassing of the space under the membrane and collected into a liquid nitrogen cooled trap. 2. An autonomous continuous flow system with an operating membrane area of 0.005 m2 (SEQUOIA) (S&R Systems LLC, Russia). Feed temperature was maintained by an automated electrical heater. Permeate was removed by vacuum degassing of the space under the membrane and collected into a cooled trap. The pressure on the permeate side was varied with the variation of trap temperature. The system is illustrated in Fig. 1.

(2)

Si / j =

Pi Pj

(3)

(5)

3. Results and discussion Although many polyimides are known to show great selectivity in terms of solvent dehydration [11], PMDA-ODA polyimide was picked for this study as it is known as the most widely used and cheap polyimide. PMDA-ODA is a well-studied polymer which provides suitable separation performance in terms of water/ethanol mixtures separation along with its wide commercial availability. Because of that it is determined as the best candidate for the selective layer material of industrially produced polyimide-based PV membranes. As shown in the literature, PMDA-ODA has relatively low Permeability [5,32] which limits its application as a material for standalone dense membranes. The PMDA-ODA backbone is considered to be moderately hydrophilic, but still, the polymer shows preferential swelling with alcohols, rather than water [5]. To enhance its performance the formation of thin-film composite (TFC) membranes on its basis could be used. The formation of a thin separation polyimide layer should provide greater fluxes with similar selectivity [1]. Previously

The permeate composition was analyzed by the measurement of a refractive index of permeate with a thermostated (20 °C) laboratory refractometer IRF-22 (Russia) (measurement accuracy ± 1⋅10−4 ( ± 0.05 wt% EtOH)). Overall and partial membrane permeation fluxes J (kg⋅m−2⋅h−1) (1) were determined as the mass of the permeate or penetrant transported through the unit area of the membrane per time unit. The separation factor (2), selectivity (3), permeance (4) and activation energy of permeation (5) were calculated using the following equations [4]:

Cip / Cjp Cif / Cjf

(4)

where α - separation factor; Cif - concentration of component i in feed, wt%; Cip - concentration of component i in permeate, wt%; Si/j - sesat lectivity; Pi - permeance of the component i, kg⋅m−2⋅h−1⋅KPa−1; pifeed pressure of saturated vapors of component i in the feed mixture, KPa; piperm - partial pressure of the component i on the permeate side of the membrane, KPa; Ti - Process temperature, K; Ea- activation energy of permeation, J·mol−1; R - gas constant, J·mol−1·K−1. All experiments were conducted at least three times with a root mean square deviation (RMSD) between values obtained during similar experiments not exceeding 0.0029. The values of components saturated pressure values under process conditions were determined by the means of NRTL (Non-Random Two Liquid) model [31].

Pervaporation performance of obtained membranes was tested using:

αi / j =

Ea 1 1 Pi (T1)·e (− R ·( T 2 − T 1 ))

3

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RomakonTM-PM 101

RomakonTM-PM 102

a

a

b

b

c

c

Fig. 2. SEM images of cross-sections of Romakon™-PM 101 and Romakon™-PM 102 membranes obtained in 2000× (a) and 50 000× (b) magnification and their top surfaces (c).

conducted through thermal (150–200 °C) or chemical [34] treatment.

3.1. Scanning electron microscopy SEM Cross-sections of obtained composite membranes were studied with Scanning electron microscopy for the investigation of the coating layer thickness, and its behaviour in interaction with the PAI support. The images are presented in Fig. 2. The cross-sections of the obtained membranes show finger-like pore morphology (Fig. 2), which is characteristic for PAI [30]. The estimated coating layer thickness for Romakon™-PM 101 membranes − 120 nm, for Romakon™-PM 102–300 nm. As could be seen from the presented images (Fig. 2a–c) suggested coating technique leads to the smooth surface of the selective layer and most likely do not induce coating solution to intensively penetrate the pores of the support to form the areas of low permeability [35]. As a result, a smooth distinguishable border between the support and the coating layer is formed. However during the study of Romakon™-PM 101 top surface small but significant defects were observed (Fig. 3), which could affect separation efficiency negatively and cause an increase in flux values as well as a great decrease in the selectivity of separation.

