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PEG blend membrane
Accepted Manuscript Sorption, diffusion and pervaporation study of thiophene/n-heptane mixture through self-support PU/PEG blend membrane Bahareh Baheri, Toraj Mohammadi PII: DOI: Reference:
S1383-5866(17)30569-5 http://dx.doi.org/10.1016/j.seppur.2017.05.026 SEPPUR 13739
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
21 February 2017 10 May 2017 10 May 2017
Please cite this article as: B. Baheri, T. Mohammadi, Sorption, diffusion and pervaporation study of thiophene/nheptane mixture through self-support PU/PEG blend membrane, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.05.026
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Sorption, diffusion and pervaporation study of thiophene/nheptane mixture through self-support PU/PEG blend membrane Bahareh Baheri, Toraj Mohammadi* Research and Technology Centre for Membrane Processes, Faculty of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran Tel: +98 21 77240496, Fax: +98 21 77240495 *
Corresponding author: E-mail address: [email protected]
Abstract The novel polyurethane (PU)/polyethylene glycol (PEG) self-support blend membranes were prepared via solution casting method to investigate their gasoline desulfurization performance. Pervaporation (PV) method was employed to separate thiophene from model gasoline consisting of a binary mixture of thiophene and n-heptane. The membrane sorption (S) and diffusion (D) coefficients were obtained from the sorption test. Characteristics and morphological attributes of the membrane were studied using FTIR and SEM analysis. SEM images exhibited a fine heterogeneous blended phase with a defect-free surface. Blending intensified the properties of individual PU and PEG membranes bringing about satisfactory permeability and desulfurization performance. A normalized flux of 25.5 kg µm/m2 h and an enrichment factor of 7.1 were obtained at 65 °C. The membranes were constructed and evaluated in self-support form, which indicates their mechanical and chemical strength when operated up to 120 hours. Both the blending method and the selection of the proper PU with appropriate blending ratio resulted in the fabrication of apposite membranes for separation of thiophene from n-heptane and also the development of a membrane material to be utilized in desulfurization industry. Keywords: Pervaporation; Sorption, Diffusion; Blending; Desulfurization; Thiophene; nheptane.
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1. Introduction The increasing effect of air pollution has spurred efforts for desulfurization of gasoline and diesel fuel. According to many scientific studies, membrane technology is one of the advancing technologies for deep desulfurization of fuel especially in separation of thiophenic compounds from gasoline or gasoline which has difficulty in other separation methods [1-5]. Dense polymeric membranes can be used in pervaporation (PV) process. PV can be applied in the case of dehydration, separation of organic compounds from aqueous-organic mixtures and separation of organic-organic components. Generally, hydrodesulfurization (HDS) is employed for reduction of sulfur compounds. However, this method reduces the octane number by saturating the olefins. Membrane desulfurization can lessen sulfur content without unfavorable side reactions [6-9]. Fabrication and investigation of an apposite membrane for deep desulfurization of fuel was initiated several years ago and efforts to make proper membranes still continue. Most of these works are about modification of neat polymers to make membranes with specific characteristics suited to remove the sulfur compounds thoroughly. For example, Lin and his coworkers modified PEG by adding CuY-zeolite particles to the casting solution. Improved results of 3.19 kg/m2h and 2.95 for flux and enrichment factor were gained respectively, in comparison with the neat PEG membrane tested at the same conditions gained 1.63 kg/m2 h and 3.05 for flux and enrichment factor correspondingly [10, 11]. Polydimethylsiloxane (PDMS) was selected and modified by Cao et al., and its properties were enhanced by embedding with silver/silica core–shell microspheres. Excellent results of 7.76 kg/m2h and 4.3 as flux and enrichment factor respectively, using a doping content of 5% for separation of synthetic solutions consisting of thiophene/n-octane were obtained [12]. Crosslinking as a technique to raise the membrane properties, in particular, diminishes the membrane swelling and improves the membrane selectivity as it was studied by Lin and coworkers [11, 13, 14].
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Blending as an advanced technique to approach fabrication of effectual membranes was also studied using two or more polymers in order to utilize their superior characteristics to obtain much better performance than those of individual polymers on account of the synergic effects of blending [15, 16]. PEG is a rubbery polymer which has many advantages in comparison with other polymers including: availability of solvents, cost efficiency and very high affinity to thiophenic compounds. By way of contrast, it has very low mechanical, chemical and thermal strength which makes it an improper candidate for industrial applications. Although crosslinking enhances its properties, some extra modifications should be applied to PEG in order to make it acceptable for industrial use. In this case, self-support membranes with the neat PEG cannot be made flawlessly because of fragile nature of the polymer. Blending is a superb alternative to elevate the properties of PEG, and PU is excellent choice for this purpose. Besides its good thermal and mechanical strength, PU has a great affinity to thiophenic compounds which is demonstrated by its solubility parameter (Table 1). Thiophene and thiophenic compounds forming a total of 80% of sulfuric pollutants are the principle sulfuric contaminants in FCC gasoline [7]. In this research, a model gasoline consisting of a binary mixture of thiophene and n-heptane was considered. Regarding the data in Table 1, it is observed that the solubility parameters of PU and PEG are more close to thiophenic species compared with the other polymers. If two components have a similar solubility parameter, then they exhibit a strong affinity to each other. Hence, PU and PEG were selected as the two proper candidates [17] to synthesize as the membrane material for desulfurization process. PUs are also rubbery polymers. The different types of PUs are defined depending on the different raw materials used in their production. Diisocyanates, long-chain diols and chain extenders are the main raw materials used in PU production. The structure of PU is formed
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by both hard and soft segments. Urethane/urea groups construct the hard parts and polyether/polyester groups build the soft parts. It should be mentioned that in PU membranes, soft segments are permeable domains, while hard segments are non-permeable domains. The length and the type of soft and hard segments, and also the length of the chain extenders affect their separation properties [18]. Investigation of the best fabrication conditions, making apposite thin membranes strong enough to operate without support and determination of the optimum weight percent of PU in the blend were the objectives of this work. Table 1. Solubility parameters of polymeric membrane materials, typical hydrocarbons and sulfur species in FCC gasoline (reprinted by permission from Elsevier) [17]. Membrane material pp
δ ((J/cm3)1/2) 21.93
Typical hydrocarbon n-Pentane
δ ((J/cm3)1/2) 14.4
Typical sulfur species Thiophene
δ ((J/cm3)1/2) 20
PEG PVC PVP
20.1 26.49 20.56
Isopentane Hexane Heptane
13.8 14.9 15.3
2-Methyl thiophene 3-Methyl thiophene Dimethyl thiophene
19.6 19.5 19.3
PVB PDMS PU
23.12 21.01 20.98
Octane Isooctane Cyclopentane
15.5 14.2 16.6
Trimethyl thiophene Diethyl thiophene Triethyl thiophene
19.2 19.2 19
CA PAN
25.06 26.61
Cyclohexane Methylcyclohexane
16.7 16
Thioether Dimethyl sulfone
16.9 29.8
PS PUU PI
18.5 20.98 32.3
Benzene Toluene m-Xylene
18.7 18.2 18.2
Sulfide Disulfide n-Butyl sulfide
16.9 17.4 28.1
CTA PVA
24.55 39.15
o-Xylene p-Xylene
18.5 18.1
n-Butyl mercaptan Benzyl mercaptan
18.4 21.1
Polystyrene
18.5
Propyl benzene
17.7
C5-Mercaptan
17.9
2. Experimental 2.1. Materials PEG (molecular weight = 20000), maleic anhydride (MA) as a crosslinking agent, n-methyl2-pyrrolidone (NMP) as a solvent, 3-methylamine (TMA) (40% solution in water) as a catalyst and thiophene were purchased from Merck (Darmstadt, Germany). An aromatic thermoplastic polyether based PU was purchased from Merquinsa (Pearlthane D16N87, Barcelona, Spain). This type of PU has good mechanical properties, low temperature 4
flexibility, outstanding erosion, weathering and biological resistance. N-heptane was purchased from Daejung (purity = 99 wt.%, Gyeonggi-Do, Korea). All chemicals used were of analytical reagent (A.R.) grade. 2.2. Blend Membrane preparation PU was dissolved in NMP at 50 °C to form a homogeneous solution. When the solution was cooled down to 40 °C, a certain amount of PEG mixed with MA as a crosslinking agent, was added to the solution. Finally, TMA as a catalyst agent was added to the mixture. The mixture was gently stirred at room temperature to form a homogeneous mixture. The mixture was put in a vacuum oven at 50 °C for 2 h to degas and then the resulting mixture was cast on a glass plate with the aid of a casting knife. The cast film was placed in an oven at 90 °C for 100 min to complete the crosslinking process and evaporate the solvent. Finally, the membrane was dried in a vacuum drying oven at room temperature for about 48 h. The membranes must be kept in a dry and dust free place. The thickness of the blend self-support membranes was approximately 50 µm measured by a micrometer (Mitutoyo, model MDC25SB). The membranes were prepared with different amounts of PU varied from 0 to 100 wt.% and were named in the specific order shown in Table 2. For example, PU20/80 corresponds to the membrane with 20 wt.% PU in the blend. Table 2. The different types of membranes prepared in this work.
