processes

Article Pervaporation of Aqueous Solutions through Rigid Composite Polyvinyl-Alcohol/Bacterial Cellulose Membranes

Tănase Dobre 1, Claudia Ana Maria Patrichi 1,* , Oana Cristina Pârvulescu 1,* and Ali A. Abbas Aljanabi 2

1 Chemical and Biochemical Engineering Department, University POLITEHNICA of Bucharest, 1-6 Gheorghe Polizu, 011061 Bucharest, Romania; [email protected] 2 Al Mussaib Technical College, Al-Furat Al-Awsat Technical University, Babylon 54003, Iraq; [email protected] * Correspondence: [email protected] (C.A.M.P.); [email protected] (O.C.P.)

Abstract: The paper focuses on synthesis, characterization and testing in ethanol-water separation by pervaporation of new membrane types based on polyvinyl alcohol (PVA) and bacterial cellulose (BC). A technology for obtaining these membranes deposited on a support is presented in the experimental section. Three PVA-BC composite membranes with different BC content were obtained and characterized by FTIR, SEM and optic microscopy. The effects of operating temperature (40–60 ◦C), permeate pressure (18.7–37.3 kPa) and feed ethanol concentration (24–72%wt) on total permeate flow rate (0.09–0.23 kg/m2/h) and water/ethanol selectivity (5–23) were studied based on an appropriate experimental plan for each PVA-BC membrane. Statistical models linking the process factors to pervaporation performances were obtained by processing the experimental data. Ethanol   concentration of the processed mixture had the highest influence on permeate flow rate, an increase in ethanol concentration leading to a decrease in the permeate flow rate. All 3 process factors and Citation: Dobre, T.; Patrichi, C.A.M.; Pârvulescu, O.C.; Aljanabi, A.A.A. their interactions had positive effects on membrane selectivity. Polynomial regression models were Pervaporation of Aqueous Ethanol used to assess the effect of BC content in the dried membrane on pervaporation performances. Values Solutions through Rigid Composite of process performances obtained in this study indicate that these membranes could be effective for Polyvinyl-Alcohol/Bacterial ethanol-water separation by pervaporation. Cellulose Membranes. Processes 2021, 9, 437. https://doi.org/10.3390/ Keywords: pervaporation; PVA-bacterial cellulose membrane; FTIR analysis; membrane selectivity; pr9030437 permeate flow rate

Academic Editor: Mohammad Javad Parnian 1. Introduction Received: 26 January 2021 The most attractive point of the pervaporation process is the separation of the organic Accepted: 25 February 2021 Published: 28 February 2021 solvents [1–3], especially the water removal. It presents a high level of performance and it is economically efficient. In the field of solvents dehydration, there have been many cases in

Publisher’s Note: MDPI stays neutral which pervaporation technology has been employed and successfully implemented on an with regard to jurisdictional claims in industrial scale [4,5]. Currently, more than 90 industrial units are in operation world-wide published maps and institutional affil- for the dehydration of ethanol, isopropanol, ethyl acetate and multipurpose solvents [6]. iations. There are many studies focused on the development of new membranes that can be used for the pervaporation process as well as for other processes involving membranes. For the pervaporation process, the focus is on the enhancement of membranes, in order to be more efficient, with longer-term stability and life, and on the economic factor, expressed in terms of the accepted cost. Although many types of materials have been used in the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. development of membranes, the composite membranes based on polyvinyl-alcohol (PVA) This article is an open access article are the ones mostly used in the pervaporation process [7–11]. Due to its numerous hydroxyl distributed under the terms and groups (Figure1), PVA exhibits a high water permeability. Moreover, dried PVA-based conditions of the Creative Commons membranes have better abrasion resistance, mechanical properties and selectivity for Attribution (CC BY) license (https:// hydrophilic species [12]. The high degree of swelling of PVA determines an increase in the creativecommons.org/licenses/by/ free volume of its network and a decrease in the chemical interactions with the permeating 4.0/). species, resulting in low membrane selectivity and stability [12].

Processes 2021, 9, 437. https://doi.org/10.3390/pr9030437 https://www.mdpi.com/journal/processes Processes 2021, 9, 437 2 of 15

PVA determines an increase in the free volume of its network and a decrease in the Processes 2021, 9, 437 2 of 14 chemical interactions with the permeating species, resulting in low membrane selectivity and stability [12].

FigureFigure 1. 1.Polyvinyl Polyvinyl alcohol alcohol (PVA) (PVA) structure. structure. Various techniques have been employed to obtain enhanced performances of PVA Various techniques have been employed to obtain enhanced performances of PVA membranes [13–16]. Praptowidodo [13] studied the influence of the cross-linking degree membranes [13–16]. Praptowidodo [13] studied the influence of the cross-linking degree on PVA-based membrane performances by adding glutaraldehyde (GA) as a cross-linking on PVA-based membrane performances by adding glutaraldehyde (GA) as a agent. An increase in the amount of GA led to a decrease in the degree of swelling, resulting cross-linking agent. An increase in the amount of GA led to a decrease in the degree of in a decrease in the permeate flow rate and an improvement of membrane selectivity. The swelling, resulting in a decrease in the permeate flow rate and an improvement of mechanical properties of composite membranes based on PVA can be highly improved by addingmembrane suitable selectivity. reinforcement The mechanical materials inprop PVA.erties Huang of composite et al. [17] prepared membranes a composite based on hydrogelPVA can containing be highly improved PVA and polyvinylby adding pyrrolidonesuitable reinforcement (PVP) with materials wrapped in multiwalled PVA. Huang carbonet al. [17] nanotubes. prepared Unlike a composite a PVA-based hydrogel membrane, containing this compositePVA and membranepolyvinyl pyrrolidone exhibited about(PVP) a with 133% wrapped increase inmultiwalled the tensile carbon strength nanotubes. and 63% inUnlike the tear a PVA-based strength. membrane, this compositeBacterial membrane cellulose (BC)exhibited is a special about cellulosea 133% in withcrease a nanoscalein the tensile structure, strength produced and 63% by in selectedthe tear aerobic strength. Gram-negative bacteria, which are active on saccharide substrates at a certain composition,Bacterial pH cellulose and temperature (BC) is a [ 18special]. BC iscellulo a veryse purewith product,a nanoscale with structure, a high degree produced of crystallinityby selected andaerobic polymerization Gram-negative and bacteria, with an which interesting are active mechanical on saccharide behavior substrates [19,20]. BC at a exhibitscertain superiorcomposition, structural pH and properties temperature compared [18]. to BC plant is a cellulose very pure [21 ].product, It has been with proven a high thatdegree BC, eitherof crystallinity pure or composite, and polymerization can be used and as awith valuable an interesting biopolymer mechanical in various behavior fields (e.g.,[19,20]. industrial, BC exhibits medical, superior food processing)structural proper [22–24ties]. The compared hydrophilic to plant nature cellulose of pure [21]. BC It and has BC-basedbeen proven composites that BC, confers either thesepure typesor composit of membranese, can be selectivityused as a invaluable water transport,biopolymer if ain binaryvarious mixture fields (e.g., containing industri wateral, medical, and another food compoundprocessing) passes[22–24]. through The hydr theophilic membrane. nature Duringof pure the BC BC and membrane BC-based pervaporation composites confer processs these of binary types ethanol-water of membranes mixture, selectivity as the in waterwater content transport, is high, if a it binary results mixture in a high containing permeate flow water rate and and another a low selectivity. compound passes throughRecently, the membrane. much research During has been the BC focused membra on PVA-BCne pervaporation composites, process especially of binary because eth- bothanol-water components mixture, present as the strong water hydrophilic content is behavior. high, it results In PVA-BC in a composites,high permeate BC appearsflow rate asand fiber, a low film selectivity. or bulk material. The aim is to obtain a membrane with good mechanical prop- erties, highRecently, chemical much stability research as wellhas asbeen good focuse selectivityd on PVA-BC and permeability composites, for especially transferable be- speciescause [both13]. Forcomponents PVA-BC nanocompositepresent strong layers,hydrop Geahilic et behavior. al. [25] presented In PVA-BC different composites, prepara- BC tionappears methods; as fiber, the resultsfilm or showedbulk material. that the The presence aim is ofto PVAobtain led a tomembrane a reduction with in Young’sgood me- moduluschanical andproperties, in an increase high chemical in toughness stability compared as well to as a good pure BCselectivity layer. Tan and et permeability al. [26] and Chiciudeanfor transferable et al. species [27,28] [13]. used For a layered PVA-BC PVA-BC nanocomposite composite layers, hydrogel, Gea et which al. [25] exhibited presented excellentdifferent mechanical preparation properties. methods; the results showed that the presence of PVA led to a re- ductionHigh in permeate Young's flow modulus rates and and acceptable in an increase water selectivitiesin toughness were compared obtained to for a pervapo- pure BC rationlayer. of Tan ethanol-water et al. [26] and mixtures Chiciudean through et BCal. [27,28] membranes used [a29 layered]. However, PVA-BC due tocomposite membrane hy- crackingdrogel, atwhich drying exhibited and also excellent excessive mechanical swelling, a properties. high pervaporation surface is not possible. PervaporationHigh permeate performances flow ofrates BC-PVA and membranes,acceptable obtainedwater selectivities by impregnating were BCobtained gel with for PVApervaporation and cross-linking of ethanol-water with GA [30 mixtures], were superiorthrough toBC those membranes of BC membranes. [29]. However, However, due to thesemembrane types ofcracking membranes at drying have and the also same excessive disadvantage swelling, of crackinga high pervaporation and swelling, surface which is doesnot notpossible. allow Pervaporation the development performances of pervaporation of BC-PVA surfaces membranes, required by obtained applications by impreg- with highernating working BC gel capacity.with PVA and cross-linking with GA [30], were superior to those of BC membranes.In this paper, However, we assess these PVA-BCtypes of membranes,membranes have with the BC same distributed disadvantage discreetly of crack- and controlleding and swelling, in PVA, appliedwhich does on supports not allow of the any development shape and size, of pervaporation and processed surfaces by cross- re- linkingquired withby applications total water with removal, higher cover working all practicalcapacity. requirements of a pervaporation process with water transport. Accordingly, hydrophilic fibers of BC were mixed with PVA and GA and the mixture was deposited on a ceramic support, to obtain rigid PVA-BC membranes with a high operational effectiveness when used for water separation from various mixtures. The main aim of the paper was the study of pervaporation performances Processes 2021, 9, 437 3 of 14