Fig. 3. SEM image of the top surface of Romakon™-PM 101.

developed techniques [33] of obtaining polyimide films through the formation of corresponding poly(amic-acid) salt films from their solutions in water with following imidization allow the formation of a selective layer on a wide range of water-stable porous supports. One of the most important requirements for the porous support is that it could withstand the PAA-N+[Et]3 salt imidization procedure which is usually 4

739 739 123 0.00002 0.00003 0.00027

2049 49 30 666 16 11 0.00008 0.00385 0.00537

0.312 0.755 1.659 0.05 0.05 0.3

0.17182 0.18971 0.16159 4.612 32.416 0.04637 0.06713 0.08718 27.238 110.806 1.163 1.700 3.809 0.05 2.00 3.00

25 25 25 (water), KJ·mol−1 (ethanol), KJ·mol−1 27 27 27 (water), KJ·mol−1 (ethanol), KJ·mol−1 40 45 65 permeation permeation 40 50 60 permeation permeation Romakon™- PM 101 Romakon™- PM 101 Romakon™- PM 101 Activation energy of Activation energy of Romakon™- PM 102 Romakon™- PM 102 Romakon™- PM 102 Activation energy of Activation energy of

Selectivity (water/ ethanol) Separation factor (water) EtOH in permeate, wt %

Overall flux, kg·m−2·h−1

Permeance of water kg·m−2·h−1·KPa−1

Permeance of ethanol kg·m−2·h−1·KPa−1

5

EtOH in feed, wt %

For preliminary characterisation, obtained membranes were tested in terms of separation of model water/ethanol mixtures of different composition at 40 °C using PC-1 pervaporation setup. Results are presented in Fig. 4. For determination of the influence of the temperature on the transport properties of studied membranes the separation of a water/ ethanol mixture containing 25–27 wt% EtOH was performed at different temperatures, the results are presented in Table 1. Activation energies of permeation were calculated as an average between every two pairs of neighbouring temperatures. During the calculation, significant deviations from Eq. (5) for Romakon™-PM 101 were observed. It could be seen, that, unlike dense PMDA-ODA films [33], obtained

Temperature, °C

3.2. Pervaporation of model mixtures

Membrane

Table 1 The separation of H2O/EtOH mixtures at different temperatures (operating membrane area 6.6⋅10–4 m2).

Fig. 4. Dependencies of total flux (solid lines) and separation factor (dotted lines) values on ethanol content in the feed mixture for Romakon™- PM 101 (a) and Romakon™- PM 102 (b) membranes at 40 °C and water content in permeate in dependence of ethanol concentration in feed (c) for the corresponding experiments. (Operating membrane area 6.6⋅10–4 m2).

2134 2056 328

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Table 2 Transport characteristics of some polymeric membranes applied for low ethanol content mixtures separation. Membrane

Temperature, °C

EtOH in feed, wt %

EtOH in permeate, wt %

Total flux, kg·m−2·h−1

Selectivity (water/ EtOH)

Separation factor (water)

PSI

Reference

Pervap™ 4100

95

53

2,6

12.8

50

42

512

Romakon™-PM 102 Romakon™-PM 102 Pervap™ 2201

40 61 60

55 27 30

2.5 0.3 3.4

0.495 1.659 0.750

58 328 40

48 123 12

23 202 8

Calculated from [12] This work This work [24]