PU0/100 PU20/80 PU40/60
Weight percent of PU in blend (wt.%) 0 20 40
PU60/40
60
Membrane
PU80/20
80
PU100/0
100
5
Fig. 1. A schematic representation of the PV setup 2.3. PV experiments and measurements A schematic view of the PV set-up is presented in Fig. 1. The permeation cell is made of stainless steel with an effective membrane area (A) of 10.4 cm2. The model feed was circulated by a centrifuge pump (MD-15R) from a feed tank, which had a capacity of 1 L, to a PV cell, and the retention stream was circulated back to the feed tank. The feed tank was made of glass and sealed by a proper closure to avoid vaporization of the feed vapor to stabilize the feed composition. A vacuum pump with a control valve and a vacuum gage maintained a maximum vacuum of 10 mmHg on the permeate side. The feed stream operated at atmospheric pressure.
The permeate stream was collected in a cold trap which was
immersed in a container filled with liquid nitrogen. The operating temperature was controlled by a heater. When the cold trap was exchanged, all permeate was then poured back to the feed tank to avoid changing the feed composition. The system equilibrated for at least 2 h. Permeation flux was calculated using the quantity of permeate collected during the experiment time and its composition was determined by a gas chromatography instrument (GC-2010, Shimadzu, Kyoto, Japan).
Separation performance of the membranes was
evaluated on the basis of total permeation flux (J) and enrichment factor (E), which were calculated using the following equations [11]: 6
(1) (2) Where W, A and t represent the weight of permeate (kg), the effective membrane area (m2), and the operating time (h), respectively. Cp and Cf are total sulfur content of permeate and feed samples (µg/g), respectively. Each experiment was performed three times and the results were reported as the average. 2.4. Immersion test Diffusion and permeation through polymer blends depend upon their composition, miscibility and phase morphology [19]. In this study, the PU40/60 membrane was chosen and prepared specifically with high thickness. Experimental errors were reduced by the use of thick films which increased the time for the membrane to reach equilibrium saturation [20]. For this experiment, slices of membranes which had the same size as used in the PV experiment were chosen. The membrane samples were weighed by a very sensitive electronic balance (KERN 770) with an accuracy of 0.0001 g. For single component sorption, the pieces of membranes were immersed in the liquid solvent within a sealed bottle and quickly removed from the container, and wiped off to remove the surface liquid then were weighted immediately. The membranes were then placed back into the sealed bottle. After a certain time interval this procedure was repeated until the weight of the samples became constant. These experiments were performed very quickly in less than 20 seconds during the times when the samples were weighed to minimize experimental errors. The experiments were carried out at 25 °C. Each experiment was performed three times and the recorded data are presented as the average. Diffusion coefficients (D) of the model compounds in the membranes were evaluated using the Fickian diffusion theory. The plot of Mt/M∞ versus t1/2 is a dynamic sorption curve which
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is usually linear initially. D was calculated using the initial linear slope of the sorption curve before 50% of the equilibrium uptake in conjunction with the following formula [21, 22]: (3) Where Mt and M∞ represent the mass adsorbed at time t and at infinite time respectively, and L is initial thickness of the membranes. The mass transport mechanism in the PV process is the solution diffusion occurring in three steps: sorption in the membrane surface, diffusion across the membrane and desorption at the permeate side. Due to the fact that the permeate pressure is very low, the third step can be neglected. Hence, the sorption and diffusion properties control the membrane performance conspicuously. Permeation coefficient (P) can be calculated using the following equation: (4) Sorption coefficient, S (mol%), is expressed as moles of solvent uptake by 100 g of membrane sample using the following formula [23]:
(5)
3. Results and discussion 3.1. Morphology of PU/PEG self-support blend membranes Fig. 2 shows cross sectional and surface SEM images of the PU60/40 membrane (see Table 2) at high magnification. The membrane morphology was examined with a CamScan SEM (Model MV2300, VEGA TESCAN) microscope. The images clearly demonstrate that PU and PEG are blended finely and the membrane surface is remarkably defect-free. Regarding the SEM images, thickness of the membranes is about 50 μm as measured with a micrometer (Mitutoyo, model MDC-25SB).
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The blend membranes become more uniform, resilient and less breakable by enhancing the PU content based on the experimental observations. Although the glass transition temperature (Tg) of PEG is -34.63 °C and it is in a rubbery form at ambient temperature, the neat PEG membrane has a flimsy and weak structure. Without a proper support, the membrane may crack or break and lose its selectivity during installation and transportation. The experimental results revealed that the membranes with PU content of less than 20 wt.% exhibit weak mechanical strength, thus are not suitable for making self-support membranes. More SEM images of this blend can be found in the supplementary content.
Fig. 2. Surface and cross sectional SEM images of the PU60/40 membrane.
3.2. Calculation of diffusion and permeation coefficient Fig. 3 shows the dynamic sorption curve for thiophene and n-heptane into the PU/PEG blend membrane. It is apparent that the time required to reach the equilibrium curve for thiophene is much less than n-heptane. It can be concluded that thiophene sorption in the blend membrane is much faster than n-heptane sorption indicating higher solubility speed and this is the key factor for separation of binary mixture of thiophene/n-heptane using this membrane.
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Fig. 3. Dynamic sorption curve for the PU40/60 membrane soaked in n-heptane and thiophene.