during the dehydration of ethanol-water solutions using rigid PVA-BC composites deposited on a ceramic support.

2. Materials and Methods 2.1. Materials

The pervaporation rigid membranes were prepared from PVA powder type Mw 89,000–98,000 (Sigma Aldrich), BC gel (0.995 g/cm3) having 96% water, fibrils with di- ameters less than 50 nm, a crystallinity index of 78−90% and a polymerization degree of 2500−4000 (UPB Mass Transfer Laboratory) and GA (Merck) as a cross-linking agent. The membranes were deposited on a high porosity ceramic tube support (type TC-00314, Research, Design and Production Center for Refractive (CCPPR), Alba Iulia, Romania).

2.2. Membrane Preparation There were two steps involved in preparing the disperse systems used to obtain PVA- BC membranes. The first step involved the preparation of a PVA solution having a PVA content of 75 g/L (by dissolving 12 g of PVA in 150 mL of distilled water, under moderate agitation, at 60 ◦C. In the second step, BC gel was ground by using a disintegrator knife and then mixed with the PVA solution to obtain a PVA/BC ratio of 2 (12 g PVA and 6 g BC), 1 (12 g PVA and 12 g BC) and 0.66 (12 g PVA and 18 g BC), respectively. Thus, three PVA-BC disperse systems were obtained, the concentrations of PVA and BC being 7.14% and 3.57% for the first system (S1), 6.90% and 6.90% for the second system (S2), 6.67% and 10% for the third system (S3), respectively. The disperse systems were stored at 5 ◦C before being used. Rigid PVA-BC membranes were obtained as follows: (i) the membrane support (ce- ramic tube) was cleaned with distilled water, dried at 105 ◦C and then weighed; (ii) 30 g of disperse system (S1, S2 or S3) was mixed with 1.2 g of GA, which was used as a cross-linking agent; (iii) the membrane mixture was applied on the ceramic tube with a brush; after the first layer was applied, the system was dried at 60 ◦C for 15–20 min and then weighed; the second layer was applied and the procedure was repeated until the thin layer reached the desired mass (1.3–1.8 g on the ceramic support with an external diameter of 0.035 m and a length of 0.25 m); (iv) supported membrane was dried at 90 ◦C for 90–120 min, then weighed to determine the final mass and thickness of the active membrane layer. Depending on the disperse system (S1–S3) from which they were prepared, the membranes were coded as follows: PVA-BC 12/6 (M1), PVA-BC 12/12 (M2) and PVA-BC 12/18 (M3). Figure2 shows the stages of preparation of rigid PVA-BC membranes applied on the ceramic tube. The computed thickness of membranes was about 45 µm, according to some tests performed with a PosiTector 6000 apparatus (DeFelsko inspection Instruments). In Processes 2021, 9, 437 order to have membrane samples for physicochemical analysis (FTIR,4 SEM),of 15 film samples were applied on a glass plate.

Figure 2. TheFigure stages 2. ofThe preparation stages of preparation of rigid polyvinyl of rigid alcohol-bacterial polyvinyl alcohol-bacterial cellulose (PVA-BC) cellulose membrane (PVA-BC) onmem- ceramic support: brane on ceramic support: (a) PVA solution, (b) wet ground BC, (c) PVA-BC-glutaraldehyde (GA) (a) PVA solution, (b) wet ground BC, (c) PVA-BC-glutaraldehyde (GA) mixture, (d) membrane applied on support. mixture, (d) membrane applied on support.

2.3. Pervaporation Tests Pervaporation experiments involving these rigid membranes were carried out in a laboratory equipment (Figure 3). The membrane (1b) deposited on the ceramic support (1c) and the glass tube (1a) of the pervaporation unit (1) were heated by an electric re- sistance heater, the heating eliminating the permeate condensation on the glass tube. The pervaporation chamber was connected to the vacuum and to the systems for measur- ing/control the process temperature and pressure as well as the permeate flow rate.

Figure 3. Laboratory equipment for pervaporation studies with rigid membranes deposited on the external surface of a ceramic tube: (1) pervaporation unit, (1a) glass tube, (1b) PVA-BC membrane, (1c) ceramic support, (1d) tube for heating the processed liquid, (1e) ethanol-water mixture, (2) electric cooler, (3) Dewar vessel with crushed ice, (4) steam condenser with central tube, (5) vacuum pump, (6) precision burette for measuring the permeate flow rate, (7) mercury manometer.

As shown in Figure 3, the ethanol-water mixture (1e) was placed in the precision burette (6), which was connected to the liquid pervaporation chamber (1). The liquid mixture inside the pervaporation unit passed through the membrane, where the water was preponderantly removed and the ethanol was concentrated in the pervaporation system. On the external side of the membrane, the water and the associated ethanol were vaporized due to the low pressure on the membrane surface. The vapor phase was con- densed in a cooling system, which consists of two parts: the first part is a glass coil re- frigerator working at a temperature of 5–6 °C, whereas the second part is a cold trap containing ice. The purpose of using this cooling system is to provide as much cold space as possible for the condensation of all permeating vapor. Pervaporation tests were performed at different levels of temperature (40, 50 and 60 °C), pressure (140, 210 and 280 mm Hg, i.e., 18.7, 28.0 and 37.3 kPa) and ethanol concen- trations in the feed solution (24, 48 and 72%wt). The ethanol concentrations in the feed and in the condensed permeate were determined by measuring the refractive index.

Processes 2021, 9, 437 4 of 15

Processes 2021, 9, 437Figure 2. The stages of preparation of rigid polyvinyl alcohol-bacterial cellulose (PVA-BC) mem- 4 of 14 brane on ceramic support: (a) PVA solution, (b) wet ground BC, (c) PVA-BC-glutaraldehyde (GA) mixture, (d) membrane applied on support.