membranes was conducted. The results are discussed further in Section 3.3. Lower values of activation energy of permeation (Table 1), their deviation from Eq. (5) and non-extremal character of flux curve (Fig. 4) observed for Romakon™- PM 101 membranes, as well as high flux values and a tendency of greater decrease in selectivity than expected from the estimation by the means of SDM model with the rise of the process temperature may be not only the consequence of the PAI support effect or difference in structural features of polymer in the coating layer, but most likely to be caused by defects presence in the coating layer. Thus, during the comparative analysis of PM-101 and PM-102 membranes we could conclude that the minimal thickness of the coating layer which is necessary to provide good values of diffusion selectivity in the selected feed composition range lies between 120 and 300 nm. Obtained results show, that composite membranes with PMDA-ODA polyimide selective layer do not show high values of selectivity in the range of high EtOH content, as reported for dense films [11], but are very effective for PV separation of mixtures containing up to 35–40 wt % EtOH, providing high values of both flux and selectivity in this range. The comparative characteristics of the obtained membranes with commercially available membranes in the case of water-rich mixtures separation are presented in Table 2. PV performance of Romakon™-PM 102 membranes surpasses that of the PERVAP 2201 membranes in application to the separation of mixtures with low ethanol content, which acts in favour of their industrial applicability in such processes in food and pharmaceutical industry. However, as was mentioned above, their transport characteristics differ significantly from those of PMDA-ODA dense membranes, which show high values of selectivity throughout all ethanol content range along with lower flux values. To successfully implement PV separation to selective water removal from organics-poor mixtures high values of selectivity and flux should be obtained to make the process of PV concentration feasible, which are provided with Romakon™-PM 102 membranes. It is also important to mention that during the model mixtures PV permeance values of both water and ethanol through Romakon™-PM 102 membrane remain constant with RMSD of 0.38% for water and 0.05% for ethanol, which makes it possible to predict its performance under various conditions with suitable accuracy by the means of mathematical models based on Fick’s law diffusion pattern.

composite membranes show good values of selectivity, only in the range between 0 and 25 wt%, and start to lose separation efficiency with increase in ethanol content in the feed mixture above 25–27 wt% (Fig. 4). From these observations, we may form two main suggestions which may explain the cause of the selectivity loss. The first and most obvious one is that the loss in selectivity is caused by the defects in the coating layer, but the defects were only observed on the surface of Romakon™-PM 101, and still, defectless PM 102 membranes behave in a similar manner and lose selectivity above 25 wt% EtOH threshold. From this, we may suggest that defects are not the main reason for the unusual PMDA-ODA transport behaviour. It was shown by other researchers, that structural features of the polymer in the coating layer may differ from those in thick films due to the influence of the support [28], the difference in a layer formation conditions [35], molecular weight of the coating polymer [29] and etc. We suggest that in conditions of thin-layer formation from a dilute solution of PAA, PMDA-ODA structure is affected by the presence of amorphous polymer of similar nature (PAI of the support). This may cause a decrease in crystallinity in comparison with the thick film. Similar effects of the forming solution concentration [36] and the influence of the support [37] on the polyimide structure are reported in the literature. It is also important to remember, that in the case of composite membrane both layers (porous support and coating layer) affect one another during swelling, and by so separation process. So it may be concluded that the loss in separation selectivity in the case of PMDAODA at PAI membranes may be caused by either the swelling of PAI, which is known to be more permeable for alcohols than water [3] and by so, most likely to show preferential swelling in them, or by the structural features of PMDA-ODA layer formed on the PAI support. Lower crystallinity rates in comparison with dense films suggested to occur in PMDA-ODA layers in membranes and small thickness of the diffusion layer accompanied by the fact that PMDA-ODA is preferentially swollen with ethanol rather than water may be the reason of selectivity loss in the terms of high ethanol content mixtures pervaporation. Crystallites or sites with the regular structure are believed to behave as physical crosslinks [11] which prevent the increase in the interchain distances during swelling, so it could be suggested that more amorphous nature of the PMDA-ODA in the selective layer is to cause the loss in selectivity. It corresponds well with data presented in the literature [5,11,33], as preferential diffusivity believed to be determining part of the polyimide membranes selectivity. The extremal character of the dependencies of the separation factor on ethanol content in the feed mixture (Fig. 4) is also worth noticing. According to the NRTL model, at 40–60 °C saturated vapour pressures of water and ethanol are equal within the mixture containing 24–27 wt% ethanol, which corresponds with the ethanol content values where the separation factor curves start to decrease. This also attributes to the fact that selectivity is lost due to the swelling of the selective layer of the membrane material which is suggested to be higher in PMDA-ODA coating layers than in dense films due to their more-amorphous nature. To prove the aforementioned suggestions an X-ray study of dense PMDA-ODA films, uncoated support and Romakon™ membranes was conducted. To see how swelling affects structural features of the composite membranes and what happens to the membranes after 25 wt% ethanol threshold an X-ray diffraction analysis of swollen composite

