1. Introduction
Solvents such as isopropyl alcohol (IPA), ethanol, butanol, benzene, and toluene are commonly produced and discharged into wastewater from industries such as the food industry, fermentation industry, chemical industry, etc. The most frequent solvent utilized in pervaporation is ethanol, since the separation of ethanol from aqueous streams is particularly essential from an industrial standpoint[1]. Global ethanol consumption was $89 billion in 2019, with a CAGR of 4.8 percent projected from 2020 through 2027. The use of ethanol as a biofuel has raised the demand with a high purity grade more than 99.8 wt%. As a result, it must be well purified for the use of transportation fuels.
Over the last few decades, pervaporation technology has been intensively explored for the separation of organic- organic, water-organic, and organic-water mixtures[ 2-6]. Furthermore, it has been proposed as a practical and effective technological substitute for traditional methods, such as the distillation process utilized for the separation of azeotropic mixtures[7-11]. PV has major benefits over traditional processes, including economic effectiveness, environmental friendliness, high selectivity, and simplicity of modifying process parameters[ 12]. These considerable advantages of pervaporation technology have prompted the adoption of PV in a variety of applications, such as PV-distillation, PV-reaction hybrid processes, and PV-fermentation[13]. After that, the use of PV to separate organic solvents has become more common.
In pervaporation, the most common membrane materials studied for the past few decades are polydimethylsiloxane (PDMS), poly (1-trimethylsilyl-1-propynen (PTMSP), polysulfone (PSF), polyether sulfone (PES), polyacrylonitrile (PAN), polyvinylidiene fluoride (PVDF), polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), cellulose acetate (CA) and Polyamide (PA). Despite the fact that there are many materials available, still none of these materials have not fulfilled the industrial requirements i.e, these materials are poor in ethanol selective properties. From the literature, polydimethylsiloxane (PDMS) is the most investigated material for evaluating ethanol pervaporation performance[ 14,15].
The permeation of a molecule through a PDMS membrane is influenced by many factors such as composition of feed mixtures, type and shape of membrane module, and processing conditions. Among physicochemical properties of feed mixtures, the polarity of components plays an important role in the mass transport through the hydrophobic PV membranes. Hydrophilic membranes have higher affinity for the polar components of the mixture and will allow it to pass through the membrane. On the other hand, the hydrophobic membrane such as PDMS will have less affinity for polar components. Furthermore, penetrant-polymer interaction governs the permeation, in other words the diffusion through the separation layer. Therefore, it is very complicated to understand the mass transfer mechanism of organic-water mixtures through a membrane[16,17].
W. Zang et al. investigated the effects of temperature and concentration on the pervaporation performance of PTFE supported PDMS membranes for dilute organic solutions. It was observed that increasing the temperature, the overall flux rises initially and subsequently declines, and selectivity increases up to 40 °C. Moreover, by increasing the feed concentration, the flux increases but the selectivity decreased[13]. S.J. Lue et al. studied PDMS/polyurethane (PU) blend membranes and revealed that when the PU amount increased, diffusion rate decreased due to the monotonically drop of the free volume of the membrane. It was also determined that the trade-off between selectivity and flux was optimized by controlling the blending ratio[ 18]. T. Mohammadi et al. studied PV performance of a PDMS membrane for different alcohol mixtures such as methanol/water and ethanol/water. It is observed that methanol had a higher flux than ethanol. This is due to the smaller size of methanol, which enables it to interact less with the membrane, resulting in high flux. L. Li et al. investigated the transport behaviors of symmetric PDMS membranes under various conditions. They elucidated that the boundary-layer transfer coefficient rose exponentially with temperature and was proportional to the Reynolds number. The Arrhenius correlation of the membrane mass transfer coefficient was observed to be independent of flow states such as laminar, transient, and turbulent flows[20].
From aforementioned literatures, it is evident that, most of the studies focused on the separation behavior based on varying the type and concentration of feed solvents, additives to PDMS, PV operation conditions such as temperature and time, but there are no systematic study on the transient PV behavior. Therefore, in this study transient separation behavior of PDMS/PSF hollow fiber membrane modules has been investigated for 50 wt% ethanol-water mixture at 50°C.
2. Experimental
2.1. PDMS hollow fiber membrane modules
PDMS hollow fiber membrane module used in this study was manufactured and supplied by Airrane Co., Ltd. It was prepared by placing 950 hollow membrane fibers coated with PDMS on the surface of Polysulfone hollow fiber in an aluminum membrane module case and both ends were sealed by potting process. Test membrane modules used for this study had a surface area of 1194 cm2 per module. Fig. 1 shows hollow fiber membrane module coated with PDMS on the surface of polysulfone manufactured by Airrane and hollow fiber bundle before potting.
2.2. Characterizations
For SEM analysis, the PDMS/PSF hollow fibers were immersed in the liquid nitrogen for about 3 min and then cut while keeping it in the liquid nitrogen in order to have clear cross-section images. Field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) connected with energy-dispersive X-ray spectroscopy was used for the membrane surface and cross-sectional thickness measurements.