2.3. Pervaporation2.3. Tests Pervaporation Tests Pervaporation Pervaporationexperiments involving experiments these involving rigid membranes these rigid were membranes carried out were in a carried out in a laboratory equipmentlaboratory (Figure equipment 3). The (Figure membrane3). The (1b) membrane deposited (1b) on deposited the ceramic on the support ceramic support (1c) (1c) and the glassand tube the glass(1a) of tube the (1a) pervaporation of the pervaporation unit (1) were unit (1)heated were by heated an electric by an electricre- resistance sistance heater,heater, the heating the heating eliminating eliminating the permeate the permeate condensation condensation on the on glass the tube. glass tube.The The pervapo- pervaporation rationchamber chamber was connected was connected to the to vacuum the vacuum and to and the to systems the systems for measur- for measuring/control ing/control the theprocess process temperature temperature and and pressu pressurere as well as well as the as thepermeate permeate flow flow rate. rate.

Figure 3. LaboratoryFigure 3. Laboratory equipment equipment for pervaporation for pervaporation studies with stud rigidies with membranes rigid membranes deposited deposited on the external on the surface of a ceramic tube:external (1) pervaporation surface of a ceramic unit, (1a tube:) glass (1 tube,) pervaporation (1b) PVA-BC unit, membrane, (1a) glass (1tube,c) ceramic (1b) PVA-BC support, membrane, (1d) tube for heating (1c) ceramic support, (1d) tube for heating the processed liquid, (1e) ethanol-water mixture, (2) the processed liquid, (1e) ethanol-water mixture, (2) electric cooler, (3) Dewar vessel with crushed ice, (4) steam condenser electric cooler, (3) Dewar vessel with crushed ice, (4) steam condenser with central tube, (5) vacuum with central tube, (5) vacuum pump, (6) precision burette for measuring the permeate flow rate, (7) mercury manometer. pump, (6) precision burette for measuring the permeate flow rate, (7) mercury manometer. As shown in Figure3, the ethanol-water mixture (1e) was placed in the precision burette As shown in Figure 3, the ethanol-water mixture (1e) was placed in the precision (6), which was connected to the liquid pervaporation chamber (1). The liquid mixture inside burette (6), whichthe pervaporationwas connected unit to passedthe liqu throughid pervaporation the membrane, chamber where (1). the waterThe liquid was preponderantly mixture inside removedthe pervaporation and the ethanol unit passed was concentrated through the in membrane, the pervaporation where the system. water On the external was preponderantlyside of removed the membrane, and the the ethano waterl was and theconcentrated associated in ethanol the pervaporation were vaporized due to the system. On the lowexternal pressure side of on the the membrane membrane, the surface. water Theand the vapor associated phase was ethanol condensed were in a cooling vaporized due system,to the low which pressure consists on ofthe two membrane parts: the surface. first part The is vapor a glass phase coil refrigerator was con- working at a densed in a coolingtemperature system, of which 5–6 ◦C, consists whereas of the two second parts: part the isfirst a cold part trap is containinga glass coil ice. re- The purpose of

frigerator workingusing at this a temperature cooling system of is5–6 to°C, provide whereas as much the coldsecond space part as is possible a cold for trap the condensation containing ice. ofThe all purpose permeating of using vapor. this cooling system is to provide as much cold space as possible for the condensationPervaporation of all tests permeating were performed vapor. at different levels of temperature (40, 50 and Pervaporation60 ◦ C),tests pressure were performed (140, 210 at and different 280 mm levels Hg, of i.e., temperature 18.7, 28.0 (40, and 50 37.3 and kPa)60 and ethanol °C), pressure (140,concentrations 210 and 280 in mm the Hg, feed i.e., solution 18.7, 28.0 (24, 48and and 37.3 72%wt). kPa) and The ethanol ethanol concen- concentrations in the trations in the feed andsolution in the (24, condensed 48 and 72 permeate%wt). The were ethanol determined concentrations by measuring in the the feed refractive index. and in the condensedCharacteristic permeate were working determined procedure by ofmeasuring a pervaporation the refractive test was index. as follows: (i) the ceramic tube of pervaporation unit was filled with ethanol-water mixture and the working liquid level in the burette was measured; (ii) working temperature and pressure levels were selected and the vacuum pump was started; (iii) the time (∆τ) after which the liquid level in the burette decreased by V = 2 mL was measured; (iv) the burette was refilled and other 3 measurements of ∆τ were performed; (v) the refractive index of the condensate collected in the steam condenser was measured.

2.4. Process Parameters

Total permeate flow rate, NA, was determined using Equation (1), where A is the effective membrane area, ρf the density of feed solution and ∆τ the time after which the liquid level in the burette decreased by V = 2 mL.

Vρ f N = (1) A A∆τ Processes 2021, 9, 437 5 of 14

Ethanol concentrations in the feed and permeate samples were estimated using an Atago Abbe refractometer (Atago, Japan). Based on the experimental data obtained for the refractive index at 25 ◦C and refractive index-concentration curves [31], the mass fractions of ethanol were obtained. Water/ethanol selectivity, αw/et, was calculated using Equation (2), where X and Y represent the mass fractions of species in the feed and permeate.

Yw 1−Yet α = Yet = Yet (2) w/et Xw 1−Xet Xet Xet

3. Results and Discussions Pervaporation data corresponding to 4 experimental runs (measurements of ∆τ) using PVA-BC 12/6 membrane (M1) are summarized in Table1, where cet is the concentration of ethanol in the feed solution, t the temperature, p the pressure, V the volume of feed solution passed through membrane in the time ∆τ, nf and np are the refractive indexes for the feed solution and permeate.

Table 1. Experimental data for PVA-BC 12/6 membrane (M1).

cet t p V ∆τ in Run nf o np (%wt) ( C) (mm Hg) (mL) I II III IV 48 1.3592 50 210 2 66 63 66 72 1.3376 24 1.3485 60 280 2 61 48 45 45 1.3345 24 1.3485 60 140 2 45 35 17 10 1.3335

Characteristic FTIR spectra of PVA-BC 12/6, PVA-BC 12/12 and PVA-BC 12/18 mem- branes used in the pervaporation experiments are shown in Figure4. Due to the fact that the FTIR spectra only analyze the vibration of the molecular groups, it is not expected to have many differences among the spectra of membranes, which differ only in BC content. For a deeper analysis, the FTIR spectra for BC, PVA and PVA cross-linked with GA are provided (Figures5 and6). It is known that characteristic peaks of PVA are revealed at 3263 cm−1 (O-H stretching), −1 −1 −1 2921 cm (C-H stretching), 1413 cm (CH2 bending), 1235 cm (OH plane bending), −1 −1 −1 1085 cm (C-O stretching), 916 cm (CH2 rocking) and 828 cm (C-C stretching). For PVA cross-linked with GA (Figure6), almost all peaks are present (with small displace- ments, due to the structure stiffening). Moreover, with small displacements, all these peaks appear for PVA-BC membranes (Figure4). According to Figures5 and6, BC and PVA cross-linked with GA have 5 peaks almost similar, i.e., those at 3343 and 3267 cm−1 (O-H stretching), 2895 and 2912 cm−1 (C-H stretching), 1314 and 1327 cm−1 (OH bending, alcohol), 1162 and 1141 cm−1 (C-O stretching, ester or ether), 1054 and 1087 cm−1 (C-O stretching, alcohol). Also, a peak at 1629 cm−1 (C-OH absorbed water), can be identified in the FTIR spectrum of BC (Figure5) and similar peaks, but of much lower intensity, appear in characteristic FTIR spectra of PVA-BC membranes (Figure4), i.e., at 1640.0 cm −1 for PVA-BC 12/6 and 1658.7 cm−1 for PVA-BC 12/18. The low intensity of these peaks, probably due to the heat treatment (60−90 min at 90 ◦C) to complete cross-linking, may indicate that these membranes do not trend to retain water and swell. The spectra of composite membranes (M1-M3) shown in Figure4 indicate a slight displacement of characteristic peaks of BC and PVA cross-linked with GA as well as the appearance of new peaks, specific to these membranes. The results plotted in Figure4 highlight 7 peaks for PVA-BC 12/6 membrane (M1), 9 peaks for PVA-BC 12/12 membrane (M2) and 11 peaks for PVA-BC 12/18 membrane (M3). Accordingly, new peaks appear as the concentration of BC in the membranes increases. This suggests that each PVA-BC membrane has its own structure, which depends on BC content. It is expected that the Processes 2021, 9, 437 6 of 15