3.3. X-Ray diffraction To investigate structural features of obtained materials, Romakon™ membranes, uncoated PAI support and PMDA-ODA dense film were studied with XRD analysis in the reflection mode. The trend curves of diffractograms were calculated on the basis of X-ray data with the leastsquares method using polynomials of the power of 6 (P6(2Θ)) as an interpolation function in the ranges of 2Θ = 5–25°. The values of 2Θ corresponding to the peak intensities were located with the use of following equation:

dP6 (2Θ) =0 d2Θ 6

(6)

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Samples of Romakon™ membranes were immersed in water or water/ ethanol mixtures (15 wt% and 35 wt% ethanol) in closed containers until the constant value of the swelling degree was achieved, then they were removed from containers, blotted with filtering paper to remove residual moisture on the surface not sorbed by the studied sample and immediately tested by the means of XRD analysis. The diffraction patterns of swollen Romakon membranes are presented in Fig. 6. The exact concentrations of water/ethanol solutions were picked because the decrease in the selectivity of separation of water/ethanol mixtures is observed at 25 wt%, where saturated vapour pressures of ethanol and water are almost equal. So, by studying the difference in structural features of the membrane below (15 wt%) and above (35 wt%) the point of selectivity loss we may observe the structural changes which caused the decrease in membrane separation efficiency and suggest why does it occur. Both samples show a shift of the halo maximum to lower values of 2Θ with an increase in ethanol content in the mixture. Besides that tendency, there are some important changes occurring to membranes during swelling which worth to be noticed. In Fig. 6b we may see that in the case of Romakon™-PM 102 swelling in 15 and 35 wt% ethanol induces splitting of single halo maximum into two independent peaks (Fig. 6c). At 15 wt% ethanol, we observe the first maximum at 2Θ = 19.15°, which suggestively corresponds to PMDA-ODA layer and second at 2Θ = 15.95° which consequently corresponds to swollen PAI support. As the position of the PMDA-ODA peak is relatively unchanged in comparison with diffraction patterns of its film (Fig. 5a) we may suggest that before the threshold of 25 wt% Ethanol PMDA-ODA layer in Romakon™-PM 102 does not show intense swelling in the feed mixture. The support does which is witnessed by an increase in interplanar distance value determined by the attributed to the PAI support peak position shift from 2Θ = 18° to 2Θ = 15.95°. In the case of 35 wt % mixture, Romakon™-PM 102 diffraction patterns show the shift of the first maximum from 2Θ = 19.15° to 2Θ = 18.20° from which we may conclude that interplanar distances in the coating layer started to increase in relation to PMDA-ODA dense film and so swelling of the coating layer occurs. In the case of PM 101 (Fig. 6a) we do not see such a picture, that could be explained either by the insufficient intensity of diffraction peaks of PMDA-ODA or by more amorphous nature of the polymer in a thin coating layer. The fact after swelling in water PM 101 its X-ray diffraction pattern shows shift of the peak maximum to the higher values of 2Θ may witness that while we could not distinguish standalone PMDA-ODA peak, because of its low intensity, its presence could contribute to the mutual PMDA-ODA at PAI peak position and create aforementioned shift of the maximum. This could false-indicate a decrease in average effective interplanar distances which most likely does not occur. Due to the low intensity of crystalline structure reflexes at 2Θ = 5–30° characteristic for obtained PMDA, we may not directly conclude that PMDA in coating layer has more amorphous nature that in the form of dense film. The observation that coating layer starts to swell in mixtures containing 35 wt% ethanol which is prevented by crystalline sites in the case of PMDA-ODA film [5], allows us to conclude that PMDA-ODA in the coating layer of Romakon™ membranes has more amorphous nature. This could explain the difference between the transport properties of studied membranes and ones reported by other researchers for dense [11] and composite dip-coated PMDA-ODA membranes [33].