The calculated diffusion, sorption and permeation coefficients under the experimental temperature are given in Table 3 along with the data of the neat PEG membrane tested by Lin et al. at the same operating conditions [14]. The higher diffusion and sorption coefficients of thiophene and n-heptane for the blend membrane are shown in Table 3. This is due to the morphological changes in the system, as a result of increasing the permeable domain and the free volume in the polymer blend. The higher sorption coefficient is obtained by thiophene, due to proximity of the thiophene solubility parameter to the solubility parameters of both the neat and blend membranes. In comparison with the neat PEG membrane, adding PU (with Tg of -48 °C) which is less crystalline than PEG, results in an increase in the free volume of the polymer blend which enlarges sorption coefficients of both thiophene and n-heptane. The higher sorption coefficient means larger numbers of penetrant molecules are dissolved in the polymer sample and able to diffuse across it. Also, higher penetrant concentration in the membrane can increase polymer chain mobility resulting in an increase in the diffusion coefficient. Altogether, the blending of PEG with PU enhances sorption, diffusion and permeation coefficients, which are important to elevate the membrane performance. These improvements result in the higher flux and the bigger enrichment factor obtained in the PV experiments. 10
Furthermore, the permeation coefficient of thiophene is higher than n-heptane confirming the ability of the membrane to separate thiophene from n-heptane, and subsequently, aromatic thiophenic compounds from other hydrocarbons in gasoline. Table 3. Diffusion (D), sorption (S) and permeation coefficient (P) for thiophene and n-heptane in to PU40/60 membrane at 25 °C. Membrane PU40/60
PEG [14]
Penetrant
D(cm2/s)×107
S(mol%)×102
P(cm2/s mol%)×109
Thiophene
11.9
1.64
19.52
N-heptane
0.613
0.038
0.023
Thiophene
8.06
0.62
4.98
N-heptane
0.013
0.013
0.00017
3.3. FTIR analysis FTIR spectra of the PU100/0 and PU60/40 membranes were obtained by PerkinElmer Spectrum RX1 spectrometer and the results are displayed in Fig. 4 and Fig. 5, respectively. The appearance of the peaks in the region of 3340-3400 cm-1 can be attributed to the hydroxyl group and in the region of 2850-2950 cm-1 to the C-H group. The presence of the peaks at 1734 cm-1 and 1546 cm-1 point out the carbamate and the amide groups in PU. Generally, the remaining isocyanate groups in PU can react with the hydroxyl groups in PEG and produce carbamate groups which is confirmed by the disappearance of the isocyanate peak at 2260 cm-1 [15]. It can be observed from the FTIR spectra of the neat PU membrane that this type of PU randomly has free isocyanate groups shown by the weak intensity of the peak at 2260 cm-1 and this indicates a low reaction yield.
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Fig. 4. FTIR spectra of PU100/0 membrane.
Fig. 5. FTIR spectra of the PU60/40 membrane.
3.4. PV performance of PU/PEG blend membranes The PV performance of self-support PU/PEG blend membranes with different content of PU is presented in Fig. 6. The experiments were conducted with sulfur content of 2000 ppm at 25 °C with the permeate pressure of 10 mmHg to separate the model gasoline composed of thiophene and n-heptane. In this experiment, the PU20/80 membrane was not investigated because it was found that membranes with high PEG content have random cracks which are a consequence of the fragile nature of this polymer. As a result, the enrichment factor was minimized and the permeation flux reached a maximum. With this ratio of blending it is
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preferable to use a proper support to increase the mechanical properties of the membrane [24].
Fig. 6. Effects of PU content on the membrane performance at 25 °C.
According to Fig. 6, by enhancing the PU content, flux decreased and enrichment factor increased which can be influenced by two factors. First, in consequence of the proximity of solubility parameter of PEG to thiophenic compounds, affinity of PEG to thiophenic compounds is higher than PU. Hence, increasing the membrane PU content, which reduces membrane swelling, results in a decrease in permeation flux and an increase in enrichment factor. Second, PU and PEG form a heterogeneous blend and PEG is dispersed in PU as a continuous phase. Fig. 7 displays a schematic model for the morphology of the PU/PEG blend membranes including an SEM image of the PU60/40 membrane. In this study, no compatibilizer was used for making this blend. The forces between the individual PU and PEG molecules are more than the forces between the PU and PEG molecules especially because there is no reaction between PU and PEG. As a result, the molecular forces in the boundary between these two phases are lower than the molecular forces in each phase separately. Hence, it is possible that very tiny voids are created in these areas. Thiophene and n-heptane can transfer through both the polymer blend and the minute voids formed in contact between the two phases due to less transportation resistance. These 13
passages enhance the transport velocity of these molecules. Density of the dispersed phase increases with an increase in PEG content in the blend and this creates more passages which results in higher permeation flux. This phenomenon has the reverse effect on enrichment factor because n-heptane can also pass through these passages easily.
Fig. 7. Schematic model for morphology of the PU/PEG blend membrane.
3.5. Effect of permeate pressure on the membrane performance Permeate pressure is a prominent parameter providing the driving force for transport of target molecules in the PV process. The main mass transfer driving force in PV separation is the chemical potential difference that occurs by either exerting vacuum or flowing sweep gas at the permeate side. Generally, exerting vacuum is preferred to flowing sweep gas due to its capability of exact regulation and convenient application especially at bench scale [15].
Fig. 8. Effects of permeate pressure on permeation flux at 25 °C.
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Fig. 9. Effect of permeate pressure on enrichment factor at 25 °C.
Effects of permeate pressure on the PU/PEG blend membrane performance are presented in Fig. 8 and Fig. 9. The experiments were carried out at 25 °C and the model gasoline feed contained 2000 ppm sulfur content. Fig. 8 displays the impact of permeate pressure on permeation flux of the membranes with different PU contents. As expected for all the membranes, decreasing permeate pressure enhances permeation flux. As observed, the effect of vacuum pressure on permeation flux is more distinguishable for the membranes with lower PU content because these membranes have more PEG as the dispersed phase and this results in the creation of more passages between the two phases for transporting thiophene and nheptane. Decreasing permeate pressure has more effect on transporting molecules through these passages. Fig. 9 shows the effect of permeate pressure on enrichment factor for the PU60/40 membrane. As observed, decreasing permeate pressure also increases enrichment factor. The activity of the downstream components is affected by permeate pressure directly. Transport of both n-heptane and thiophene become faster due to the larger PV driving force. Solubility and diffusivity of thiophene in this membrane are much greater than those of nheptane and by enhancing the PV driving force, more thiophene can pass through the membrane in comparison to n-heptane. This, results in increasing enrichment factor of the membrane. Accordingly, diminishing permeate pressure enhances both permeation flux and
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enrichment factor, strongly affects the PV performance, and the maximum efficiency can be obtained for the minimum permeate pressure [11, 17]. 3.6. Effect of feed temperature on the membrane performance Temperature has also a significant impact on the membrane permeability. Fig. 10 reveals the effect of temperature on performance of the PU60/40 membrane with feed sulfur content of 2000 ppm and permeate pressure of 10 mmHg. As expected, the results demonstrate that with increasing temperature permeation flux also increases. This can be explained by two mechanisms. First, the improved saturated vapor pressure of the feed molecules by enhancing
Fig. 10. Effects of feed temperature on performance of PU60/40 membrane.
the feed temperature result in a larger driving force for molecular transport. Second, the enhanced polymer chain motions at elevated feed temperatures, which increases the free volume of polymer matrix. These two phenomena improve membrane permeation flux. In order to accurately investigate the impact of temperature on permeation flux, the Arrhenius equation was applied to calculate the permeation activation energy of two components in the membrane. (6) Where, A is the pre-exponential factor, Ep is the permeation activation energy of PV, R is the universal gas constant and T is absolute temperature [7, 13, 14].
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Fig. 11. Arrhenius plots of thiophene and n-heptane partial fluxes versus feed temperature (PU60/40 membrane).
Both partial permeation fluxes and activation energies of thiophene and n-heptane were calculated. The linear Arrhenius plots for thiophene and n-heptane are presented in Fig. 11. The results clearly reveal that activation energy of n-heptane is greater than that of thiophene (30.68 kJ/mol and 10.14 kJ/mol for n-heptane and thiophene, respectively). As a result of this assessment, n-heptane is more sensitive to temperature than thiophene. At higher temperatures more n-heptane molecules receive activation energy for permeation through the membrane. The influence of temperature on enrichment factor is also displayed in Fig. 10. As exhibited here, a reverse effect on enrichment factor is observed. By enhancing temperature, sorption and diffusion coefficients of both thiophene and n-heptane increase. This can be due to enhancing the mobility of permeating molecules and polymer chains, and also increasing the free volume of polymer which causes the more membrane swelling. This eases transport of the smaller molecules of n-heptane through the membrane which ultimately leads to a decrease of its enrichment factor. As described above, increasing temperature increases permeation flux of n-heptane more than that of thiophene. 3.7. Stability of the membrane in long time operation
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Investigation of operation time on membrane performance is very important in industrialization of membrane processes, and indicates mechanical and chemical strength of the membrane. In this experiment, effects of long time operation up to 120 h on performance of the PU60/40 membrane with feed sulfur content of 2000 ppm at 45°C were examined. The results are presented in Fig. 12 showing that there is a gradual change at the beginning of the experiment, while the PV performance is steady after that, and no considerable fluctuation is observed during the length of the experiment. The first gradual change in the membrane performance can be attributed to the membrane swelling [11, 13]. These results reveal that the membrane has the capability to be used in a scaled up plant.