Processes 2021, 9, 437 6 of 15

According to Figures 5 and 6, BC and PVA cross-linked with GA have 5 peaks al- most similar, i.e., those at 3343 and 3267 cm−1 (O-H stretching), 2895 and 2912 cm−1 (C-H stretching), 1314 andAccording 1327 cm to−1 Figures(OH bending, 5 and 6, alcohol), BC and PVA1162 cross-linkedand 1141 cm with−1 (C-O GA stretch-have 5 peaks al- ing, ester or ether),most similar, 1054 and i.e., 1087 those cm at− 13343 (C-O and stretching, 3267 cm −alcohol).1 (O-H stretching), Also, a peak 2895 at and 1629 2912 cm -1cm −1 (C-H −1 −1 (C-OH absorbedstretching), water), 1314 can and be identified1327 cm (OHin the bending, FTIR spectrumalcohol), 1162 of BC and (Figure 1141 cm 5) (C-Oand stretch- −1 -1 similar peaks,ing, but ester of ormuch ether), lower 1054 andintensity, 1087 cm appear (C-O stretching,in characteristic alcohol). FTIR Also, spectra a peak atof 1629 cm (C-OH absorbed water), can be identified in the FTIR spectrum of BC (Figure 5) and PVA-BC membranes (Figure 4), i.e., at 1640.0 cm−1 for PVA-BC 12/6 and 1658.7 cm−1 for similar peaks, but of much lower intensity, appear in characteristic FTIR spectra of PVA-BC 12/18. The low intensity of these peaks, probably due to the heat treatment PVA-BC membranes (Figure 4), i.e., at 1640.0 cm−1 for PVA-BC 12/6 and 1658.7 cm−1 for (60−90 min at 90 °C) to complete cross-linking, may indicate that these membranes do not PVA-BC 12/18. The low intensity of these peaks, probably due to the heat treatment trend to retain water and swell. (60−90 min at 90 °C) to complete cross-linking, may indicate that these membranes do not The spectratrend ofto compositeretain water membranes and swell. (M1-M3) shown in Figure 4 indicate a slight displacement of characteristicThe spectra ofpeaks composite of BC membranesand PVA cross-linked (M1-M3) shown with inGA Figure as well 4 indicate as the a slight appearance ofdisplacement new peaks, of specific characteristic to these peaks membranes. of BC and The PVA results cross-linked plotted with in FigureGA as well4 as the highlight 7 peaksappearance for PVA-BC of new 12/6 peaks, membrane specific (M1),to these 9 peaks membranes. for PVA-BC The results12/12 membrane plotted in Figure 4 Processes 2021, 9, 437 (M2) and 11 peakshighlight for 7PVA-BC peaks for 12/18 PVA-BC membrane 12/6 membrane (M3). Accordingly, (M1), 9 peaks new for peaksPVA-BC appear 12/12 asmembrane 6 of 14 the concentration(M2) andof BC 11 peaksin the for membranes PVA-BC 12/18 increases. membrane This (M3). suggests Accordingly, that each new PVA-BC peaks appear as membrane hasthe its concentration own structure, of BC which in the depends membranes on BC increases. content. This It is suggests expected that that each the PVA-BC behavior of themembrane membranes has itsin ownthe pervaporation structure, which process depends will on depend BC content. on the It BCis expected content that the behavior of the membranes in the pervaporation process will depend on the BC content in in the membrane,behavior which of thewill membranes be considered in the as pervaporationa process factor process in the will future. depend on the BC content the membrane, which will be considered as a process factor in the future. in the membrane, which will be considered as a process factor in the future.

Figure 4. FTIRFigureFigure spectra 4. 4. FTIR forFTIR PVA-BC spectraspectra 12/6 forfor PVA-BC (top), PVA-BC 12/6 ( (toptop 12/12),), PVA-BC PVA-BC (middle 12/12 12/12) and (middle ( middlePVA-BC) and) and12/18 PVA-BC PVA-BC (bellow 12/18 12/18) (bellow (bellow) ) composite membranescompositecomposite after membranesmembranes preparing after and preparing drying at and and 90 drying°C. drying at at 90 90 °C.◦C.

Processes 2021, 9, 437 7 of 15

Figure 5. FTIR spectrum of BC used for composite membranes synthesis. Figure 5. FTIR spectrum of BC used for composite membranes synthesis.

Figure 6. FTIR spectra for PVA ((toptop)) andand PVAPVA cross-linkedcross-linked withwith GA GA ( bellow(bellow).).

Optic microscope images of BC fibrils in PVA solutions are shown in Figure 7. We observed the BC distribution as micro agglomerates with concentrations consistent with BC content in the mixture. SEM images presented in Figure 8 show composites membrane in which the com- pact PVA film is furrowed by BC fibers having diameters less than 2 μm. It appears that the density of fibrils in the films is proportional to the BC content of the disperse system used to prepare the membranes.

Figure 7. Optic microscope images (×10) of PVA-BC 12/6 (1), PVA-BC 12/12 (2) and PVA-BC 12/18 (3) mixtures before the start of membrane cross-linking.

Processes 2021, 9, 437 7 of 15

Figure 5. FTIR spectrum of BC used for composite membranes synthesis.

Processes 2021, 9, 437 7 of 14

Figure Optic6. FTIR microscopespectra for PVA images (top) and of PVA BC cross-linked fibrils inPVA with GA solutions (bellow). are shown in Figure7. We observed the BC distribution as micro agglomerates with concentrations consistent with Optic microscope images of BC fibrils in PVA solutions are shown in Figure 7. We observedBC content the inBC the distribution mixture. as micro agglomerates with concentrations consistent with BC contentSEM in images the mixture. presented in Figure8 show composites membrane in which the compact PVASEM film images is furrowed presented by in BC Figure fibers 8 havingshow composites diameters membrane less than in 2 whichµm. It the appears com- that the pactdensity PVA offilm fibrils is furrowed in the films by BC is fibers proportional having diameters to the BC less content than 2 ofμm. the It disperseappears that system used theto preparedensity of the fibrils membranes. in the films is proportional to the BC content of the disperse system used to prepare the membranes.

FigureFigure 7. 7.OpticOptic microscope microscope images images (×10) (× of10) PVA-BC of PVA-BC 12/6 (1), 12/6 PVA-BC (1), PVA-BC 12/12 (2) 12/12 and PVA-BC (2) and PVA-BC 12/18 Processes 2021, 9, 437 8 of 15 12/18(3) mixtures (3) mixtures before before the the start start of of membrane membrane cross-linking.cross-linking.

Figure 8. SEM images of PVA-BC 12/6 (1), PVA-BC 12/12 (2) and PVA-BC 12/18 (3) mem- Figure 8. 1 2 3 branes. SEM images of PVA-BC 12/6 ( ), PVA-BC 12/12 ( ) and PVA-BC 12/18 ( ) membranes.

Pervaporation tests for each membrane used in the experimental study were orga- nized according to a factorial plan with 2 levels for each factor, i.e., pervaporation tem- perature (t), pervaporation pressure (p) and ethanol concentration in the feed solution (cet). Process response variables in terms of permeate flow rate and membrane selectivity depending on process factors for PVA-BC 12/12 membrane (M2) are summarized in Ta- ble 2. The dimensionless factors (x1, x2 and x3) were calculated using Equations (3)–(5), where the c subscript indicates the center of experimental plan. Characteristic experi- mental data matrices of 23 factorial experiments for PVA-BC 12/6 (M1), PVA-BC 12/12 (M2) and PVA-BC 12/18 (M3) membranes, containing the values of process responses at various levels of dimensionless factors, are given in Tables S1–S3. Levels of process re- sponse variables for all pervaporation measurements, i.e., 0.09–0.23 kg/m2/h for total permeate flow rate and 5–23 for water/ethanol selectivity, are summarized in Figure 9. These levels are in the same ranges as those characterizing the cellulose-silica membranes [21,32] or some special cellulose membranes [33,34]. The results presented in Figure 9 highlight the following issues: (i) pervaporation flow rate increased with an increase in t and p as well as it de- creased with an increase in cet for all M1–M3 membranes, the effect of cet being higher [for cet = 24%wt (runs 1-4), the values of pervaporation flow rate (0.14-0.23 kg/m2/h) were al- most double than those corresponding to cet = 72%wt (runs 5-8), i.e., 0.09-0.11 kg/m2/h]; in all experimental runs 1-4 (cet = 24%wt), the pervaporation flow rate was higher for M1 membrane (having the lowest content of BC) and lower for M3 membrane (having the highest content of BC), whereas in all experimental runs 5-8 (cet = 72%wt), it was almost constant for M1–M3 membranes; (ii) for M3 membrane, water/ethanol selectivity increased with an increase in t, p and cet;

