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water separation by pervaporation dewatering
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Novel porous ceramic tube-supported polymer layer membranes for acetic acid/water separation by pervaporation dewatering Mi Lu, Michael Z. Hu
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Energy and Transportation Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane material synthesis Dewatering membrane Acetic acid dehydration Polybenzimidazole polymer coating membrane Polymer coating on ceramics High flux membranes
New polymer-ceramic hybrid membranes consisting of sulfonated polybenzimidazole (sPBI) coating layer on porous ceramic tube supports have been successfully developed for acetic acid/water separation by selective pervaporation removal of water from the liquid mixtures. The enabling preparation procedure that were critical to form crack/defect-free polymer layer coated intimately on the wall surface of ceramic support tubes is reported. Air humidity during dip coating, the dying method, and pre-mixing of polymer solution were found critical to the intimate adherence, crack-free, and uniformity of the coating membrane layer on the wall of porous ceramic tube. Our hybrid membranes have shown a high-flux selective separation performance data: 2.5–10 times higher flux than previously reported state-of-the-art value (~0.15 LMH) for free-standing sPBI polymer membranes. Also, the ceramic tube-supported membranes provide better mechanical ruggedness, pervaporation stability, and a unique platform for large scale multi-tube membrane module applications.
1. Introduction Acetic acid is one of the most important intermediates in the chemical and the food industries, and as well, an essential intermediate in various aqueous streams generated during biofuel processing via thermochemical (hydrothermal liquefaction and pyrolysis) pathways or biochemical pathways [23,29]. Membrane pervaporation technology for the molecular separation of organic/organic mixtures with close boiling points (especially for azeotropic mixtures) has attracted significant attention since it only requires mild temperatures and pressures [8,15,17,24]. Most recently reported work prepared free-standing polymeric film membranes or small polymeric disc type membranes for pervaporation dehydration of acetic acid-water mixtures studies [1,19,25]. To date, the commercially available polymeric membranes and small disc type polymer-film membranes evaluated in academia have not attained widespread industrial acceptance due to material defects and instability nature of the thin layer membrane in organic/ water mixtures that often cause swelling, corrosion, degradation of membrane performance, and poor mechanical ruggedness [14,23]. An alternate approach, as reported in this paper, is to develop a new type of inorganic supported polymer hybrid nanostructured membranes that may overcome above-mentioned issues [16,21,26]. In the academic testing of a polymer film type of membranes, the application typically requires a porous solid frit flat disc to support the
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fragile free-standing polymer film membrane to reach better mechanical ruggedness or higher-pressure tolerance. It would be better for easy use and long-term membrane performance stability if we could prepare a ceramic-polymer membrane that intrinsically coat the polymer layer onto a rugged porous ceramic support tube. The emergence of robust inorganic membranes of controlled nano pore size has also motivated the development of ceramic-supported polymer membranes in which the coated polymer layer governs the separation performance (e.g., selectivity) of the membrane [22]. In our previous study for ethanol/ water separation, we called the surface-modified porous inorganic membranes as high-performance architecture surface selective (HiPAS) membranes [9]. The polymer surface chains are expected to have greater mobility than fully cross-linked chains, yet the polymer phase is stable even when contacted by liquid mixtures, while the rugged porous inorganic support provides the desired mechanical integrity for the coated polymer layer. The ceramic-polymer hybrid membranes can be suitable not only for the separation of chemical mixtures, but also for catalysis, nanoscience, and electrolyte membrane fuel cells [6,11,20]. For dehydration of acetic acid-water solution mixtures, the hybrid membranes also need to provide sufficient chemical corrosion resistance and thermal stability as well as good mechanical strength to withstand acidic conditions, high pressures, and higher temperatures [5,27,28]. Furthermore, it offers more opportunities to enable large scale membrane fabrication via mature ceramic membrane fabrication,
Corresponding author. E-mail address: [email protected] (M.Z. Hu).
