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Effect of coagulation medium on PVDF pervaporation membrane morphology for the separation of butanol/water mixture.

INTRODUCTION

The renewable and sustainable fuels had been ruling the scene all around the globe ever since automotive transportation was introduced early twenties of last century. The plenty resources of this fuel made it quite logical to avoid thinking of any other form of fuel until it was time for researchers and scientists to do so due to high surges in crude oil prices, fluctuation in demand and consumption rates, catastrophes related to the inflammable nature of these fuels as well as political and military disputes and conflicts leading to wars and economic stalling. All these shortcomings urged the chemical industry sector to seek alternative fuels that could serve both renewability and sustainability in this field (Niloofar et al., 2014). In contrast with gasoline, biobutanol excels in aspects of volatility risks, corrosion resistance, flashpoint and vapour pressure issues, hence the well distinguished importance given to this fuel lately (Veronica et al., 2011).

Quite relevantly, the talk about biofuels mandates mentioning industrial membranes which had been proven to be very crucial in the field of chemical process industry. Membrane-based technology is very essential in chemical engineering when biofuels are the main focus since purification, concentration and fractioning of fluid mixtures rely on it (Richard, 2012).

While there is a great need to recover fuels and chemicals: namely acetone, butanol and ethanol, a technique of separation should be there to implement this target. Pervaporation comes first in this aspect (Johanna et al., 2013). Right before condensation of products commences, there is a need for a membrane that can allow "selective" diffusion of these products as vapors. Already there are several recovery methods including "Distillation", but the alternative "Pervaporation" technique has many advantages in contrast; ease of operation, low energy consumption and offering no harm to microbes are only a few of those advantages. Additionally; pervaporation does not allow salts, sugar and reaction intermediates to diffuse through it leading to a much cleaner product. However, more research is needed to improve the selectivity and flux of these membranes(Enu et al., 2012).

Recently, all industrial membranes are made from organic polymers and/or inorganic materials; the latter being the one that dominates the existing membrane market. Polysulfone (PSF), poly (ethersulfone) (PES), poly-acrylonitrile (PAN), polyamide, polyimide, poly (vinylidene fluoride) (PVDF) and polytetrafluoroethylene (PTFE) are the popular organic polymers in membranes fabrication (Guo-dong and Yi-ming, 2014). Due to its superior properties such as: chemical resistance, mechanical strength, thermal stability, and high hydrophobicity; poly (vinylidene fluoride) (PVDF) has gained a great consideration as a membrane material compared to other polymeric materials and has been paid much attention by researchers and manufacturers (Yee et al., 2011).This is caused by the fact that PVDF dissolves easily in common organic solvents. As a result, it is possible to produce porous PVDF membranes via phase inversion method through a modest immersion precipitation process. PVDF is soluble in some common solvents such as N-methyl-2-pyrrolidone (NMP), N, N-dimethyl acetamide (DMAc), and dimethyl formamide (DMF) (Guo-dong and Yi-ming, 2014).

Early eighties of last century marked the introduction of PVDF membranes (Fu Liu et al., 2011). As with most cases of materials fabrication, more than one method was available on the course of preparation of these membranes. These methods included the well famed phase inversion, sintering, track etching and incorporating inorganic particles either as additives or as fillers, but the first method (phase inversion) remains the leading and most dominant approach in the field of membrane manufacturing so far, due to reasons of being a simple and flexible production method, compared to other methods mentioned above (Boor et al., 2013).

Many experimental parameters including coagulation medium, polymer concentration, type of solvent and non-solvent, membrane thickness, evaporation time and temperature, relative humidity, and the post-treatment, all have a significant effect on membrane morphology, and eventually on the membrane performance (Brown et al., 2002).

The objective of this work is to investigate the effect of ethanol concentration in water coagulation bath on pervaporation PVDF membrane morphology and performance in the separation of Butanol/Water mixture.

MATERIAL AND METHODS

Experimental:

Materials:

Poly((vinylidene fluoride)) PVDF (average Mwt~534,000 by PC, powder) (Sigma Aldrich), and (1-Methyl-2-Pyrrolidinone (NMP)) (Anhydrous 99.5%) (Sigma Aldrich) as a solvent without any purification were used in this study for the purpose of PVDF membrane preparation.