Processes 2021, 9, 437 8 of 14

Pervaporation tests for each membrane used in the experimental study were organized according to a factorial plan with 2 levels for each factor, i.e., pervaporation temperature (t), pervaporation pressure (p) and ethanol concentration in the feed solution (cet). Process response variables in terms of permeate flow rate and membrane selectivity depending on process factors for PVA-BC 12/12 membrane (M2) are summarized in Table2. The dimensionless factors (x1, x2 and x3) were calculated using Equations (3)–(5), where the c subscript indicates the center of experimental plan. Characteristic experimental data matrices of 23 factorial experiments for PVA-BC 12/6 (M1), PVA-BC 12/12 (M2) and PVA- BC 12/18 (M3) membranes, containing the values of process responses at various levels of dimensionless factors, are given in Tables S1–S3. Levels of process response variables for all pervaporation measurements, i.e., 0.09–0.23 kg/m2/h for total permeate flow rate and 5–23 for water/ethanol selectivity, are summarized in Figure9. These levels are in the same ranges as those characterizing the cellulose-silica membranes [21,32] or some special cellulose membranes [33,34]. The results presented in Figure9 highlight the following issues: (i) pervaporation flow rate increased with an increase in t and p as well as it de- creased with an increase in cet for all M1–M3 membranes, the effect of cet being higher 2 [for cet = 24%wt (runs 1-4), the values of pervaporation flow rate (0.14–0.23 kg/m /h) were 2 almost double than those corresponding to cet = 72%wt (runs 5-8), i.e., 0.09–0.11 kg/m /h]; in all experimental runs 1-4 (cet = 24%wt), the pervaporation flow rate was higher for M1 membrane (having the lowest content of BC) and lower for M3 membrane (having the highest content of BC), whereas in all experimental runs 5-8 (cet = 72%wt), it was almost constant for M1–M3 membranes; (ii) for M3 membrane, water/ethanol selectivity increased with an increase in t, p and cet; (iii) for M1 and M2 membranes, the values of selectivity in runs 3 and 4 (cet = 24%wt and p = 37.3 kPa), i.e., 4.5–9.7, were lower than those in runs 7 and 8 (cet = 72%wt and p = 37.3 kPa), i.e., 14.1–18.4; characteristic selectivities of M1 in runs 3 and 4 were almost double than those of M2, whereas selectivities of M1 in runs 7 and 8 were slightly lower (up to 5%) than those of M2; except for runs 5 and 6 (cet = 72%wt and p = 18.7 kPa), water/ethanol selectivities increased with an increase in t for M1 and M2 membranes; (iv) M1-M3 membranes had similar selectivities in runs 1 and 2 (cet = 24%wt and p = 18.7 kPa), i.e., 7.5–9.5, whereas in runs 3-8 the selectivities of M3 (9.5–22.9) were higher than those of M1 (6.8–17.9) and M2 (4.5–18.42).

Table 2. Process factors and responses for PVA-BC 12/12 membrane (M2).

t p c N c No. x x et x A Etp α (◦C) 1 (kPa) 2 (%wt) 3 (kg/m2/h) (%wt) w/et 1 40 −1 18.7 −1 24 −1 0.182 3.18 8.6 2 60 1 18.7 −1 24 −1 0.203 2.92 9.5 3 40 −1 37.3 1 24 −1 0.195 5.24 4.7 4 60 1 37.3 1 24 −1 0.218 4.92 5.1 5 40 −1 18.7 −1 72 1 0.089 20.63 8.9 6 60 1 18.7 −1 72 1 0.097 24.71 6.8 7 40 −1 37.3 1 72 1 0.092 13.95 14.8 8 60 1 37.3 1 72 1 0.112 11.50 18.8 9 50 0 28 0 48 0 0.151 8.30 9.2 10 50 0 28 0 48 0 0.148 8.05 9.6 11 50 0 28 0 48 0 0.147 7.81 9.9

t − t x = c , t = 50 ◦C , ∆t = 10 ◦C (3) 1 ∆t c p − p x = c , p = 28 kPa , ∆p = 9.3 kPa (4) 2 ∆p c Processes 2021, 9, 437 9 of 15

(iii) for M1 and M2 membranes, the values of selectivity in runs 3 and 4 (cet = 24%wt and p = 37.3 kPa), i.e., 4.5–9.7, were lower than those in runs 7 and 8 (cet = 72%wt and p = 37.3 kPa), i.e., 14.1–18.4; characteristic selectivities of M1 in runs 3 and 4 were almost double than those of M2, whereas selectivities of M1 in runs 7 and 8 were slightly lower (up to 5%) than those of M2; except for runs 5 and 6 (cet = 72%wt and p = 18.7 kPa), wa- ter/ethanol selectivities increased with an increase in t for M1 and M2 membranes; (iv) M1-M3 membranes had similar selectivities in runs 1 and 2 (cet = 24%wt and p = 18.7 kPa), i.e., 7.5–9.5, whereas in runs 3-8 the selectivities of M3 (9.5–22.9) were higher than those of M1 (6.8–17.9) and M2 (4.5–18.42).

Table 2. Process factors and responses for PVA-BC 12/12 membrane (M2).

t p cet NA cEtp No. x1 x2 x3 αw/et (°C) (kPa) (%wt) (kg/m2/h) (%wt) 1 40 −1 18.7 −1 24 −1 0.182 3.18 8.6 2 60 1 18.7 −1 24 −1 0.203 2.92 9.5 3 40 −1 37.3 1 24 −1 0.195 5.24 4.7 4 60 1 37.3 1 24 −1 0.218 4.92 5.1 5 40 −1 18.7 −1 72 1 0.089 20.63 8.9 6 60 1 18.7 −1 72 1 0.097 24.71 6.8 7 40 −1 37.3 1 72 1 0.092 13.95 14.8 8 60 1 37.3 1 72 1 0.112 11.50 18.8 9 50 0 28 0 48 0 0.151 8.30 9.2 10 50 0 28 0 48 0 0.148 8.05 9.6 11 50 0 28 0 48 0 0.147 7.81 9.9

t − t x = c , t = 50º C, Δt =10º C (3) 1 Δt c

− p pc Processes 2021, 9, 437 x = , p = 28 kPa , Δp = 9.3 kPa (4) 9 of 14 2 Δp c

c − c x = et et,c , c = 48%wtcet −, cet,cΔc = 24%wt 3 Δ et,cx3 = , et cet,c = 48%wt , ∆cet = 24%wt (5) (5) cet ∆cet