https://doi.org/10.1016/j.seppur.2019.116312 Received 10 September 2019; Received in revised form 8 November 2019; Accepted 8 November 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Mi Lu and Michael Z. Hu, Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116312
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(overnight), then take out of the oven and let it cool down to room temperature in a desiccator to avoid humidity uptake from air. (5) Second coating (to balance the coating thickness on the top and the bottom portion of the tube): Reverse the tube upside down, repeat above dip coating (step 3 and 4). (6) Sulfonation: Soak the entire coated tube in a 5 wt% sulfuric acid aqueous solution at 50 °C for overnight. The sulfuric acid solution is contained in a top-covered volumetric cylinder to avoid acid solution evaporation during sulfonation. After the sulfonation step, there is no observed significant amount of superficial acid drops left on the surface of the tube. (7) Thermal treatment: Insert the sulfonated tube with an inverted T shaped stainless-steel rack, the rack allows the tube stand vertically without contacting the outside coated layer. Place the tube and rack into a furnace (pre-heated at 450 °C) for 90 s. After cooling down to room temperature, the coated tube is soaked in about 1-L roomtemperature water overnight to wash out unreacted sulfuric acid from the sPBI coating. Then, typically the sPBI polymer-coated membrane tube is stored by soaking in water prior to pervaporation testing.
more polymer selection, and even new 3D printing technologies for more membrane module configurations. The common approach proposed in literatures has been to utilize cross-linked and/or blended polymeric membrane materials that can be optimized for specific chemical mixtures and operating conditions [3,5,12,18,22,26,28]. In this work, we prepared a new polymer-ceramic combination of supported polymer membranes for molecular level separations (pervaporation), by permeating (water) and rejecting the dissolved molecular species (acetic acid). The ceramic tube-supported polymer layer membranes were made by dip-coating method, followed by room temperature air drying, vacuum oven-drying, sulfonation, and heat treatment. A thin layer of polybenzimidazole (PBI) polymer solid were coated onto the surface of different porous ceramic supports after the entire coating and treatment procedure. The surface chemistry of the polymer could be tailored, for example, by sulfonation of polybenzimidazole (PBI) to provide hydrophilic property of sPBI that favors more stable higher flux performance for dehydration or dewatering applications with less polymer swelling. Controlling air humidity exposure after dip coating and vacuum drying (instead of regular convective oven drying) were found critical to enable the desirable adhesion between the crack/defect-free polymer layer and the surface of ceramic support.
The graphic summary of synthesis can be found in Fig. 1. Different pore sizes of tubular supports (1, 10, 50, 100 KD titania and 50 KD titania-zirconia) , coating compositions (7 wt% PBI, 10 wt% PBI and 15 wt% PBI in DMAc), and processing conditions (drying in the air and vacuum oven) have been studied for improvement of adherence.
2. Materials and methods 2.1. Materials The commercially formulated viscous liquid of 10 wt% Polybenzimidazole (PBI) in n,n-dimethylacetimide (DMAc) was purchased from PBI Performance Products Inc. Two types of ceramic tubular supports (titania and titania/zirconia based, 250-mm in length, OD 10 mm. with different pore sizes (molecular weight cutoff of 1, 10, 50, 100 KD for titania, and 50 KD for titania/zirconia) are purchased from Sterlitech Corporation. Acetic acid (99.9%), acetone, and ethanol are purchased from Sigma Aldrich.
2.3. Membrane pervaporation evaluation and sample analysis Membrane pervaporation experiments were conducted in a crossflow mode. The typical outer wall sPBI-coated ceramic tubular membrane was assembled into a concentric stainless-steel tube holder. The membrane-holder assembly contains shell side (feed flow) and membrane tube inner lumen side (permeate side), which was connected with an air vaccum pump that can supply vacuum in the range of 1–29 inHg. The temperature-controlled feed (acetic acid aqueous solution) in a reservoir (300-mL three-neck round bottom glass flask) were pumped out by a peristaltic pump and recirculated flowing through the shell side of the membrane-holder assembly so that the flowing feed solution contacts the outer wall of the membrane tube. The acetic acid concentrations of aqueous samples, collected either from the permeate side or the feed side, were determined by a high-performance liquid chromatography (HPLC) analysis using a Shimadzu HPLC system equipped with a UV–VIS Detector (Shimadzu SPD 10-AV) at 208 nm. For the analysis of acetic acid, the samples were separated in an phenomenex column (Rezex ROA organic acid LC column), using 5 mM H2SO4 as the mobile phase at 0.5 mL/min flow and a column temperature of 30 °C. The separation factor of the membrane was calculated using the formula:
2.2. Synthesis methods for polymer-ceramic hybrid membranes The material synthesis method and condition for the sulfonated polybenzimidazole (sPBI) coating on tubular ceramic support were modified according to those for free-standing sPBI polymer membranes [25]. sPBI on ceramic tubular membrane were developed on the different nano porous ceramic tube supports. The sulfonation involves soaking the entire PBI-coated ceramic tube in a 5 wt% H2SO4 aqueous solution. The procedure for preparing sPBI-coated ceramic tubular membranes, in contrast to the free standing PBI membrane fabrication procedure [25], is described as follows: (1) Cleaning of support tube: Wash and dry the porous ceramic tube and heat up to 700 °C to remove any pre-existing organic or polymeric residues. Wash the tube with soap water, ethanol, acetone, and dry in air at room temperature for overnight. (2) Pre-mixing of polymer coating solution: Take a bottle of 10% PBIDMAc liquid out of a refrigerator and prepare it by homogenizing on a horizontal roller for overnight, and then degas by sitting still the bottle for another overnight. (3) Dip coating: Dip coat a dry ceramic support tube (either outer or inner wall) with the homogenized and degassed 10 wt% PBI-DMAc liquid contained in a volumetric cylinder. a) First time dip coating: submerge the ceramic tube vertically into the polymer liquid, soak for 10 min, pull out upward at a controlled speed at 200 mm/mins, let the liquid flowing down the wall until no obvious liquid dripping from the bottom of the tube. (4) Drying treatment after dip coating: a) Air-dry the coated ceramic tube in a chemical fume hood for 10–30 mins until no obvious liquid flow down the surface of the vertically held ceramic tube. b) Dry the coated tube at 125 °C in a vacuum oven for ~16 h
α = (XH2O/X acetic
acid ) perm /(XH2O/X acetic acid ) feed
in which XH2O is the fraction (in wt%) of water and Xacetic acid is the fraction (in wt%) of acetic acid. The concentration of acetic acid in the samples was determined by a HPLC analysis. 2.4. Membrane characterization 2.4.1. SEM SEM (Zeiss Merlin VP) were used for imaging of the polymer film on the ceramic supports. The membrane samples were cut into small pieces with gold sputtering to be able to conduct SEM measurements on the cross-sections. 2.4.2. Contact angle measurement The membrane surface wetting properties were checked by measuring the contact angle by tantec contact angle meter after applying a 2
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Fig. 1. Membrane tube synthesis summary and pervaporation setup.
vacuum oven drying had produced intimate adherence of the PBI solid layer to the ceramic wall. In contrast, the regular oven dried PBI layer detached from the ceramic wall with a big gap. Titania supported sPBI has better adherence with vacuum oven heat treatment than the regular oven. As seen in Fig. 3 (SEM image on the right side), there is no gap between the ceramic porous support and polymer coating layer (~40μm thickness). This drying method study indicates that the moisture in the air during the air drying and regular oven drying process affected the microstructure and density of the dried PBI coating solid. The longer humid air exposure tends to produce a pale whitish colored (less dense) dry PBI layer material in contrast to the dense dark brown colored PBI coating material. This humid-air-drying phenomenon has not been reported elsewhere to our best knowledge, but it is critical to ensure intimate PBI layer adherence to the porous ceramic wall. Second, the thickness of the thin polymer film needs to be controlled very well. We use relatively low concentration 10 wt% PBI-DMAc solution and a large-dimension commercial dip coater with well-controlled submerging and lifting rate at 200 mm/mins. The consistently prepared polymer-ceramic tube membranes were evaluated for their pervaporation dewatering performance (flux and selectivity) on aqueous acidic acid solution feed. All the permeate/feed samples were analyzed by HPLC in determining the acetic acid (AA) concentration for permeate or feed samples generated during the evaluation of various prepared membranes. Membrane pervaporation for 1 h evaluation results on different pore sizes of Titania supports are summarized in Table 1. Major findings from the pervaporation evaluation include: Both 10 KD and 50 KD titania tube supported sPBI layer appeared to be crackfree, intimately adhered coatings to the ceramic tube wall surface. No liquid permeate samples can be obtained at 20 °C and 29 inHg, indicating that the dense polymer coating has little crack or defect pinholes to leak feed solution undesirably. At 60 °C, the sPBI coated 10 KD (Cera10) tubular membranes processed a high acid concentration feed (30 wt% acetic acid) and produced a lower concentration (~10.1 wt%) permeate sample, with high flux ~6.95 LMH. The crack-free sPBI coating on the wall of 50 KD titanic tube is uniform and strongly bonded to form a reflective membrane, normally means the polymers are highly crosslinked, so the acetic acid concentration for the permeate sample was reduced down to 3.26% (separation factor = 12.7) at 60 °C with a flux of ~0.3 LMH. This indicates that 50 KD titania provides the best support for coating a defect-free sPBI polymer layer. The 10 wt% sPBI coating on 1 KD and 100 KD ceramic tubes tend to peel off easily.