2.3. Pervaporation studies
Pervaporation tests were performed using PDMS/PSF hollow fiber membrane modules to evaluate the separation behavior of EtOH/H2O mixtures. The schematic diagram of the pervaporation system employed in this work for EtOH/H2O separation is shown in Fig. 2.
The feed concentration of 50wt% EtOH/H2O mixture was prepared by combining deionized water and absolute ethanol (99.9.% Conc.). The feed-tank had a total volume of 2 liters, and it was kept at 50°C, with constant stirring (200 rpm). Initially, the membrane was given a stabilization time for 1.5 hr in order to achieve proper wetting and 50°C temperature so that it can perform at the desired conditions. This was achieved by allowing the feed solution to circulate at a constant flow rate of 0.5 L/min across the membrane module. A rotary pump was used to pump the feed at a constant rate of 0.5 L/min, and a vacuum was applied to the permeate side. The sample was collected using liquid nitrogen traps to measure the EtOH/H2O separation compositions. The permeate composition was analyzed using a gas chromatography (GC) fitted with a thermal conductivity detector (TCD). The retentate was recovered by injecting it back into the feed tank, and the feed tank overall temperature was maintained at 50°C. The total flux (J) and the separation factor (α) was calculated by
Whereas Wi and Wj are the mass fractions of the compounds i and j on the permeate side, m is the weight of defrosted traps (kg), t is the time (min) and A is the membrane area (m2) respectively.
3. Results and Discussion
3.1. PDMS/PSF hollow fiber membrane
Fig. 3 shows a surface and cross-sectional SEM-images of a single PSF supported PDMS hollow fiber membrane. Fig. 3(a) depicts that the structure of the PSF-support is in the microstructure region and the selective PDMS separation layer, has a thickness of 28 μm. Fig. 3(b) exhibits the defect-free and uniform surface of PDMS/PSF membrane.
3.2. Pervaporation performance of PDMS/PSF membrane modules
Table 1 shows the PV transient behavior of PDMS/ PSF hollow fiber membrane Module 1 for a 50 wt% EtOH/H2O mixture. The concentration of ethanol on the feed side reduced from 47.75 to 38.9 wt%, while the concentration in the permeate has decreased from 87.8 to 84.7 wt%. Moreover, the separation factor initially increased followed by gradual decrease and finally remains stable. Similarly, in the case of Modules 2, 3, 4, and 5, shown in the Tables 2, 3, 4 and 5, the ethanol concentration on the feed side decreased from 49.15 to 40.1 wt%, 47.8 to 41.8 wt%, 47.8 to 39.6 wt%, 45.1 to 41.1 wt%, and 45.19 to 41.18 wt%. Also, on the permeate side, the same phenomena of decrease in the ethanol concentration was observed. In the case of water concentrations on the feed side increased from 50.69 to 59.52 wt% as shown in Table 1. This increase in the water concentration clearly suggested a decrease in the concentration of ethanol on the feed side. Whereas, on the permeate side the concentration of water increased from 12.14 to 15.30 wt%. The percentage increase in the concentration of water on the permeate side was very low as compared to the concentration increase on the feed side because the PDSM/PSF hollow fiber membrane is hydrophobic in nature and has more affinity towards ethanol molecules to adsorb. The same trend of percentage increase of water concentrations on the feed side and permeate side was observed in the other Modules 2, 3, 4 and 5 exhibited in Table 2, 3, 4 and 5 respectively.
In Table 1 the total flux varies from 1.98 to 1.69 Kg/m2 hr. Even though it shows an overall decrease, in the middle i.e., at 0.75 hr and 1 hr it exhibits a high total flux of 2.17 and 2.08 Kg/m2 hr respectively, which is a deviation from the normal behavior. The separation factor (α) also followed the trend similar to flux, it fluctuated from 7.68 to 8.74. Table 2 summarizes the PV performance of Module 2 in which the total flux changes from 1.72 to 1.69 Kg/m2 hr and the separation factor (α) from 8.03 to 7.56. After 0.5 hr separation factor (α) suddenly increased to 10.09 then dropped to 6.08 followed by a further increase up to 7.56. Module 3 shows the pervaporation performance which is somewhat similar to the behaviors observed in the case of Module 1 and 2. Initially it increased and the maximum total flux obtained was 2.06 Kg/m2 hr at 0.5 hr and separation factor (α) increased from 6.51 to 7.46. In this case, the trend observed was different as compared to first two Modules 1 and 2. In Module 1 and 2 the trend was more inconsistent but in this Module 3 the separation performance was more consistent and gradually increased as pervaporation time increased as exhibited in Table 3. In addition, the PV transient behavior of Module 4 presented in Table 4, the total flux obtained altered from 1.82 to 1.72 Kg/m2 hr and separation behavior (α) observed was similar to Module 1 and 2 and values varied from 7.43 to 8.53. Module 5 showed overall low performance in terms of total flux as compared to all other Modules (1, 2, 3 and 4) but in case of separation factor it followed the pattern of Module 1, 2 and 4 and the maximum value obtained was 9.14 at 0.5 hr.