M1 0.25 M2 0.2 M3

0.15 N A 2 (kg/m /h) 0.1

0.05 Processes 2021, 9, 437 10 of 15

0 12345678

M1 25 M2 20 M3

15 α w/et 10

5

0 12345678

Figure 9. Permeate flow rate and membrane selectivity for pervaporation through PVA-BC membranes (M1: PVA-BC 12/6, M2: PVA-BCFigure 12/12, M3:9. Permeate PVA-BC flow 12/18). rate and membrane selectivity for pervaporation through PVA-BC mem- branes (M1: PVA-BC 12/6, M2: PVA-BC 12/12, M3: PVA-BC 12/18). Data presented in Figure9 were used to quantify the effect of pervaporation factors on Data presentedresponse in variables. Figure 9 Experimental were used to data quantify for each the membrane effect of pervaporation (Mk, k = 1..3) were factors processed as fol- lows: (i) characteristic regression analysis of a 23 experimental plan was used to obtain values on response variables. Experimental data for each membrane (Mk, k = 1..3) were processed of significant regression coefficients in dependences N (x ,x ,x ) and α (x ,x ,x ) given as follows: (i) characteristic regression analysis of a 23 experimentalAk 1 plan2 3 was usedw/etk to1 2 3 by Equations (6) and (7) [35]; (ii) significant regression coefficients in Equations (6) and (7), obtain values of significant regression coefficients in dependences NAk(x1,x2,x3) and βik and αjk, were processed resulting in 2 polynomial regression models, βi(x4) = b0i + b1ix4 αw/etk(x1,x2,x3) given by Equations (6) and (7) [35]; (ii) significant regression coefficients in + b x 2 and α (x ) = a + a x + a x 2, where x represents BC concentration in the dried Equations (6) and2i 4(7), βik andj 4 αjk, 0werej 1 jprocessed4 2j 4 resulting4 in 2 polynomial regression membrane; analytical2 expressions NA(x1,x2,x23,x4) and αw/et(x1,x2,x3,x4) were determined models, βi(x4) = b0i + b1ix4 + b2ix4 and αj(x4) = a0j + a1jx4 + a2jx4 , where x4 represents BC con- based on these polynomial regression models. centration in the dried membrane; analytical expressions NA(x1,x2,x3,x4) and αw/et(x1,x2,x3,x4) NAk(werex1, x2 determined, x3) = β0k based+ β1k xon1 + theseβ2kx polynomial2 + β3kx3 + regressionβ12kx1x2 + models.β13kx1x3 + β23kx2x3 + β123kx1x2x3 (6) ()= β + β + β + β + β + β + β + β N Ak x1, x2 , x3 αw0k/etk (x1k1x,1x2, x32k)x2= α03kk x+3 α1k12xk1x+1xα2 2kx213+k xα1x3k3x3 +23αk12x2kxx31x2 +123αk13x1kxx21xx33 + α23kx2x3 + α123kx1x(6)2x 3 (7) Tables S1−S3 contain values of significant regression coefficients in Equations (6) and (7) α ()= α + α +α +α +α +α +α +α w / etk x1, x2 , x3 0k 1k x1 2k xfor2 the3k x membranes3 12k x1x2 used13k x in1x the3 pervaporation23k x2 x3 123k x experiments.1x2 x3 The significance(7) of regression coefficients was assessed based on 3 center-point runs performed for each membrane (Table2 and Table S1–S3) according to the procedure detailed in our previous papers [36,37]. Tables S1−S3 contain values of significant regression coefficients in Equations (6) Taking into account these significant coefficients, Equations (6) and (7), which express the and (7) for the membranes used in the pervaporation experiments. The significance of dependences N (x ,x ,x ) and α (x ,x ,x ), turn into Equations (8) and (9) for PVA- regression coefficients was assessedAk 1 2 based3 onw/etk 3 center-point1 2 3 runs performed for each BC 12/6 membrane (M1), Equations (10) and (11) for PVA-BC 12/12 membrane (M2), membrane (Tables 2 and S1–S3) according to the procedure detailed in our previous pa- Equations (12) and (13) for PVA-BC 12/18 membrane (M3). Data presented in Tables S1–S3 pers [36,37]. Taking into account these significant coefficients, Equations (6) and (7), indicate that the experimental values of process performances and those calculated using which express the dependences NAk(x1,x2,x3) and αw/etk(x1,x2,x3), turn into Equations (8) and Equations (8)–(13) are in a good agreement. (9) for PVA-BC 12/6 membrane (M1), Equations (10) and (11) for PVA-BC 12/12 mem- brane (M2), Equations (12) and (13) for PVA-BC 12/18 membrane (M3). Data presented in NA1(x1, x2, x3) = 0.151 + 0.008x1 − 0.057x3 (8) Tables S1–S3 indicate that the experimental values of process performances and those calculated using Equationsαw (8)–(13)/et1(x1, xare2, x in3) a= good10.3 agreement.+ 2.1x2 + 1.55 x3 + 0.825x1x2 + 2.05x2x3 (9) ()= + − N A1 x1, x2 , x3 0.151 0.008x1 0.057x3 (8)

α ()= + + + + w / et1 x1, x2 , x3 10.3 2.1x2 1.55x3 0.825x1x2 2.05x2 x3 (9)

()= + + − N A2 x1, x2 , x3 0.149 0.009x1 0.0057x2 0.051x3 (10)

α ()= + + + + + w / et2 x1, x2 , x3 9.58 1.1x2 2.65x3 0.68x1x2 3.25x2 x3 0.75x1x2 x3 (11)

()= + − − N A3 x1, x2 , x3 0.127 0.012x1 0.034x3 0.0062x1x3 (12)

α ()= + + + + w / et3 x1, x2 , x3 12.4 1.225x1 2.475x2 3.275x3 1.55x2 x3 (13)

Processes 2021, 9, 437 10 of 14

NA2(x1, x2, x3) = 0.149 + 0.009x1 + 0.0057x2 − 0.051x3 (10)

αw/et2(x1, x2, x3) = 9.58 + 1.1x2 + 2.65x3 + 0.68x1x2 + 3.25x2x3 + 0.75x1x2x3 (11)

NA3(x1, x2, x3) = 0.127 + 0.012x1 − 0.034x3 − 0.0062x1x3 (12)

αw/et3(x1, x2, x3) = 12.4 + 1.225x1 + 2.475x2 + 3.275x3 + 1.55x2x3 (13) Equations (8)–(13) highlight the following issues: (i) NA1, NA2 and NA3, which are expressed by Equations (8), (10) and (12), depend linearly on temperature (x1) and ethanol concentration in the feed mixture (x3) and x3 has a higher effect; an increase in x1 and a decrease in x3 lead to an increase in NAk (k = 1..3); for PVA-BC 12/12 membrane (M2), there is a low linear influence of the pressure from the per- vaporation unit [the term + 0.0057x2 in Equation (10)]; for PVA-BC 12/18 membrane (M3), the interaction between temperature and composition of processed mixture diminishes the permeate flow rate [the term − 0.0062x1x3 in Equation (12)]; (ii) αw/et1, αw/et2 and αw/et3, which are expressed by Equations (9), (11) and (13), depend on process factors and their interactions; all regression coefficients are positive, conse- quently, an increase in x1, x2 and x3 leads to an increase in αw/etk (k = 1..3). These findings are similar to those obtained for pervaporation of ethanol-water sys- tem or isopropanol-water mixture through BC [29], BC-PVA [30] or cellulose based mem- branes [38]. The regression coefficients in Equations (8)–(13) depend on the membrane type, which is related to BC concentration in the dried membrane (cBC = 2.4–7.2%wt). The values of regression coefficients in Equations (8), (10) and (12), βik (i = 0, 1, 2, 3, 13, k = 1..3), and Equations (9), (11) and (13), αjk (j = 0, 1, 2, 3, 12, 23, 123, k = 1..3), are summarized in Tables3 and4 , where dimensionless BC concentration (x4) is defined by Equation (14). 2 2 Polynomial regression models, i.e., βi(x4) = b0i + b1ix4 + b2ix4 and αj(x4) = a0j + a1jx4 + a2jx4 , obtained by processing the values of βik and αjk (k = 1..3) are also given in Tables3 and4 . Taking into account these polynomial regression models, Equations (8), (10) and (12) turn into Equation (15) and Equations (9), (11) and (13) into Equation (16), which express the permeate flow rate (NA) and membrane efficiency (αw/et) depending on 4 process factors, i.e., temperature (x1), pressure (x2), ethanol concentration in the feed solution (x3) and BC concentration in the dried membrane (x4).

cBC − cBC,c x4 = , cBC,c = 4.8%wt , ∆cBC = 2.4%wt (14) ∆cBC

Table 3. Regression coefficients βik (i = 0, 1, 2, 3, 13, k = 1..3) depending on BC content in the membrane.

Membrane Type Polynomial Regression Model Parameter 2 PVA-BC 12/6 PVA-BC PVA-BC βi(x4) = b0i + b1ix4 + b2ix4 (k = 1) 12/12 (k = 2) 12/18 (k = 3)

cBC (%wt) 2.4 4.8 7.2 -

x4 –1 0 1 - 2 β0k 0.151 0.149 0.127 0.149 – 0.012x4 – 0.014x4 2 β1k 0.00825 0.009 0.012 0.009 + 0.056x4 + 0.055x4 2 β2k 0 0.00575 0 0.00575 – 0.00575x4 2 β3k –0.057 –0.051 −0.034 –0.051 + 0.012x4 + 0.0055x4 2 β13k 0 0 0.00625 0.0031x4 + 0.0031x4 Processes 2021, 9, 437 11 of 14

Table 4. Regression coefficients αjk (j = 0, 1, 2, 3, 12, 23, 123, k = 1..3) depending on BC content in the membrane.