~2 mm diameter water droplet. The contact angle degree numbers were averaged by three repeatable measurements. 2.4.3. Parametric sensitivity modeling studies Assume we have initial 1 L of 30 wt% acetic acid in water, and running the continuous pervaporation process, the processing time were calculated by known tested flux and separation factor to see whether selectivity or flux will have more impact on processing time. 3. Results and discussion Modified [25] sPBI coatings on ceramic tubular membrane were developed on the different nano porous ceramic tube supports. Effects of sPBI coating process, such as drying methods, different pore sizes and materials of support on the membrane pervaporation performance were investigated. Ceramic tube materials (titania and/or zirconia) have shown good chemical stability in the sulfuric acid solution during the sulfonation process. The PBI coating is somewhat hydrophilic in nature. The sulfonation showed effective improvement of polymer surface hydrophilicity. This can be verified by contact angle data: 72°for PBI coating surface vs. 43°for sPBI coating surface (Fig. 2). Initial coating trials with the 10 wt% PBI polymer solution to dip coat the wall surfaces of porous ceramic (Titania) tubes proved to be very difficult due to the adherence and crack issues of the polymer coating layer. Thus, the membrane development work was first focused on overcoming the adhesion problem on the corrosion-resistant ceramic support tubes. The adhesion between polymer and porous ceramic surface is always a big challenge to many common coating applications besides membrane fabrication. During air drying of dip coated PBI-DMAc liquid on the tube wall, the polymer tends to absorb water from the humid air and thus produce a dried PBI solid layer with varied porosity, mechanical property, and bonding/detachment behavior. First, we observed that the air exposure and drying method played an important role in determining whether or not a dried PBI coating layer form cracks and peel off from the ceramic tube wall. Herein we compared two different heat drying methods for the same PBI-DMAc coating solution, one with a regular convention oven (connected with room air) and the other one with a vacuum oven. After cutting the 50 KD (kilodalton molecular weight cutoff) Titania tube supported sPBI into smaller sample sizes, the dense 35–45 um thick coated polymer layer can be observed by SEM cross-section imaging (Fig. 3). It was found that the 3
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Fig. 2. (a) Experimental setup for contact angle measurement of polymer coated surface on flat ceramic support material. (b) A water droplet on PBI-coated ceramic plate. (c) A water droplet on sPBI-coated ceramic plate.
zirconia porous support to understand the interaction between the support and polymer. The efficiency of pervaporation with different membranes depends significantly on the pervaporation temperature. The separation performance results of water permeation flux as a function of pervaporation temperatures have been evaluated for both Titania supported sPBI and Zirconia supported sPBI. We found that permeate concentration and the flux of water depends exponentially on the feed temperature. The value of apparent activation energies was calculated based on Arrhenius equation and plotted in Fig. 4:
The average molecular weight of PBI is 35 KD [30], we believe 50 KD pores can selective trap and covered by the highly crosslinked PBI while PBI molecules cannot diffuse into smaller molecular cutoff, such as 1 or 10 KD supports. Note that we also tried 7 wt% and 15 wt% PBI in DMAc as the coating solutions on 50 KD titania, the coating layer with 7 wt% PBI-DMAc (dip coating twice) didn’t seem to crack however had serious leaking when perform pervaporation therefore no selectivity, while the coating layer can be easily peeled off with 15 wt% PBI-DMAc coating. Besides titania support, we also investigated coatings onto titania-
Fig. 3. Cross-section SEM images of tubular ceramic (Titania) supported sPBI layer membrane. The SEM image on the left is for Titania supported sPBI layer dried with a regular convection oven; The SEM image on the right is for Titania supported sPBI layer dried by a vacuum oven after dip coating. The sPBI coating layers in both images are around 40 µm thick.