Fig. 5. X-ray diffraction patterns of PMDA-ODA dense film (a), uncoated PAI support (b), Romakon™ PM-101 (c) and Romakon™ PM-102 (d).

X-ray diffraction patterns of PMDA-ODA dense film (a), PAI support (b), dry Romakon™ membranes PM 101 (c), and PM 102 (d) are presented in Fig. 5. The diffractogram of PMDA-ODA film (Fig. 5a) obtained from the same polymer as a coating layer shows the maximum at 2Θ = 19° and a number of overlapping reflexes in the range 2Θ = 5–30° which witness for the semi-crystalline nature of the obtained sample, and correlates with the available data reported by other researchers [38]. It could be seen that unlike the case of the dense film [3] PAI shows more amorphous character in the form of the porous support (Fig. 5b), which is witnessed by the broadening of the halo and a shift of the diffraction maximum to 2Θ = 18°. This observation could be explained by more rapid formation of the structure during NIPS, where the solvent is forced to come out rapidly from the forming solution, unlike the case of the film formation process, where the solvent evaporates slowly, which may give the necessary time for polymer chains to pack in a more dense and regular manner. As the penetration depth of the X-ray radiation is significantly greater than the thickness of the coating layer, on the diffractograms of the composite membranes are observed amorphous halos. The peaks of intensities at 2Θ = 18.05° for Romakon™-PM 101 (Fig. 5c) and 2Θ = 18.95° for Romakon™-PM 102 (Fig. 5d) correspond to the effective average interplanar distances in the sample caused by diffraction on both PMDA-ODA and PAI chains. The peaks intensity position and broadness of the diffraction patterns for Romakon™-PM101 and Romakon™-PM 102 may be attributed either to the difference in the PMDA-ODA layer thickness and/or their more amorphous nature in the case of a thinner layer of Romakon™-PM 101. Comparing with uncoated PAI support halo maximums of Romakon™ diffraction patterns shift from 2Θ = 18° characteristic to PAI support, closer to 2Θ = 19° characteristic for PMDA-ODA with an increase in coating layer thickness, which is expected. To investigate the influence of swelling on the structure of the obtained TFC membranes the experimental procedure similar to the one described by Xiangyi Qiao and Tai-Shung Chung [39] was used.

3.4. AFM studies of the coating layer To confirm suggestions about the coating layer structure made on the basis of X-ray diffraction analysis and pervaporation performance data Romakon™ membranes were subjected to AFM analysis. AFM images of the coating layer in Romakon™ -PM 101 and 102 are given in Fig. 7. Comparative analysis of the images shows that the morphology of the coating layer differs significantly between 101 and 102 samples. 7

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RomakonTM- PM 101

RomakonTM- PM 102

0

(2Θ = 5-250) (b)

(2Θ = 5-25 ) (a)

RomakonTM- PM 102 (2Θ = 12-220) (c)

Fig. 6. X-ray diffraction patterns of swollen Romakon™ in comparison with dry samples.

as seen in the surface line patterns shown in Fig. 7(b and f). Thus the depth of the caverns for 101 sample is defined to be between 40 and 60 nm while the cavern depth for 102 sample is around 20 nm (Fig. 7c and g). The fact of cavern formation and that their depth is dependent

The surface of both samples replicates the pattern of the PAI support pores through the formation of the nano dimensional caverns which have honeycomb-like morphology (Fig. 7a and e). It is seen that the depth of the caverns for 101 sample is much lower than for 102 sample 8

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Fig. 7. AFM images of the surface Romakon™-PM 101 (a–d) and Romakon™-PM 102 (e–h) membranes: a, e −3D-images, b, f, d, h- height images, c, g -profiles of the selected area on the surface.