Membrane type Polynomial Regression Model Parameter PVA-BC 12/6 2 PVA-BC 12/12 (k = 2) PVA-BC 12/18 (k = 3) αj(x4) = a0j + a1jx4 + a2jx4 (k = 1)

cBC (%wt) 2.4 4.8 7.2 -

x4 −1 0 1 - 2 α0k 10.3 9.58 12.4 9.58 + 1.05x4 + 1.77x4 2 α1k 0 0 1.225 0.613x4 + 0.613x4 2 α2k 2.1 1.27 2.475 1.127 + 0.188x4 + 1.161x4 2 α3k 1.55 2.65 3.275 2.65 + 0.862x4 − 0.238x4 2 α12k 0.825 0.68 0.05 0.68 − 0.388x4 − 0.243x4 2 α23k 2.05 3.25 1.55 3.25 − 0.25x4 − 1.45x4 2 α123k 0 0.75 0 0.75 − 0.75x4

2 2 NA(x1, x2, x3, x4) = 0.149 − 0.012x4 − 0.014x4 + 0.009 + 0.056x4 + 0.055x4 x1+ 2 2 2 (15) 0.00575 − 0.00575x4 x2 + −0.051 + 0.012x4 + 0.0055x4 x3 + 0.0031x4 + 0.0031x4 x1x3

2 2 αw/et(x1, x2, x3, x4) = 9.58 + 1.05x4 + 1.77x4 + 0.613x4 + 0.613x4 x1+ 2 2 1.127 + 0.188x4 + 1.161x4 x2 + 2.65 + 0.862x4 − 0.238x4 x3+ (16) 2 2 2 0.68 − 0.388x4 − 0.243x4 x1x2 + 3.25 − 0.25x4 − 1.45x4 x2x3 + 0.75 − 0.75x4 x1x2x3 In order to validate the statistical models described by Equations (15) and (16), 5 ex- ◦ perimental runs were performed at x1 = x2 = x3 = x4 = 0.5, i.e., t = 55 C, p = 32.7 kPa, cet = 60%wt and cBC = 6%wt (PVA-BC 12/15 membrane). Experimental values of pro- cess performances for these replicates (n = 5), NA,ex and αw/et,ex, and those predicted by Equations (15) and (16), NA,pr and αw/et,pr, are summarized in Table5. A test statistic for one sample T-test, denoted T and defined by Equation (17), was calculated for each process performances (P) to assess whether the difference between the mean of experimental values and predicted value (Pex.mn − Ppr) is statistically significant. Values of Pex.mn, standard deviation (SD), standard error of the mean (SE), coefficient of variance (CV = 100SD/Pex.mn), degrees of freedom (df = n − 1) and T are specified in Table5. The difference Pex.mn − Ppr is statistically non-significant for each process performance [the values of T (1.147 and 1.236) are lower than its critical value for two tails at a significance level of 0.05 and df = 4, i.e., Tcr(0.05,4) = 2.776], which demonstrates the model validity. Accordingly, both Equations (15) and (16) can be used to design and simulate pervaporation units equipped with supported PVA-BC membranes, at levels of process factors in the ranges selected in the experimental study.

Pex,mn − Ppr Pex,mn − Ppr T = = (17) √SD SE n Processes 2021, 9, 437 12 of 14

Table 5. Experimental and calculated data for replicates used to validate the statistical models described by Equations (15) and (16).

N (kg/m2/h) α t p c c A w/et Run et BC (◦C) (kPa) (%wt) (%wt) Experimental Predicted (pr) by Experimental Predicted (pr) by (ex) Equation (15) (ex) Equation (16) 1 0.153 13.9 2 0.139 15.1 355 32.7 60 6 0.1460.1458 14.9 13.911 4 0.161 13.4 5 0.151 14.2 2 Pex,mn 0.150 kg/m /h 14.3 - SD 0.0082 kg/m2/h 0.7036 - SE 0.0037 kg/m2/h 0.3146 - CV 5.46 % 4.92 % df 4 4 T 1.147 1.236

4. Conclusions The composite PVA-BC membranes deposited on ceramic supports can be very ef- fective to remove water from solvents. Preparation, characterization and testing of rigid PVA-BC membranes applied on a ceramic support are presented in this paper. Three membranes types, i.e., M1, M2 and M3, with different concentration of BC in the dried membrane (2.4, 4.8 and 7.2%wt) were prepared and then tested in pervaporation exper- iments. An experimental equipment at a laboratory pilot scale was used to study the pervaporation of ethanol-water mixtures through supported PVA-BC membranes. A two-level experimental plan based on 3 factors, i.e., temperature (x1), permeation pressure (x2) and ethanol concentration in the feed solution (x3), was used in the experi- mental investigation of the pervaporation behavior of each membrane type. Regression equations linking these 3 factors to process performances in terms of permeate flow rate (NAk) and membrane selectivity (αw/etk) were obtained according to characteristic procedure 3 of a 2 factorial experiment for each membrane type (Mk, k = 1..3). Regression coefficients in these equations, βik (i = 0, 1, 2, 3, 13) and αjk (j = 0, 1, 2, 3, 12, 23, 123), were processed 2 resulting in 2 polynomial regression models, βi(x4) = b0i + b1ix4 + b2ix4 and αj(x4) = a0j + 2 a1jx4 + a2jx4 , where x4 represents BC concentration in the dried membrane. Analytical expressions NA(x1,x2,x3,x4) and αw/et(x1,x2,x3,x4) were determined based on these poly- nomial regression models. These relationships can be used to predict the pervaporation performances at levels of process factors in the ranges selected in the experimental study.

Supplementary Materials: The following are available online at https://www.mdpi.com/2227-971 7/9/3/437/s1, Table S1: Data and results of regression analysis (23 experimental plan) for ethanol- water pervaporation through PVA-BC 12/6 membrane (M1), Table S2: Data and results of regression analysis (23 experimental plan) for ethanol-water pervaporation through PVA-BC 12/12 membrane (M2), Table S3: Data and results of regression analysis (23 experimental plan) for ethanol-water pervaporation through PVA-BC 12/18 membrane (M3). Author Contributions: Methodology, investigation, formal analysis, writing-original draft prepara- tion, T.D., C.A.M.P., O.C.P. and A.A.A.A.; writing-review and editing, T.D., O.C.P. and C.A.M.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are contained within the article. Processes 2021, 9, 437 13 of 14

Conflicts of Interest: The authors declare no conflict of interest.