Preparation Method:

Flat sheet polymeric PVDF membrane was fabricated using 16 wt% of poly(vinylidene fluoride) powder and 84 wt% of (1-Methy-2-Pyrrolidinone) (NMP) as a solvent. The PVDF powder was first dissolved in the solvent (NMP) for 6h under dissolution temperature of (30-40[degrees]C) followed by cooling the solution at room temperature for 3h to remove all bubbles formed upon dissolution. Casting solution with thickness of 250pm was then immersed with the glass plate in the coagulation bath with different ethanol concentrations (15-35 %) until the membrane was formed. The formed membrane was then left immersed in the coagulation bath for 24h followed by suitable drying at room temperature for another 24h before characterization. Three flat sheet PVDF membranes (M1, M2, and M3) were fabricated with different ethanol concentrations in a deionized water coagulation bath (15, 25, and 35%), respectively. The composition and preparation conditions of the prepared membranes are shown in Table 1.

Pervaporation Experiment:

Pervaporation experiment was conducted using a lab scale apparatus shown in Fig.1. (3L) of Butanol/Water mixture was pumped from a feed tank to the rectangular membrane cell by peristaltic pump. A controlled water bath was used to maintain the feed mixture temperature for each run. The flat sheet membrane was contained in a rectangular stainless steel cell with an effective area of 18.75 [cm.sup.2]. Permeate side pressure was kept constant using a vacuum pump. Retentate was recycled back to the feed tank. The permeate passing through the membrane was collected in a liquid nitrogen cold trap. The pervaporation permeate and the feed were analysed using a direct reading refractometer. The prepared membranes were tested in a pervaporation experiment at the operating conditions of: feed temperature of 30[degrees]C, flow rate of 40L/h, butanol in feed of 5wt%, and experiment time of 2h.

The permeate flux (J) at steady state is calculated using the following equation (Liu et al., 2011):

J = M/At (1)

where M is the mass of the collected permeation, gm ; A is the effective area of the membrane, [m.sup.2]; and t is the collection time for the pervaporation, h.

The separation factor (a ) of the PV membrane is defined as:

[alpha] = ([Y.sub.A]/[Y.sub.B])/([X.sub.A]/[X.sub.B]) (2)

where [Y.sub.A], [Y.sub.B] and [X.sub.A], [X.sub.B] are the mass fractions of butanol and water in the permeate and feed side, respectively.

Characterization of Prepared Membranes:

The morphology of the prepared membranes was characterized in terms of its structure, pore size and porosity measurements.

SEM Microscopy:

Scanning Electron Microscope (SEM) (Hutachi TM-300) was used to characterize the morphology of the prepared PVDF membranes. In order to reduce sample charging under the electron beam the membranes were coated with platinum using Auto line cutter, JEOL: JFC-1600.

Porosity:

Membranes porosity was measured according to the following procedure: the membrane samples were first immersed in octanol (ACS, ISO, Reag Ph Eur, Merck) for 2h then the membrane surface was dried by filter paper. The membranes were weighed before and after immersion in octanol and the porosity was calculated using Eq. (3) (Ahmad et al., 2012).

[epsilon] = [([m.sub.b]/[p.sub.b])/{([m.sub.b]/[p.sub.b]) + ([m.sub.p]/[p.sub.p])}] x 100 (3)

where [epsilon] is the porosity of membrane (%), [m.sub.b] is the mass of the observed octanol (gm), mp is the mass of dry membrane (gm), [[rho].sub.b] is the density of octanol (gm/[cm.sup.3]), and pp is the density of membrane (gm/[cm.sup.3]).

Pore Size Measurement:

Pore size distribution of the prepared membranes was measured using a Porolux 1000 Porometer (IB-FT GmbH, Germany).

RESULTS AND DISCUSSION

Effect of Ethanol Concentration in Water Coagulation Bath on Membrane Morphology and Performance:

In order to study the effect of ethanol concentration in a coagulation bath on membrane morphology and its performance, three samples of PVDF membrane (M1, M2, and M3) were prepared with the composition shown in Table 1.