3.5. Pervaporation of real mixtures

on coating layer thickness witnesses that forming solution is not likely to penetrate the pores of the support to form the areas of low permeability, as was suggested on the basis of SEM images of the cross-section of Romakon™-PM membranes. The mean arithmetic (Ra) and root mean square (Rq) roughness values of sample surfaces also vary and are defined to be Ra = 11.5 nm Rq = 14.7 nm for 101 sample and Ra = 6.4 nm Rq = 8.2 nm for 102 sample. The fine structure of the coating layer is well illustrated in the images with the scanning matrix of 4 × 4 μm (Fig. 7d and h). It is seen that the surface of 101 samples has granular (nanodomain) morphology between the caverns, while the average grain size is estimated to be equal to 200 nm. In the case of 102 samples, the surface remains smooth and does not show any sign of nanodomain formation. This observation acts in favour of structural difference between 101 and 102 samples observed with X-ray diffraction and pervaporation experiments. It seems that in the case of 101 sample (1 wt% PAA in the forming solution) the polymer in coating layer is likely to form distinguished aggregates that are attached to each other by intermolecular interaction forces, which explains greater loss in separation selectivity with an increase in process temperature in comparison with 102 sample (Table 1) and the absence of distinguishable peaks at 2Θ = 19 characteristic for PMDA-ODA during XRDswelling experiments.

As the coating layer in Romakon™- PM 102 membranes has been considered to be defectless, and by so they provided good values of PSI in the selected feed composition and temperature range, they have been considered as perspective in terms of industrial applicability, and were tested in terms of separation of real mixtures using autonomous continuous flow system with an operating membrane area of 0.005 m2 (SEQUOIA). One of the most attractive commercial application of this type of membranes was found to be in the processes of selective removal of water in the technology of beer production. Usually, to obtain beers with high ethanol content, extract [40] and density values (such as Russian imperial stout [41] and British barley wine ale [42]), energyand time-consuming processes are usually unavoidable [42]. The implementation of PV removal of water into the process allows reaching similar values of the aforementioned parameters and beverage taste and texture, providing a cost-effective alternative for brewers. As compounds which affect the beverage's unique taste and aroma are not able to penetrate the membrane, dehydration of the beer induces an increase in their concentration, so suggestively increase in the value of the beverage on the market [43]. As polyimides are considered to be resistant to microbiological contamination [44] and could withstand dry-heat sterilisation [11], 9

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microbiological contamination and simulate the industrial process conditions. Membranes after operation were studied using SEM microscopy and no sign of microbiological contamination or damage of the selective layer was observed.

Table 3 Initial ethanol content, extract, extracted sugars and density of the studied beverage; process temperature 60 °C; average flux 1.10 kg·m−2·h−1; initial sample volume − 225 ml, process time 12 h.

EtOH, wt% Extract, wt% Extracted sugars, wt% Density g·cm−3

Before

After

Russian imperial stout A

6.38 10.27 9.96 1.028

9.32 15.2 14.8 1.048

8.82 11.59 10.31 1.029

4. Conclusions Commercially available Romakon™-PM thin-film composite PV membranes were characterised in terms of separation of dilute water/ ethanol model and real multicomponent mixtures and studied by the means of SEM, AFM and X-ray diffraction analysis. Membranes showed high values of flux and selectivity in the range below 30 wt% ethanol content in the feed mixture, which makes them applicable to the processes of selective water removal from dilute aqueous mixtures. The difference between the selective properties of Romakon™ membranes and PMDA-ODA dense films in terms of water/ethanol mixtures separation is explained by more amorphous nature of polyimide in the coating layer in comparison with dense thick film observed with X-ray diffraction analysis and AFM. The implementation of X-ray diffraction to analyse swollen samples of Romakon™ membranes allowed to show the difference in swelling behaviour of the coating layer and the support and show that swelling of the coating PMDA-ODA layer in feed mixtures containing more than 25 wt% ethanol, which is suggested to occur due to the difference of structural features in comparison with dense polyimide films, induces the loss in separation selectivity. Romakon™-PM 102 membranes were successfully applied to the process of beer concentration to obtain the beverage with high market potential. Thermal and chemical stability of Romakon™-PM and its ability to withstand sterilisation procedures allows using it in direct contact with the products in the food and pharmaceutical industry. The possibility of using flowing water (17 °C) as a cooling agent for permeate condensation with insignificant loss in overall flux values makes the proposed PV separation process economically feasible.