Total flux and separation factor of all the PDMS/PSF hollow fiber membrane modules are displayed in Fig. 4. In terms of total flux the maximum performance was showed by the Module 1 and 3 and the lowest performance was shown by Module 5. However, similar pattern was not observed in terms of separation factor. In this case, Module 5 shows the better separation performance than the other modules. Moreover, Module 3 showed a gradual increase in the performance instead of fluctuating performance as observed in the Modules 1, 2, 4 and 5.
As time passes, flux begins to decrease, as seen in Fig. 4, with the exception of Module 3, which exhibits a slight increase, nevertheless this increase may be attributed to the aforementioned factors. Overall, all modules exhibit a drop in total flux, Apart from that, it could also happen due to cluster formation by a decrease in the ethanol concentration on the feed side.
This type of effect is not visible at lower concentrations, but in this study, the concentration is sufficiently high, i.e., 50 wt%, to produce this type of phenomena. During cluster formation, ethanol and water create hydrogen bonds with one other, increasing the overall size of the molecule. Therefore, aggregates into the polymer matrix and the diffusion coefficient decreases[23].
This kind of PV transient behavior give us an idea about the transport phenomena of PDMS/PSF hollow fiber membrane. Fig. 5 depicts the schematic representation of solution-diffusion mechanism which explains the separation behavior according to the results observed from the modules. The mass transport of non-porous PDMS membrane is recognized by so-called solutiondiffusion mechanism which generally involves three steps: (a) Diffusion of the mixture species at the feed/active surface; (b) diffusion through the membrane; and (c) transport across the membrane permeate interface[17]. In the Fig. 5 the red color line represents the concentration of ethanol with respect to time. It shows high concentration of ethanol on the feed side followed by the saturation of the membrane surface, then decrease in the concentration of ethanol on the feed side. There are various factors that influence the solution diffusion mechanism that can be considered in order to explain such a kind of PV transient behavior and some of them are discussed below.
The transient behavior of flux and selectivity might have occurred for the following reasons: Specifically, because of the high concentration polarization at the feed-membrane interface rather than the permeatemembrane interface. The concentration is high in the upstream side, and it is nearly zero in the downstream side. Furthermore, as this process was operating at a higher temperature of 50°C, so, it brings up another possible factor that could be considered as a contributor to changes in flux, named “membrane plasticization’’. At high operating temperature the membrane swells, causing the free volume to increase, hence the flux increases[21,22]. Another possible explanation for the increase in flux, is that the ethanol and water sorption is an endothermic process, so, as the temperature increases flux increases.
Additional possible reason for the PV transient behaviors might be due to the dimerization of the ethanol- water molecules. The definition of dimerization is “an addition reaction in which molecules are linked by either strong or weak interactions”. Because of the hydrogen bonding, the ethanol-water dimer clogs the channel for both molecules, causing the diffusion coefficient to drop and, as a result, influencing the fluxes and separation factor. The high separation factor in Module 2, can be understood by the degree of dimerization. If the dimerization of ethanol-water molecules increases, the average size of the ethanol molecule increases, and the diffusion coefficient decreases, resulting in a negative impact on membrane performance. On the contrary, when the dimerization of ethanolwater molecules decreases, the average size of ethanol also decreases, and hence the diffusion coefficient of ethanol increases. In case of Module 2 performance, the dimerization might be decreased, which has subsequently increase in separation performance[23].
All the aforementioned possible reasons i.e., concentration polarization (due to high ethanol con.), plasticization (due to temperature i.e., 50°C), cluster for-mation (due to hydrogen-bonding) and dimerization (due to additional reaction) broadens the understanding of the solution-diffusion mechanism. Which helps to understand the transient behavior observed during the PV of ethanol/water mixture through PDMS/PSF hollow fiber membranes. In this study, the concentration polarization factor mainly influence the solution diffusion mechanism, as the concentration of ethanol decreases on the feed side and increases on the permeate side because of the selective adsorption of ethanol on PDMS/PSF hydrophobic membrane.
4. Conclusion
In this study, transient pervaporation behavior of 50 wt% EtOH/H2O mixture through PDMS/PSF hollow fiber membrane module was investigated. Initially the hollow fiber modules were dry and upon the interaction with the ethanol/water mixture (1:1) it started to dissolve into the separation layer of the membrane. So, it caused an increase in the performance but eventually with the passage of time it started to decline especially in case of permeate flow rate. Moreover, with the passage of time the increase in the concentration of ethanol on the feed side becomes low as compared to water concentration. The water concentration on the permeate side gets very low as compared to the concentration increase on the feed side because the PDSM/PSF hollow fiber membrane is hydrophobic in nature and has more affinity towards ethanol molecules to adsorb. As a result, it was confirmed that the solution-diffusion mechanism governs the ethanol permeation across a PDMS membrane.