Fig. 12. Effects of operation time on performance of the PU60/40 membrane at 45°C.
4. Conclusion This work is an attempt to fabricate a proper membrane for deep desulphurization of gasoline. PU and PEG were selected because of their high affinity to sulfuric components estimated by their solubility parameters. PU/PEG blend membrane characteristics and performance were investigated by various experiments and the results revealed a very outstanding and effective separation in comparison to previous works. Blending improved characteristics of both PU and PEG, moreover, using this type of PU resulted in an incredible enrichment factor up to 26.1. In this research membranes were prepared without a support, 18
which is a new research technique in the desulfurization process. Due to the membrane thickness in self-support form, permeation flux values are not exceptional, but using a proper support reduces the membranes thickness and enhances permeation flux. The impacts of the blending ratio on the membrane performance were also investigated. As observed, with increasing the PU content permeation flux diminishes and enrichment factor enhances. The influences of permeation pressure and operation temperature on the membrane performance were also examined. Decreasing permeate pressure increased both permeation flux and enrichment factor, while enhancing feed temperature increased permeation flux and reduced enrichment factor. In Table 4, PV performance of different membranes prepared by other researchers including this work is compared. In this table, permeation flux values were normalized in order to compare the membranes with different thicknesses. It was observed that prepared PU60/40 membrane with its superior performance at 65 °C, which gives the highest pervaporation separation index (PSI) among all fabrication and performance conditions of this work, is a promising membrane for separation of sulfuric compounds from gasoline. Table 4. Comparison between PV performance of the self-support PU60/40 membrane and other works. Feed
Sulfur content (ppm)
Normalized flux (kg µm/m2 h)a
Enrichment factor
Reference
PU/PEG
Thiophene/Nheptane
2000
25.5
7.1
This work
PEG
FCC gasoline
1227
16.3
3.05
[11]
PI
Refinery naphtha
807
9.75
1.44
[25]
PDMS/PEI
Thiophene/Heptane
100
3.2
7.6
[26]
PEBAX/PVDF
Thiophene/Heptane
1000
41.8
4
[7]
PEG/PU
FCC gasoline
1200
37.5
4.03
[15]
a
Membrane
All the fluxes are normalized to the thickness of the membranes.
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Figure Captions:
Fig. 1. A schematic representation of the PV setup. Fig. 2. Surface and cross sectional SEM images of the PU60/40 membrane. Fig. 3. Diffusion (D), sorption (S) and permeation coefficient (P) for thiophene and n-heptane in to the PU40/60 membrane at 25 °C. Fig. 4. FTIR spectra of the PU100/0 membrane. Fig. 5. FTIR spectra of the PU60/40 membrane. Fig. 6. Effects of PU content on the membrane performance at 25 °C. Fig. 7. Schematic model for morphology of the PU/PEG blend membrane. Fig. 8. Effects of permeate pressure on permeation flux at 25 °C. Fig. 9. Effect of permeate pressure on enrichment factor at 25 °C. Fig. 10. Effects of feed temperature on performance of the PU60/40 membrane. Fig. 11. Arrhenius plots of thiophene and n-heptane partial fluxes versus feed temperature (PU60/40 membrane). Fig. 12. Effects of operation time on performance of the PU60/40 membrane at 45°C.
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Table Captions: Table 1. Solubility parameters of polymeric membrane materials, typical hydrocarbons and sulfur species in FCC gasoline. Table 2. The different types of membranes prepared in this work. Table 3. Diffusion (D), sorption (S) and permeation coefficient (P) for thiophene and nheptane in to the PU40/60 membrane at 25 °C. Table 4. Comparison between PV performance of the self-support the PU60/40 membrane and other works.
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References
[1] R. Konietzny, T. Koschine, K. Rätzke, C. Staudt, POSS-hybrid membranes for the removal of sulfur aromatics by pervaporation, Separation and Purification Technology, 123 (2014) 175-182. [2] Z. Yang, W. Zhang, T. Wang, J. Li, Improved thiophene solution selectivity by Cu2+, Pb2+ and Mn2+ ions in pervaporative poly[bis(p-methyl phenyl) phosphazene]desulfurization membrane, Journal of Membrane Science, 454 (2014) 463-469. [3] C.-J. Li, Y.-J. Li, J.-N. Wang, L. Zhao, J. Cheng, Ag+-loaded polystyrene nanofibrous membranes preparation and their adsorption properties for thiophene, Chemical Engineering Journal, 222 (2013) 419-425. [4] P. Li, L. Zhao, P. Wei, S. Li, H. Qu, J. Lee, Y. Kong, H. Sun, Mass transfer of hydroxyethyl cellulose membranes for desulfurization of FCC gasoline: Experimental and modeling, Chemical Engineering Journal, 231 (2013) 255-261. [5] G.O. Yahaya, F. Hamad, A. Bahamdan, V.V.R. Tammana, E.Z. Hamad, Supported ionic liquid membrane and liquid–liquid extraction using membrane for removal of sulfur compounds from diesel/crude oil, Fuel Processing Technology, 113 (2013) 123-129. [6] M. Shahverdi, B. Baheri, M. Rezakazemi, E. Motaee, T. Mohammadi, Pervaporation study of ethylene glycol dehydration through synthesized (PVA–4A)/polypropylene mixed matrix composite membranes, Polymer Engineering & Science, 53 (2013) 1487-1493. [7] K. Liu, C.-J. Fang, Z.-Q. Li, M. Young, Separation of thiophene/n-heptane mixtures using PEBAX/PVDF-composited membranes via pervaporation, Journal of Membrane Science, 451 (2014) 24-31. [8] H. Matuschewski, U. Schedler, MSE — modified membranes in organophilic pervaporation for aromatics/aliphatics separation, Desalination, 224 (2008) 124-131. [9] B. Baheri, M. Shahverdi, M. Rezakazemi, E. Motaee, T. Mohammadi, Performance of PVA/NaA Mixed Matrix Membrane for Removal of Water from Ethylene Glycol Solutions by Pervaporation, Chemical Engineering Communications, 202 (2014) 316-321. [10] L. Lin, Y. Zhang, H. Li, Pervaporation and sorption behavior of zeolite-filled polyethylene glycol hybrid membranes for the removal of thiophene species, Journal of Colloid and Interface Science, 350 (2010) 355-360. [11] L. Lin, Y. Kong, G. Wang, H. Qu, J. Yang, D. Shi, Selection and crosslinking modification of membrane material for FCC gasoline desulfurization, Journal of Membrane Science, 285 (2006) 144151. [12] R. Cao, X. Zhang, H. Wu, J. Wang, X. Liu, Z. Jiang, Enhanced pervaporative desulfurization by polydimethylsiloxane membranes embedded with silver/silica core–shell microspheres, Journal of Hazardous Materials, 187 (2011) 324-332. [13] L. Lin, G. Wang, H. Qu, J. Yang, Y. Wang, D. Shi, Y. Kong, Pervaporation performance of crosslinked polyethylene glycol membranes for deep desulfurization of FCC gasoline, Journal of Membrane Science, 280 (2006) 651-658. [14] L. Lin, Y. Kong, Y. Zhang, Sorption and transport behavior of gasoline components in polyethylene glycol membranes, Journal of Membrane Science, 325 (2008) 438-445. [15] L. Lin, Y. Kong, K. Xie, F. Lu, R. Liu, L. Guo, S. Shao, J. Yang, D. Shi, Y. Zhang, Polyethylene glycol/polyurethane blend membranes for gasoline desulphurization by pervaporation technique, Separation and Purification Technology, 61 (2008) 293-300. [16] P. Das, S.K. Ray, Analysis of sorption and permeation of acetic acid–water mixtures through unfilled and filled blend membranes, Separation and Purification Technology, 116 (2013) 433-447. [17] H.R. Mortaheb, F. Ghaemmaghami, B. Mokhtarani, A review on removal of sulfur components from gasoline by pervaporation, Chemical Engineering Research and Design. [18] M.M. Talakesh, M. Sadeghi, M.P. Chenar, A. Khosravi, Gas separation properties of poly(ethylene glycol)/poly(tetramethylene glycol) based polyurethane membranes, Journal of Membrane Science, 415–416 (2012) 469-477.