Effect on Membrane Morphology: SEM & Cross Section Morphology:

Figure 2 shows the SEM images of the prepared PVDF membranes (M1, M2, and M3) with different ethanol concentration in a water coagulation bath (15, 25, and 35%), respectively. As shown in both Fig.2 and Table 2, all membranes showed a similar morphology but with different pore sizes and numbers in the order of M1>M3>M2, with pore size of values of 1.021 > 0.3868 > 0.3427 [micro]m and porosities 72% > 65% > 55%, respectively.

The membrane formation via the phase inversion method is strongly affected by the coagulation that is related to the coagulation ability of the non-solvent. Furthermore, the coagulation ability of a coagulant can affect the demixing rate of PVDF casting film in coagulation bath. It was found that the effect of alcohol addition in a coagulation bath had been to slow down the liquid-liquid demixing according to the sequence of isopropanol > ethanol > methanol > water (Sukitpaneenit and Chung, 2009).

Two different types of membranes are usually applied in pervaporation: symmetric and asymmetric membranes. The symmetric is also called "homogeneous", in which the cross section shows a uniform porous structure, while in the asymmetric membrane the cross section shows two different structures; a thin and dense top layer on a porous structure of the same material. Comparing pervaporation characteristics of asymmetric to symmetric membranes, asymmetric membranes combine a higher permeation rate, due to the very thin separation layer, and a good mechanical resistance provided by the porous support (Angelo et al, 2015).

Figure 3 shows the cross section images of PVDF membranes (M1, M2, and M3) with the different ethanol concentration in a water coagulation bath of 15, 25, and 35%, respectively.

As shown in Fig.3, asymmetric structure consisting of a dense top layer and a porous sublayer with finger-like cavities is very clear for the membranes M1 and M3 with ethanol concentration of 15 and 35%, respectively, while the membrane M2 with ethanol concentration of 25% shows a very low asymmetricity with a sponge-like structure.

The concept of membrane formation can be described in terms of three components: polymer, solvent, and non-solvent. After dissolving the polymer in a suitable solvent, the polymer solution then is cast as a thin film upon a support (e.g. a glass plate) followed by immersion in a non-solvent bath. The exchange of solvent and non-solvent will occur after a period of time and the demixing takes place. Finally a solid polymeric film is obtained with an asymmetric structure (Nur et al., 2015).

Instantaneous and delayed demixing are the main two types of liquid-liquid demixing process involved in the phase inversion membrane formation. Instantaneous demixing means that the membrane is formed immediately after immersion in the non- solvent bath whereas it takes some time before the ultimate membrane is formed in the case of delayed demixing. In the first case where the demixing occurs instantaneously, membranes with relatively porous top layers are obtained resulting in the formation of a porous membrane (micro-filtration/ultrafiltration type). On the other hand, delayed demixing process produces a membrane with a relatively dense top layer resulting in the formation of an asymmetric membrane having a dense nonporous top layer with pervaporation properties. It was also known that the thermodynamic behavior of each solvent-non-solvent system has a significant effect on the formation of the phase inversion membrane. In other words, a high mutual affinity contributes to creating a porous membrane whereas an asymmetric membrane with a dense nonporous top layer is obtained in the case of low mutual affinity system (Shimizu et al, 2002) and (Hao and Wang, 2003).

The structure becomes longer for membranes Ml and M3 with ethanol concentration of 15 and 35%. When the concentration of ethanol in water coagulation bath was increased, a gradual change in the membrane morphology from finger-like structure to sponge-like structure was very clear. This is attributed to the decrease of both of the effective membrane porosity and the precipitation rate (Fu Liu et al., 2011).

Furthermore, the sponge like structure in the case of M2 membrane is due to slow precipitation rate when the concentration of ethanol in the coagulation bath was 25%. Generally, the rapid precipitation results in finger-like structure while the slow precipitation produces the sponge-like structure. The finger-like structure was changed to the sponge- like structure due to the delayed phase separation. In other words, the restricted movement of molecules in the coagulation bath (which makes it difficult to form more pores in membranes) increased the proportion of the sponge-like structure in this case (Qinglei et al, 2014).