Romakon™-PM 102 membranes could be successfully applied to this type of processes, in direct contact with the product. Craft-brewed stout samples were used as feed mixture to test the performance of Romakon™-PM 102 in terms of beer dehydration, the results are presented in Table 3 as beverage parameters before and after the process in comparison with the parameters of locally-obtained Russian imperial stout A. The ethanol content in permeate did not exceed 0.08 wt% during all the experiments. During the organoleptic study of the obtained samples by various independent beer-tasters, the craft-brewed stout concentrate showed balanced taste and rich texture. As Romakon™-PM 102 did not show significant deviations from Fick's law pattern in the studied range of mixture compositions its transport performance under various process parameters could be calculated with sufficient accuracy with Eqs. (2)–(5). To define optimal process parameters for the dehydration process mathematical model on the basis of experimental data using Eqs. (2)–(5) and NRTL model was developed. The model could predict the permeance of the component at the modelled process temperature from the data obtained during the PV of model mixtures, by the means of Arrhenius plot function (Eq. (5)), with the introduction of correction between model and real mixture fluxes. Using NRTL (beer is assumed to behave as a model binary mixture with the same ethanol content in terms of saturated vapour pressures of water and ethanol), feed mixture composition, permeate temperature and permeance values we may predict the overall flux and permeate composition using Eq. (5). To check the accuracy of mathematical model prediction membranes were tested by the means of autonomous continuous flow system with an operating membrane area of 0.005 m2 (SEQUOIA) in terms of variation of permeate condenser temperature using craft-brewed stout as feed mixture. The process parameters (overall flux and ethanol content in permeate) calculated for different temperatures of permeate in the condenser and their values obtained during the experiment are demonstrated in Table 4. It could be seen from the data presented in Table 4, that the value of total flux during the process with liquid-nitrogen permeate cooling is only 6.2% higher than in the case of cooling with running water (17 °C), while selectivity remains unchanged. The cost of the process decreases dramatically with the change of the cooling agent, which makes the process of PV dehydration of beer economically feasible and effective even for small production capacity. Romakon™-PM 102 membranes showed stable continuous operation for more than 8 months with no change in both flux and selectivity values. During the operation, they were washed with P3-COSA CIP 72 (Ecolab, USA) and thermally sterilised at 200 °C for 10 min (directly in the SEQUOIA module assembly) every two weeks to prevent their

CRediT authorship contribution statement D.A. Sapegin: Writing - original draft, Conceptualization. G.N. Gubanova: Data curation, Methodology. S.V. Kononova: Writing review & editing. E.V. Kruchinina: Investigation. N.N. Saprykina: Investigation. A.Ya. Volkov: Investigation. M.E. Vylegzhanina: Investigation.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement The authors would like to thank Pivmasteria 17M LLC (SaintPetersburg, Russia) for provision of craft-brewed stout and conduction of beverage parameters measurements.

Table 4 Predicted and experimentally obtained process parameters for PV of craft-brewed stout (EtOH 6.38 wt%) at 60 °C through Romakon™-PM 102. Total flux (Model), kg·m−2·h−1

Total flux (Experimental), kg·m−2·h−1

Ethanol in permeate (Model), wt%

Ethanol in permeate (Experimental), wt %

Condenser temperature, °C

1.115 1.078 1.050

1.193 1.080 1.000

0.07 0.08 0.08

0.08 0.08 0.08

−193 (liquid nitrogen) 3 17

10

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