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[19] S.C. George, S. Thomas, Transport phenomena through polymeric systems, Progress in Polymer Science, 26 (2001) 985-1017. [20] R. Ponangi, P.N. Pintauro, D. De Kee, Free volume analysis of organic vapor diffusion in polyurethane membranes, Journal of Membrane Science, 178 (2000) 151-164. [21] P.L. Ritger, N.A. Peppas, A simple equation for description of solute release I. Fickian and nonfickian release from non-swellable devices in the form of slabs, spheres, cylinders or discs, Journal of Controlled Release, 5 (1987) 23-36. [22] P.L. Ritger, N.A. Peppas, A simple equation for description of solute release II. Fickian and anomalous release from swellable devices, Journal of Controlled Release, 5 (1987) 37-42. [23] A. Joseph, A.E. Mathai, S. Thomas, Sorption and diffusion of methyl substituted benzenes through cross-linked nitrile rubber/poly(ethylene co-vinyl acetate) blend membranes, Journal of Membrane Science, 220 (2003) 13-30. [24] Y. Kong, L. Lin, Y. Zhang, F. Lu, K. Xie, R. Liu, L. Guo, S. Shao, J. Yang, D. Shi, Studies on polyethylene glycol/polyethersulfone composite membranes for FCC gasoline desulphurization by pervaporation, European Polymer Journal, 44 (2008) 3335-3343. [25] L.S.C. White, MD, US), Wormsbecher, Richard Franklin (Dayton, MD, US), Lesemann, Markus (Baltimore, MD, US), Membrane separation for sulfur reduction, in, W.R. Grace & Co.-Conn. (Columbia, MD, US), United States, 2006. [26] J. Chen, J. Li, R. Qi, H. Ye, C. Chen, Pervaporation Separation of Thiophene–Heptane Mixtures with Polydimethylsiloxane (PDMS) Membrane for Desulfurization, Applied Biochemistry and Biotechnology, 160 (2010) 486-497.
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Graphical Abstract:
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Research highlights 1. Novel self-support blend membranes for separation of sulfuric components from gasoline for long time operation were prepared. 2. Transcendent enrichment factor and fluxes for membrane desulfurization were obtained. 3. Effects of blending ratio, operation temperatures and permeate pressure on PV performance were investigated. 4. Diffusion coefficient, solubility and permeation coefficient in blend membrane were calculated via sorption test.
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S1383-5866(17)30569-5 http://dx.doi.org/10.1016/j.seppur.2017.05.026 SEPPUR 13739
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
21 February 2017 10 May 2017 10 May 2017
Please cite this article as: B. Baheri, T. Mohammadi, Sorption, diffusion and pervaporation study of thiophene/nheptane mixture through self-support PU/PEG blend membrane, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.05.026
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Sorption, diffusion and pervaporation study of thiophene/nheptane mixture through self-support PU/PEG blend membrane Bahareh Baheri, Toraj Mohammadi* Research and Technology Centre for Membrane Processes, Faculty of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran Tel: +98 21 77240496, Fax: +98 21 77240495 *
Corresponding author: E-mail address: [email protected]
Abstract The novel polyurethane (PU)/polyethylene glycol (PEG) self-support blend membranes were prepared via solution casting method to investigate their gasoline desulfurization performance. Pervaporation (PV) method was employed to separate thiophene from model gasoline consisting of a binary mixture of thiophene and n-heptane. The membrane sorption (S) and diffusion (D) coefficients were obtained from the sorption test. Characteristics and morphological attributes of the membrane were studied using FTIR and SEM analysis. SEM images exhibited a fine heterogeneous blended phase with a defect-free surface. Blending intensified the properties of individual PU and PEG membranes bringing about satisfactory permeability and desulfurization performance. A normalized flux of 25.5 kg µm/m2 h and an enrichment factor of 7.1 were obtained at 65 °C. The membranes were constructed and evaluated in self-support form, which indicates their mechanical and chemical strength when operated up to 120 hours. Both the blending method and the selection of the proper PU with appropriate blending ratio resulted in the fabrication of apposite membranes for separation of thiophene from n-heptane and also the development of a membrane material to be utilized in desulfurization industry. Keywords: Pervaporation; Sorption, Diffusion; Blending; Desulfurization; Thiophene; nheptane.
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1. Introduction The increasing effect of air pollution has spurred efforts for desulfurization of gasoline and diesel fuel. According to many scientific studies, membrane technology is one of the advancing technologies for deep desulfurization of fuel especially in separation of thiophenic compounds from gasoline or gasoline which has difficulty in other separation methods [1-5]. Dense polymeric membranes can be used in pervaporation (PV) process. PV can be applied in the case of dehydration, separation of organic compounds from aqueous-organic mixtures and separation of organic-organic components. Generally, hydrodesulfurization (HDS) is employed for reduction of sulfur compounds. However, this method reduces the octane number by saturating the olefins. Membrane desulfurization can lessen sulfur content without unfavorable side reactions [6-9]. Fabrication and investigation of an apposite membrane for deep desulfurization of fuel was initiated several years ago and efforts to make proper membranes still continue. Most of these works are about modification of neat polymers to make membranes with specific characteristics suited to remove the sulfur compounds thoroughly. For example, Lin and his coworkers modified PEG by adding CuY-zeolite particles to the casting solution. Improved results of 3.19 kg/m2h and 2.95 for flux and enrichment factor were gained respectively, in comparison with the neat PEG membrane tested at the same conditions gained 1.63 kg/m2 h and 3.05 for flux and enrichment factor correspondingly [10, 11]. Polydimethylsiloxane (PDMS) was selected and modified by Cao et al., and its properties were enhanced by embedding with silver/silica core–shell microspheres. Excellent results of 7.76 kg/m2h and 4.3 as flux and enrichment factor respectively, using a doping content of 5% for separation of synthetic solutions consisting of thiophene/n-octane were obtained [12]. Crosslinking as a technique to raise the membrane properties, in particular, diminishes the membrane swelling and improves the membrane selectivity as it was studied by Lin and coworkers [11, 13, 14].