Pore Size & Porosity:

Figure 4 shows the relationship between ethanol concentration in water bath and both pore size and porosity of PVDF membranes (M1, M2, and M3).

Membrane pore size plays a significant role in the evaluation of the membrane performance. When the ethanol concentration in water bath was 15%, membrane (M1) showed the widest pore size (1.021 [micro]m) and the highest number of pores too, whereas the narrowest pore size (0.3427 pm) was shown for the membrane (M2) with ethanol concentration of 25% combined with the lowest pores number. In other words, the pore size distribution changes with the increase of ethanol concentration in water coagulation bath.

Due to their lower diffusion rates, the increase of the molar volume of the external non-solvent species resulted in the increase of the top layer thickness, and as a result the porosity of the membrane decreased (Buonomenna et al., 2007). It was also found that the decrease in the membrane pore size with the increase of ethanol concentration in water coagulation bath resulted in the decrease of porosity in the order of M1 > M3> M2 with values of 15% > 35% > 25%, respectively. Furthermore, the highest porosity (72%) was found for the membrane (M1) with ethanol concentration in water coagulation bath of (15%) combined with a high pore size of (1.021[micro]m). On the other hand, the lowest values of pore size (0.3427[micro]m) and porosity (55%) were found for the membrane (M2) with the ethanol concentration of 25%.

Effect on Membrane Performance:

In order to study the effects of ethanol concentration in water bath on membrane performance, the prepared membranes in Table 1 were tested in a pervaporation experiment and the pervaporation performance was evaluated in terms of membrane selectivity and permeation flux.

Figures 5 and 6 show the relation between ethanol concentration in water bath and both of the permeation flux and selectivity of the prepared PVDF membranes (M1, M2, and M3).

The experimental results indicate that the pore formation from coagulation bath has a significant effect on the membrane permeability. Both of the permeation flux and selectivity decreased with the increase of ethanol concentration in water coagulation bath from (15% to 35%) in the order of M1> M3> M2, respectively as shown in Figs.5 and 6.

The decrease of the permeation flux and selectivity with the increase of ethanol concentration in water coagulation bath is attributed to the decrease of the membrane pore size, and as a result, decreasing the porosity which in turn affects solution penetration into the pores, in other words, increasing membrane resistance to species. The higher permeation flux obtained by M1 and M3 is a result of their larger pore size as well as their higher porosity.

The highest selectivity and total permeation flux (1.41 and 11.73 kg/[m.sup.2] x h) were found to be for membrane (M1) with ethanol concentration of 15% ,while the lowest values (1.2 and 6.66 kg/[m.sup.2] x h) were recorded for membrane (M2) with ethanol concentration of 25%. On the other hand, as a result of the decrease of total permeation flux, the partial flux of both butanol and water also decreased with the increase of ethanol concentration in coagulation bath from (15 to 35%) in the order of M1> M3> M2 (as shown in Figs.7 and 8). The highest butanol flux (0.81 kg/[m.sup.2] x h) was found to be for the membrane (M1) with ethanol concentration of 15%, while the lowest one (0.3866 kg/[m.sup.2] x h) was found to be for the membrane (M2) with ethanol concentration of 25%. Water flux showed a decrease (11 to 6.3 kg/[m.sup.2] x h) with the increase of ethanol concentration from (15 to 35%). The highest flux of water (11 kg/[m.sup.2] x h) was also found for the membrane (M1) with ethanol concentration of 15%, and the lowest value (6.3 kg/[m.sup.2] x h) was found to be for the membrane (M2) with ethanol concentration of 25%.