References 1. Smitha, B.; Suhanya, D.; Sridhar, S.; Ramakrishna, M. Separation of organic-organic mixtures by pervaporation—A Review. J. Membr. Sci. 2004, 241, 1–21. [CrossRef] 2. Shao, P.; Huang, R.Y.M. Polymeric membrane pervaporation. J. Membr. Sci. 2007, 287, 162–179. [CrossRef] 3. Chapman, P.D.; Oliveira, T.; Livingston, A.G.; Li, K. Membranes for the dehydration of solvents by pervaporation. J. Membr. Sci. 2008, 318, 5–37. [CrossRef] 4. Leland, M.V. Review: Membrane materials for the removal of water from industrial solvents by pervaporation and vapour permeation. J. Chem. Technol. Biotechnol. 2019, 94, 343–365. [CrossRef] 5. Strathmann, H. Membrane separation processes: Current relevance and future opportunities. AIChE J. 2001, 47, 1077–1087. [CrossRef] 6. Malekpour, A.; Mostajeran, B.; Koohmareh, G.A. Pervaporation dehydration of binary and ternary mixture acetone, isopropanol and water using polyvinyl alcohol/zeolite membranes. Chem. Eng. Process 2017, 118, 47–53. [CrossRef] 7. Ruckenstein, E.; Liang, L. Pervaporation of ethanol-water mixtures through polyvinyl alcohol-polyacrylamide interpenetrating polymer network membranes unsupported and supported on polyethersulfone ultrafiltration membranes: A comparison. J. Membr. Sci. 1996, 110, 99–107. [CrossRef] 8. Win, N.N.; Bosch, P.; Friedl, A. Separation of ethanol-water mixture by pervaporation with organic composite membrane: Modelling of separation performance using model parameters derived from experimental data. ASEAN Eng. J. Part B 2013, 2, 28–36. [CrossRef] 9. Dmitrenko, M.; Penkova, A.; Kuzminova, A.; Missyul, A.; Ermakov, S.; Roizard, D. Development and characterization of new pervaporation PVA membranes for the dehydration using bulk and surface modifications. Polymers 2018, 10, 571. [CrossRef] [PubMed] 10. Hieu, N.H.; Duy, N.N.P. Fabrication, characterization, and pervaporation performance of graphene/poly(vinyl alcohol) nanocom- posite membranes for ethanol dehydration. Chem. Eng. Trans. 2017, 56, 1693–1698. [CrossRef] 11. Huang, Z.; Shi, Y.; Wen, R.; Guo, Y.H.; Su, J.F.; Matsuura, T. Multilayer poly(vinyl alcohol)–zeolite 4 a composite membranes for ethanol dehydration by means of pervaporation. Sep. Purif. Technol. 2006, 51, 126–136. [CrossRef] 12. Figoli, A.; Santoro, S.; Galiano, F.; Basile, A. Pervaporation membranes: Preparation, characterization, and application. In Pervaporation, Vapour Permeation and Membrane ; Basile, A., Figoli, A., Khayet, M., Eds.; Woodhead Publishing: Oxford, UK, 2015; pp. 19–63. 13. Praptowidodo, V. Influence of swelling on water transport through PVA-based membrane. J. Mol. Struct. 2005, 739, 207–212. [CrossRef] 14. Xu, S.; Shen, L.; Li, C.; Wang, Y. Properties and pervaporation performance of poly(vinyl alcohol) membranes crosslinked with various dianhydrides. J. Appl. Polym. Sci. 2018, 135, 46–159. [CrossRef] 15. Rao, K.S.V.K.; Subha, M.C.S.; Sairam, M.; Mallikarjuna, N.N.; Aminabhavi, T.M. Blend membranes of chitosan and poly(vinyl alcohol) in pervaporation dehydration of isopropanol and tetrahydrofuran. J. Appl. Polym. Sci. 2007, 103, 1918–1926. [CrossRef] 16. Cai, W.; Cheng, X.; Chen, X.; Li, J.; Pei, J. Poly(vinyl alcohol)-Modified membranes by Ti3C2Tx for ethanol dehydration via pervaporation. ACS Omega 2020, 5, 6277–6287. [CrossRef][PubMed] 17. Huang, Y.; Zheng, Y.; Song, W.; Ma, Y.; Wu, J.; Fan, L. Poly(vinyl pyrrolidone) wrapped multi-walled carbon nanotube/poly(vinyl alcohol) composite hydrogels. Compos. Part A Appl. Sci. 2011, 42, 1398–1405. [CrossRef] 18. Dobre, T.; Stoica, A.; Pârvulescu, O.C.; Stroescu, M.; Iavorschi, G. Factors influence on bacterial cellulose growth in static reactors. Rev. Chim. 2008, 59, 591–594. [CrossRef] 19. Stoica-Guzun, A.; Stroescu, M.; Jinga, S.; Jipa, I.; Dobre, T.; Dobre, L. Ultrasound influence upon calcium carbonate precipitation on bacterial cellulose membranes. Ultrason. Sonochem. 2012, 19, 909–915. [CrossRef] 20. Stroescu, M.; Stoica-Guzun, A.; Jinga, S.I.; Dobre, T.; Jipa, I.M.; Dobre, L.M. Influence of sodium dodecyl sulfate and cetyl trimethy- lammonium bromide upon calcium carbonate precipitation on bacterial cellulose. Korean J. Chem. Eng. 2012, 29, 1216–1223. [CrossRef] 21. Aljanabi, A.A.A.; Pârvulescu, O.C.; Dobre, T.; Ion, V.A. Pervaporation of aqueous ethanol solutions through pure and composite cellulose membranes. Rev. Chim. 2016, 67, 150–156. 22. Pandey, L.K.; Saxena, C.; Dubey, V. Studies on pervaporative characteristics of bacterial cellulose membrane. Sep. Purif. Technol. 2005, 42, 213–218. [CrossRef] 23. Dubey, V.; Pandey, L.K.; Saxena, C. Pervaporative separation of ethanol/water using a novel chitosan-impregnated bacterial cellulose membrane and chitosan–poly (vinyl alcohol) blends. J. Membr. Sci. 2005, 251, 131–136. [CrossRef] 24. Suratago, T.; Panitchakarn, P.; Kerdlarpphon, P.; Rungpeerapong, N.; Burapatana, V.; Phisalaphong, M. Bacterial cellulose-alginate membrane for dehydration of biodiesel-methanol mixtures. Eng. J. 2016, 20, 144–153. [CrossRef] 25. Gea, S.; Bilotti, E.; Reynolds, C.T.; Soykeabkeaw, N.; Peijs, T. BC/PVA nanocomposite prepared by in-stiu process. Mater. Lett. 2010, 64, 901–904. [CrossRef] 26. Tan, J.; Zheng, Y.; Peng, J.; Wu, J.; Gao, S.; Tian, R.; Chen, H.Y. Preparation and mechanical properties of layered BC nano-cellulose membrane/PVA composite hydrogels. Acta Polym. Sin. 2012, 4, 351–356. [CrossRef] Processes 2021, 9, 437 14 of 14

27. Chiciudean, T.G.; Stoica-Guzun, A.; Dobre, T.; van Tooren, M. High-strength epoxy bacterial cellulose based composites. Mater. Plast. 2011, 48, 159–163. 28. Chiciudean, T.G.; Stoica, A.; Dobre, T.; van Tooren, M. Synthesis and characterization of poly(vinyl alcohol)-bacterial cellulose nanocomposite. UPB Sci. Bull. Ser. B 2011, 73, 17–30. 29. Dubey, V.; Saxena, C.; Singh, L.; Ramana, V.K.; Chauhan, S.R. Pervaporation of binary water-ethanol mixtures through bacterial cellulose membrane. Sep. Purif. Technol. 2002, 27, 163–171. [CrossRef] 30. Jewprasat, S.; Saratago, T.; Phisalaphong, M. Pervaporation of ethanol/water mixtures using bacterial cellulose poly(vinyl alcohol) mebrane. Int. J. Chem. Environ. Biol. Sci. 2015, 3, 175–178. 31. Kadlec, P.; Henke, S.; Bubnik, Z. Properties of ethanol-water solutions—Tables and Equations. Zucker Iindustrie/Sugar Ind. 2010, 135, 607–613. [CrossRef] 32. Puspasari, T.; Chakrabarty, T.; Genduso, G.; Peinemann, K.V. Unique cellulose/polydimethylsiloxane blends as an advanced hybrid material for organic solvent nanofiltration and pervaporation membranes. J. Mater. Chem. A 2018, 28, 13685–13695. [CrossRef] 33. Peng, P.; Shi, B.; Lan, Y.A. Review of membrane materials for ethanol recovery by pervaporation. Sep. Sci. Technol. 2010, 46, 234–246. [CrossRef] 34. Néel, J.; Nguyen, Q.T.; Clément, R.; Lin, D.J. Influence of downstream pressure on the pervaporation of water-tetrahydrofurane mixtures through a regenerated cellulose membrane (Cuprophan). J. Membr. Sci. 1986, 27, 217–232. [CrossRef] 35. Dobre, T.; Sanchez, J.M. Chemical Engineering: Modelling, Simulation and Similitude; Wiley-VCH: Weinheim, Germany, 2007; pp. 323–460. 36. Cioroiu, D.R.; Pârvulescu, O.C.; Koncsag, C.I.; Dobre, T.; Răducanu, C. Rheological characterization of algal suspensions for bioethanol processing. Rev. Chim. 2017, 68, 2311–2316. [CrossRef] 37. Ion, V.A.; Pârvulescu, O.C.; Dobre, T. Volatile organic compounds adsorption onto neat and hybrid cellulose. Appl. Surf. Sci. 2015, 335, 137–146. [CrossRef] 38. Mao, Z.; Cao, Y.; Jie, X.; Kang, G.; Zhou, M.; Yuan, Q. Dehydration of isopropanol-water mixtures using a novel cellulose membrane prepared from cellulose/N-methylmorpholine-N-oxide/H2O solution. Sep. Purif. Technol. 2010, 72, 28–33. [CrossRef]