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Blending as an advanced technique to approach fabrication of effectual membranes was also studied using two or more polymers in order to utilize their superior characteristics to obtain much better performance than those of individual polymers on account of the synergic effects of blending [15, 16]. PEG is a rubbery polymer which has many advantages in comparison with other polymers including: availability of solvents, cost efficiency and very high affinity to thiophenic compounds. By way of contrast, it has very low mechanical, chemical and thermal strength which makes it an improper candidate for industrial applications. Although crosslinking enhances its properties, some extra modifications should be applied to PEG in order to make it acceptable for industrial use. In this case, self-support membranes with the neat PEG cannot be made flawlessly because of fragile nature of the polymer. Blending is a superb alternative to elevate the properties of PEG, and PU is excellent choice for this purpose. Besides its good thermal and mechanical strength, PU has a great affinity to thiophenic compounds which is demonstrated by its solubility parameter (Table 1). Thiophene and thiophenic compounds forming a total of 80% of sulfuric pollutants are the principle sulfuric contaminants in FCC gasoline [7]. In this research, a model gasoline consisting of a binary mixture of thiophene and n-heptane was considered. Regarding the data in Table 1, it is observed that the solubility parameters of PU and PEG are more close to thiophenic species compared with the other polymers. If two components have a similar solubility parameter, then they exhibit a strong affinity to each other. Hence, PU and PEG were selected as the two proper candidates [17] to synthesize as the membrane material for desulfurization process. PUs are also rubbery polymers. The different types of PUs are defined depending on the different raw materials used in their production. Diisocyanates, long-chain diols and chain extenders are the main raw materials used in PU production. The structure of PU is formed
3
by both hard and soft segments. Urethane/urea groups construct the hard parts and polyether/polyester groups build the soft parts. It should be mentioned that in PU membranes, soft segments are permeable domains, while hard segments are non-permeable domains. The length and the type of soft and hard segments, and also the length of the chain extenders affect their separation properties [18]. Investigation of the best fabrication conditions, making apposite thin membranes strong enough to operate without support and determination of the optimum weight percent of PU in the blend were the objectives of this work. Table 1. Solubility parameters of polymeric membrane materials, typical hydrocarbons and sulfur species in FCC gasoline (reprinted by permission from Elsevier) [17]. Membrane material pp
δ ((J/cm3)1/2) 21.93
Typical hydrocarbon n-Pentane
δ ((J/cm3)1/2) 14.4
Typical sulfur species Thiophene
δ ((J/cm3)1/2) 20
PEG PVC PVP
20.1 26.49 20.56
Isopentane Hexane Heptane
13.8 14.9 15.3
2-Methyl thiophene 3-Methyl thiophene Dimethyl thiophene
19.6 19.5 19.3
PVB PDMS PU
23.12 21.01 20.98
Octane Isooctane Cyclopentane
15.5 14.2 16.6
Trimethyl thiophene Diethyl thiophene Triethyl thiophene
19.2 19.2 19
CA PAN
25.06 26.61
Cyclohexane Methylcyclohexane
16.7 16
Thioether Dimethyl sulfone
16.9 29.8
PS PUU PI
18.5 20.98 32.3
Benzene Toluene m-Xylene
18.7 18.2 18.2
Sulfide Disulfide n-Butyl sulfide
16.9 17.4 28.1
CTA PVA
24.55 39.15
o-Xylene p-Xylene
18.5 18.1
n-Butyl mercaptan Benzyl mercaptan
18.4 21.1
Polystyrene
18.5
Propyl benzene
17.7
C5-Mercaptan
17.9
2. Experimental 2.1. Materials PEG (molecular weight = 20000), maleic anhydride (MA) as a crosslinking agent, n-methyl2-pyrrolidone (NMP) as a solvent, 3-methylamine (TMA) (40% solution in water) as a catalyst and thiophene were purchased from Merck (Darmstadt, Germany). An aromatic thermoplastic polyether based PU was purchased from Merquinsa (Pearlthane D16N87, Barcelona, Spain). This type of PU has good mechanical properties, low temperature 4
flexibility, outstanding erosion, weathering and biological resistance. N-heptane was purchased from Daejung (purity = 99 wt.%, Gyeonggi-Do, Korea). All chemicals used were of analytical reagent (A.R.) grade. 2.2. Blend Membrane preparation PU was dissolved in NMP at 50 °C to form a homogeneous solution. When the solution was cooled down to 40 °C, a certain amount of PEG mixed with MA as a crosslinking agent, was added to the solution. Finally, TMA as a catalyst agent was added to the mixture. The mixture was gently stirred at room temperature to form a homogeneous mixture. The mixture was put in a vacuum oven at 50 °C for 2 h to degas and then the resulting mixture was cast on a glass plate with the aid of a casting knife. The cast film was placed in an oven at 90 °C for 100 min to complete the crosslinking process and evaporate the solvent. Finally, the membrane was dried in a vacuum drying oven at room temperature for about 48 h. The membranes must be kept in a dry and dust free place. The thickness of the blend self-support membranes was approximately 50 µm measured by a micrometer (Mitutoyo, model MDC25SB). The membranes were prepared with different amounts of PU varied from 0 to 100 wt.% and were named in the specific order shown in Table 2. For example, PU20/80 corresponds to the membrane with 20 wt.% PU in the blend. Table 2. The different types of membranes prepared in this work.
PU0/100 PU20/80 PU40/60
Weight percent of PU in blend (wt.%) 0 20 40
PU60/40
60
Membrane
PU80/20
80
PU100/0
100
5
Fig. 1. A schematic representation of the PV setup 2.3. PV experiments and measurements A schematic view of the PV set-up is presented in Fig. 1. The permeation cell is made of stainless steel with an effective membrane area (A) of 10.4 cm2. The model feed was circulated by a centrifuge pump (MD-15R) from a feed tank, which had a capacity of 1 L, to a PV cell, and the retention stream was circulated back to the feed tank. The feed tank was made of glass and sealed by a proper closure to avoid vaporization of the feed vapor to stabilize the feed composition. A vacuum pump with a control valve and a vacuum gage maintained a maximum vacuum of 10 mmHg on the permeate side. The feed stream operated at atmospheric pressure.
The permeate stream was collected in a cold trap which was
immersed in a container filled with liquid nitrogen. The operating temperature was controlled by a heater. When the cold trap was exchanged, all permeate was then poured back to the feed tank to avoid changing the feed composition. The system equilibrated for at least 2 h. Permeation flux was calculated using the quantity of permeate collected during the experiment time and its composition was determined by a gas chromatography instrument (GC-2010, Shimadzu, Kyoto, Japan).
Separation performance of the membranes was
evaluated on the basis of total permeation flux (J) and enrichment factor (E), which were calculated using the following equations [11]: 6
(1) (2) Where W, A and t represent the weight of permeate (kg), the effective membrane area (m2), and the operating time (h), respectively. Cp and Cf are total sulfur content of permeate and feed samples (µg/g), respectively. Each experiment was performed three times and the results were reported as the average. 2.4. Immersion test Diffusion and permeation through polymer blends depend upon their composition, miscibility and phase morphology [19]. In this study, the PU40/60 membrane was chosen and prepared specifically with high thickness. Experimental errors were reduced by the use of thick films which increased the time for the membrane to reach equilibrium saturation [20]. For this experiment, slices of membranes which had the same size as used in the PV experiment were chosen. The membrane samples were weighed by a very sensitive electronic balance (KERN 770) with an accuracy of 0.0001 g. For single component sorption, the pieces of membranes were immersed in the liquid solvent within a sealed bottle and quickly removed from the container, and wiped off to remove the surface liquid then were weighted immediately. The membranes were then placed back into the sealed bottle. After a certain time interval this procedure was repeated until the weight of the samples became constant. These experiments were performed very quickly in less than 20 seconds during the times when the samples were weighed to minimize experimental errors. The experiments were carried out at 25 °C. Each experiment was performed three times and the recorded data are presented as the average. Diffusion coefficients (D) of the model compounds in the membranes were evaluated using the Fickian diffusion theory. The plot of Mt/M∞ versus t1/2 is a dynamic sorption curve which
7
is usually linear initially. D was calculated using the initial linear slope of the sorption curve before 50% of the equilibrium uptake in conjunction with the following formula [21, 22]: (3) Where Mt and M∞ represent the mass adsorbed at time t and at infinite time respectively, and L is initial thickness of the membranes. The mass transport mechanism in the PV process is the solution diffusion occurring in three steps: sorption in the membrane surface, diffusion across the membrane and desorption at the permeate side. Due to the fact that the permeate pressure is very low, the third step can be neglected. Hence, the sorption and diffusion properties control the membrane performance conspicuously. Permeation coefficient (P) can be calculated using the following equation: (4) Sorption coefficient, S (mol%), is expressed as moles of solvent uptake by 100 g of membrane sample using the following formula [23]:
(5)
3. Results and discussion 3.1. Morphology of PU/PEG self-support blend membranes Fig. 2 shows cross sectional and surface SEM images of the PU60/40 membrane (see Table 2) at high magnification. The membrane morphology was examined with a CamScan SEM (Model MV2300, VEGA TESCAN) microscope. The images clearly demonstrate that PU and PEG are blended finely and the membrane surface is remarkably defect-free. Regarding the SEM images, thickness of the membranes is about 50 μm as measured with a micrometer (Mitutoyo, model MDC-25SB).