Conclusion:

Membrane preparation conditions significantly affect both membrane structure and its properties. In this study, the effect of ethanol concentration in water coagulation bath was investigated. Membrane performance in terms of its selectivity and permeation flux is strongly affected by the type of non-solvent used in the coagulation bath. Using different types of non-solvents in the coagulation bath produces membranes with different morphologies; namely: asymmetric and symmetric membranes. In this study, three flat sheet PVDF membranes (M1, M2, and M3) with different ethanol concentration in water coagulation bath of 15, 25, and 35%, respectively were fabricated to investigate the effect of ethanol concentration in water coagulation bath on pervaporation PVDF membrane morphology and performance in the separation of Butanol/Water mixture. Lower pore size and porosity values were found for the membrane (M2) with ethanol concentration of 25% while the highest values were found for the membrane (M1) with the ethanol concentration of 15%. The decrease of the membrane selectivity and total permeation flux was in the order of M1> M3> M2 with values of 1.41> 1.2> 1.32 and 11.73> 6.66> 8.8 kg/[m.sup.2] x h, respectively. The highest selectivity and total permeation flux (1.41 and 11.73 kg/[m.sup.2] x h) were recorded for membrane (M1) with ethanol concentration of 15%, while the membrane (M2) with ethanol concentration of 25% exhibited the lowest values of selectivity (1.2) and total permeation flux (6.66 kg/[m.sup.2] x h). This is due to the decrease of the membrane pore size and porosity, that which affects solution penetration into the pores, in other words; increasing membrane resistance to species. Type of membrane structure has also showed a significant effect on pervaporation membrane performance. Furthermore, the membrane (M2) with ethanol concentration of 25% showed less asymmetricity compared to other membranes (M1 and M3), that which led to decrease in both permeation flux and selectivity.

Nomenclature:

[epsilon]: Porosity of the membrane (%)

[m.sub.b]: Mass of the observed octanol (gm)

[[rho].sub.b]: Density of octanol (gm/[cm.sup.3])

[m.sub.p]: Mass of the dry membrane (gm)

[[rho].sub.p]: Density of PVDF membrane (gm/[cm.sup.3])

[alpha]: Selectivity (-)

J: Total flux (kg/[m.sup.2] x h)

Abbreviations:

PV: Pervaporation

M: The mass of the collected permeation (gm)

A: The effective area of the membrane ([m.sup.2])

t: The collection time for the pervaporation (h)

[Y.sub.A]: The mass fraction of butanol in the permeate

[Y.sub.B]: The mass fraction of water in the permeate

[X.sub.A]: The mass fraction of butanol in feed

[X.sub.B]: The mass fraction of water in feed

ARTICLE INFO

Article history:

Received 1 June 2015

Accepted 28 June 2015

Available online 22 July 2015

REFERENCES

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(1,2) Adil Hatem Rashid, (1) M.D. Irfan Hatem and Mohd, (3) Hafiz Dzarfan Othman

(1) School of Bioprocess Engineering. University Malaysia Perlis. kompleks pusat pengajian, Jejawi 3, 02600 Arau, Perlis, Malaysia

(2) Faculty of Chemical Engineering. Al-Muthanna University. Alsamawah, Iraq

(3) Advanced Membrane Technology Research Centre (AMTEC), Faculty of Petroleum and Renewable Energy Engineering. University Technology Malaysia, 81310 UTM Johor Bahru, Malaysia

Corresponding Author: Adil Hatem Rashid, School of Bioprocess Engineering. University Malaysia Perlis. Malaysia. kompleks pusat pengajian, Jejawi 3, 02600 Arau, Perlis, Malaysia.

E-mail: adil.hatem@yahoo.com

Table 1: Compostition and preparation conditions
of the prepared membranes.

              Polymer          Solvent      Dissolution
Membrane   Concentration    Concentration      Temp.
                t %             wt %        ([degrees]C)

M1               16              84              40
M2               16              84              40
M3               16              84              40

           Dissolution         Coagulation
Membrane       Time              Medium
               (h)

M1              6         15% Eth. + 85% Water
M2              6         25% Eth. + 75% Water
M3              6         35% Eth. + 65% Water

Table 2: Pore size and porosity results for the prepared
PVDF membranes (M1, M2, and M3) with different ethanol
concentration in water coagulation bath.

              Ethanol
           concentration   Pore size    Porosity
Membrane        (%)        ([micro]m)     (%)

M1              15           1.021         72
M2              25           0.3427        55
M3              35           0.3868        65
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Title Annotation:poly(vinylidene fluoride)
Author:Rashid, Adil Hatem; Hatem, M.D. Irfan; Mohd; Othman, Hafiz Dzarfan
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Date:Jul 1, 2015
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