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The blend membranes become more uniform, resilient and less breakable by enhancing the PU content based on the experimental observations. Although the glass transition temperature (Tg) of PEG is -34.63 °C and it is in a rubbery form at ambient temperature, the neat PEG membrane has a flimsy and weak structure. Without a proper support, the membrane may crack or break and lose its selectivity during installation and transportation. The experimental results revealed that the membranes with PU content of less than 20 wt.% exhibit weak mechanical strength, thus are not suitable for making self-support membranes. More SEM images of this blend can be found in the supplementary content.
Fig. 2. Surface and cross sectional SEM images of the PU60/40 membrane.
3.2. Calculation of diffusion and permeation coefficient Fig. 3 shows the dynamic sorption curve for thiophene and n-heptane into the PU/PEG blend membrane. It is apparent that the time required to reach the equilibrium curve for thiophene is much less than n-heptane. It can be concluded that thiophene sorption in the blend membrane is much faster than n-heptane sorption indicating higher solubility speed and this is the key factor for separation of binary mixture of thiophene/n-heptane using this membrane.
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Fig. 3. Dynamic sorption curve for the PU40/60 membrane soaked in n-heptane and thiophene.
The calculated diffusion, sorption and permeation coefficients under the experimental temperature are given in Table 3 along with the data of the neat PEG membrane tested by Lin et al. at the same operating conditions [14]. The higher diffusion and sorption coefficients of thiophene and n-heptane for the blend membrane are shown in Table 3. This is due to the morphological changes in the system, as a result of increasing the permeable domain and the free volume in the polymer blend. The higher sorption coefficient is obtained by thiophene, due to proximity of the thiophene solubility parameter to the solubility parameters of both the neat and blend membranes. In comparison with the neat PEG membrane, adding PU (with Tg of -48 °C) which is less crystalline than PEG, results in an increase in the free volume of the polymer blend which enlarges sorption coefficients of both thiophene and n-heptane. The higher sorption coefficient means larger numbers of penetrant molecules are dissolved in the polymer sample and able to diffuse across it. Also, higher penetrant concentration in the membrane can increase polymer chain mobility resulting in an increase in the diffusion coefficient. Altogether, the blending of PEG with PU enhances sorption, diffusion and permeation coefficients, which are important to elevate the membrane performance. These improvements result in the higher flux and the bigger enrichment factor obtained in the PV experiments. 10
Furthermore, the permeation coefficient of thiophene is higher than n-heptane confirming the ability of the membrane to separate thiophene from n-heptane, and subsequently, aromatic thiophenic compounds from other hydrocarbons in gasoline. Table 3. Diffusion (D), sorption (S) and permeation coefficient (P) for thiophene and n-heptane in to PU40/60 membrane at 25 °C. Membrane PU40/60
PEG [14]
Penetrant
D(cm2/s)×107
S(mol%)×102
P(cm2/s mol%)×109
Thiophene
11.9
1.64
19.52
N-heptane
0.613
0.038
0.023
Thiophene
8.06
0.62
4.98
N-heptane
0.013
0.013
0.00017
3.3. FTIR analysis FTIR spectra of the PU100/0 and PU60/40 membranes were obtained by PerkinElmer Spectrum RX1 spectrometer and the results are displayed in Fig. 4 and Fig. 5, respectively. The appearance of the peaks in the region of 3340-3400 cm-1 can be attributed to the hydroxyl group and in the region of 2850-2950 cm-1 to the C-H group. The presence of the peaks at 1734 cm-1 and 1546 cm-1 point out the carbamate and the amide groups in PU. Generally, the remaining isocyanate groups in PU can react with the hydroxyl groups in PEG and produce carbamate groups which is confirmed by the disappearance of the isocyanate peak at 2260 cm-1 [15]. It can be observed from the FTIR spectra of the neat PU membrane that this type of PU randomly has free isocyanate groups shown by the weak intensity of the peak at 2260 cm-1 and this indicates a low reaction yield.
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Fig. 4. FTIR spectra of PU100/0 membrane.
Fig. 5. FTIR spectra of the PU60/40 membrane.
3.4. PV performance of PU/PEG blend membranes The PV performance of self-support PU/PEG blend membranes with different content of PU is presented in Fig. 6. The experiments were conducted with sulfur content of 2000 ppm at 25 °C with the permeate pressure of 10 mmHg to separate the model gasoline composed of thiophene and n-heptane. In this experiment, the PU20/80 membrane was not investigated because it was found that membranes with high PEG content have random cracks which are a consequence of the fragile nature of this polymer. As a result, the enrichment factor was minimized and the permeation flux reached a maximum. With this ratio of blending it is
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preferable to use a proper support to increase the mechanical properties of the membrane [24].
Fig. 6. Effects of PU content on the membrane performance at 25 °C.
According to Fig. 6, by enhancing the PU content, flux decreased and enrichment factor increased which can be influenced by two factors. First, in consequence of the proximity of solubility parameter of PEG to thiophenic compounds, affinity of PEG to thiophenic compounds is higher than PU. Hence, increasing the membrane PU content, which reduces membrane swelling, results in a decrease in permeation flux and an increase in enrichment factor. Second, PU and PEG form a heterogeneous blend and PEG is dispersed in PU as a continuous phase. Fig. 7 displays a schematic model for the morphology of the PU/PEG blend membranes including an SEM image of the PU60/40 membrane. In this study, no compatibilizer was used for making this blend. The forces between the individual PU and PEG molecules are more than the forces between the PU and PEG molecules especially because there is no reaction between PU and PEG. As a result, the molecular forces in the boundary between these two phases are lower than the molecular forces in each phase separately. Hence, it is possible that very tiny voids are created in these areas. Thiophene and n-heptane can transfer through both the polymer blend and the minute voids formed in contact between the two phases due to less transportation resistance. These 13
passages enhance the transport velocity of these molecules. Density of the dispersed phase increases with an increase in PEG content in the blend and this creates more passages which results in higher permeation flux. This phenomenon has the reverse effect on enrichment factor because n-heptane can also pass through these passages easily.
Fig. 7. Schematic model for morphology of the PU/PEG blend membrane.
3.5. Effect of permeate pressure on the membrane performance Permeate pressure is a prominent parameter providing the driving force for transport of target molecules in the PV process. The main mass transfer driving force in PV separation is the chemical potential difference that occurs by either exerting vacuum or flowing sweep gas at the permeate side. Generally, exerting vacuum is preferred to flowing sweep gas due to its capability of exact regulation and convenient application especially at bench scale [15].
Fig. 8. Effects of permeate pressure on permeation flux at 25 °C.
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Fig. 9. Effect of permeate pressure on enrichment factor at 25 °C.
Effects of permeate pressure on the PU/PEG blend membrane performance are presented in Fig. 8 and Fig. 9. The experiments were carried out at 25 °C and the model gasoline feed contained 2000 ppm sulfur content. Fig. 8 displays the impact of permeate pressure on permeation flux of the membranes with different PU contents. As expected for all the membranes, decreasing permeate pressure enhances permeation flux. As observed, the effect of vacuum pressure on permeation flux is more distinguishable for the membranes with lower PU content because these membranes have more PEG as the dispersed phase and this results in the creation of more passages between the two phases for transporting thiophene and nheptane. Decreasing permeate pressure has more effect on transporting molecules through these passages. Fig. 9 shows the effect of permeate pressure on enrichment factor for the PU60/40 membrane. As observed, decreasing permeate pressure also increases enrichment factor. The activity of the downstream components is affected by permeate pressure directly. Transport of both n-heptane and thiophene become faster due to the larger PV driving force. Solubility and diffusivity of thiophene in this membrane are much greater than those of nheptane and by enhancing the PV driving force, more thiophene can pass through the membrane in comparison to n-heptane. This, results in increasing enrichment factor of the membrane. Accordingly, diminishing permeate pressure enhances both permeation flux and
15
enrichment factor, strongly affects the PV performance, and the maximum efficiency can be obtained for the minimum permeate pressure [11, 17]. 3.6. Effect of feed temperature on the membrane performance Temperature has also a significant impact on the membrane permeability. Fig. 10 reveals the effect of temperature on performance of the PU60/40 membrane with feed sulfur content of 2000 ppm and permeate pressure of 10 mmHg. As expected, the results demonstrate that with increasing temperature permeation flux also increases. This can be explained by two mechanisms. First, the improved saturated vapor pressure of the feed molecules by enhancing
Fig. 10. Effects of feed temperature on performance of PU60/40 membrane.
the feed temperature result in a larger driving force for molecular transport. Second, the enhanced polymer chain motions at elevated feed temperatures, which increases the free volume of polymer matrix. These two phenomena improve membrane permeation flux. In order to accurately investigate the impact of temperature on permeation flux, the Arrhenius equation was applied to calculate the permeation activation energy of two components in the membrane. (6) Where, A is the pre-exponential factor, Ep is the permeation activation energy of PV, R is the universal gas constant and T is absolute temperature [7, 13, 14].
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Fig. 11. Arrhenius plots of thiophene and n-heptane partial fluxes versus feed temperature (PU60/40 membrane).
Both partial permeation fluxes and activation energies of thiophene and n-heptane were calculated. The linear Arrhenius plots for thiophene and n-heptane are presented in Fig. 11. The results clearly reveal that activation energy of n-heptane is greater than that of thiophene (30.68 kJ/mol and 10.14 kJ/mol for n-heptane and thiophene, respectively). As a result of this assessment, n-heptane is more sensitive to temperature than thiophene. At higher temperatures more n-heptane molecules receive activation energy for permeation through the membrane. The influence of temperature on enrichment factor is also displayed in Fig. 10. As exhibited here, a reverse effect on enrichment factor is observed. By enhancing temperature, sorption and diffusion coefficients of both thiophene and n-heptane increase. This can be due to enhancing the mobility of permeating molecules and polymer chains, and also increasing the free volume of polymer which causes the more membrane swelling. This eases transport of the smaller molecules of n-heptane through the membrane which ultimately leads to a decrease of its enrichment factor. As described above, increasing temperature increases permeation flux of n-heptane more than that of thiophene. 3.7. Stability of the membrane in long time operation
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Investigation of operation time on membrane performance is very important in industrialization of membrane processes, and indicates mechanical and chemical strength of the membrane. In this experiment, effects of long time operation up to 120 h on performance of the PU60/40 membrane with feed sulfur content of 2000 ppm at 45°C were examined. The results are presented in Fig. 12 showing that there is a gradual change at the beginning of the experiment, while the PV performance is steady after that, and no considerable fluctuation is observed during the length of the experiment. The first gradual change in the membrane performance can be attributed to the membrane swelling [11, 13]. These results reveal that the membrane has the capability to be used in a scaled up plant.
Fig. 12. Effects of operation time on performance of the PU60/40 membrane at 45°C.
4. Conclusion This work is an attempt to fabricate a proper membrane for deep desulphurization of gasoline. PU and PEG were selected because of their high affinity to sulfuric components estimated by their solubility parameters. PU/PEG blend membrane characteristics and performance were investigated by various experiments and the results revealed a very outstanding and effective separation in comparison to previous works. Blending improved characteristics of both PU and PEG, moreover, using this type of PU resulted in an incredible enrichment factor up to 26.1. In this research membranes were prepared without a support, 18
which is a new research technique in the desulfurization process. Due to the membrane thickness in self-support form, permeation flux values are not exceptional, but using a proper support reduces the membranes thickness and enhances permeation flux. The impacts of the blending ratio on the membrane performance were also investigated. As observed, with increasing the PU content permeation flux diminishes and enrichment factor enhances. The influences of permeation pressure and operation temperature on the membrane performance were also examined. Decreasing permeate pressure increased both permeation flux and enrichment factor, while enhancing feed temperature increased permeation flux and reduced enrichment factor. In Table 4, PV performance of different membranes prepared by other researchers including this work is compared. In this table, permeation flux values were normalized in order to compare the membranes with different thicknesses. It was observed that prepared PU60/40 membrane with its superior performance at 65 °C, which gives the highest pervaporation separation index (PSI) among all fabrication and performance conditions of this work, is a promising membrane for separation of sulfuric compounds from gasoline. Table 4. Comparison between PV performance of the self-support PU60/40 membrane and other works. Feed
Sulfur content (ppm)
Normalized flux (kg µm/m2 h)a
Enrichment factor
Reference
PU/PEG
Thiophene/Nheptane
2000
25.5
7.1
This work
PEG
FCC gasoline
1227
16.3
3.05
[11]
PI
Refinery naphtha
807
9.75
1.44
[25]
PDMS/PEI
Thiophene/Heptane
100
3.2
7.6
[26]
PEBAX/PVDF
Thiophene/Heptane
1000
41.8
4
[7]
PEG/PU
FCC gasoline
1200
37.5
4.03
[15]
a
Membrane
All the fluxes are normalized to the thickness of the membranes.
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Figure Captions:
Fig. 1. A schematic representation of the PV setup. Fig. 2. Surface and cross sectional SEM images of the PU60/40 membrane. Fig. 3. Diffusion (D), sorption (S) and permeation coefficient (P) for thiophene and n-heptane in to the PU40/60 membrane at 25 °C. Fig. 4. FTIR spectra of the PU100/0 membrane. Fig. 5. FTIR spectra of the PU60/40 membrane. Fig. 6. Effects of PU content on the membrane performance at 25 °C. Fig. 7. Schematic model for morphology of the PU/PEG blend membrane. Fig. 8. Effects of permeate pressure on permeation flux at 25 °C. Fig. 9. Effect of permeate pressure on enrichment factor at 25 °C. Fig. 10. Effects of feed temperature on performance of the PU60/40 membrane. Fig. 11. Arrhenius plots of thiophene and n-heptane partial fluxes versus feed temperature (PU60/40 membrane). Fig. 12. Effects of operation time on performance of the PU60/40 membrane at 45°C.
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Table Captions: Table 1. Solubility parameters of polymeric membrane materials, typical hydrocarbons and sulfur species in FCC gasoline. Table 2. The different types of membranes prepared in this work. Table 3. Diffusion (D), sorption (S) and permeation coefficient (P) for thiophene and nheptane in to the PU40/60 membrane at 25 °C. Table 4. Comparison between PV performance of the self-support the PU60/40 membrane and other works.
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Graphical Abstract:
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Research highlights 1. Novel self-support blend membranes for separation of sulfuric components from gasoline for long time operation were prepared. 2. Transcendent enrichment factor and fluxes for membrane desulfurization were obtained. 3. Effects of blending ratio, operation temperatures and permeate pressure on PV performance were investigated. 4. Diffusion coefficient, solubility and permeation coefficient in blend membrane were calculated via sorption test.
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