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Table 1 Pervaporation (PV) testing on sPBI-coated porous ceramic support tube. Sample ID of ceramic tube
Pore size of ceramic tube
Pervaporation temperature, °C
Vacuum pressure, in Hg
Feed conc., wt% of AA
Permeate conc., wt% of AA [at 1 h]
Flux, LMH
Separation factor
Cera1 Cera10
1KD [~ 1 nm] 10 KD [~ 5 nm]
Cera50
50KD [~ 4–25 nm]
NA 10.1% 11.03% NA 3.26% 4.3%
0 6.95 12.5 0 0.3 0.36
NA 3.8 3.5 NA 12.7 9.5
100KD [~ 50–80 nm]
leaked water. 27 27 27 27 27 27 leaked water.
30% 30% 30% 30% 30% 30%
Cera100
Coating peeled off easily and 20 60 70 20 60 70 Coating peeled off easily and
Note: No flux at 20 °C means the liquid feed didn’t penetrate through the coated membrane tube, indicating sPBI-coated ceramic tube has little defects in the dense coating layer.
J = J0 exp (−Ea/ RT ),
factor than the titania-zirconia ceramic tube-supported membranes, while the latter showed a better flux than the former. With titania-zicronia tube-supported sPBI membrane, the water flux remains high even at high concentration of acetic acid as feed. Based on our experimental data, we conducted parametric sensitivity modeling studies to see which factor (flux or selectivity) could reduce more the processing time in Fig. 6. In both graphs, y axis is processing time. For the x axis, the top one is flux (the volumetric flow rate of the permeate), the bottom one is acetic acid concentration of permeate, the more concentrated acetic acid we get from the permeate, the less selectivity and/or separation factor the membrane has. As we all know the flux and selectivity is always a trade-off for membrane separation, but compare with two figures, we can see by increase flux from 0.1 LMH (literature data) to 0.4 LMH (this study) the processing time reduced from 8 days to 2 days. While higher separation factor or selectivity, even when the permeate is pure water, the processing time didn’t reduce more than 1 day. The comparison indicated that at this stage of the HiPAS membrane development, permeation Flux (instead of Selectivity or Separation Factor) is a more sensitive performance parameter which impacts the HiPAS enrichment processing efficiency (i.e., the time needed for concentrating an initial feed of 30 wt% acetic acid to higher concentration). The simulation and our preliminary data provide the guidance on the need of the next generation-new interfaces and HiPAS material chemistry research to address the challenge for water-transport property enhancement. The long term-stability of the membranes is an important parameter to determine their durability in industrial applications. However, this is rarely reported. We further tested long time continuous pervaporation with the 50 KD titania supported sPBI at 70 °C with 27 inHg for 60 h, during which the feed concentration was enriched from 30 wt% to
where J is for flux, also know as volumetric flow rate, with the unit LMH (Liter/m2/h), R for gas constant, and T for temperature. The calculated apparent activation energy Ea for titania-supported sPBI is 62.5 kJ/mol, while titania-zirconia-supported sPBI membrane is 28.3 kJ/mol. Therefore, titania-supported sPBI membrane is more sensitive to pervaporation temperature effect, in other word, flux can be improved significantly by increasing temperature. The values of apparent activation energy for pervaporation transport is usually in the range of 50–60 kJ/mol, depending on the nature of the membrane [10,2]. In literatures, many studies have evaluated titania and zirconia membranes for nanofiltration [4,7], to the best of our knowledge, none of them explained clearly that why these two ceramic materials have different dewatering performance from organic solvent, however, titania-supported membranes show better flux and selectivity than zirconia supported membranes, which is consistent with our results. Dewatering performance data of prepared membranes were collected with starting high feed concentrations (80–10 wt% acetic acid in water). Several findings can be summarized from the coating process and pervaporation experiments: For the titania ceramic tube-supported sPBI membranes (Fig. 5a), permeation flux tends to decrease slightly when the water content in feed decreases (feed concentration increases). In contrast, the titania-zirconia ceramic tube-supported sPBI membranes (Fig. 5b) showed different trend: water flux reached to 0.72 LMH when the water content in feed is 20 wt% (feed concentration is increased to 80 wt%). This is probably because ZrO2 in the ceramic tube is favorable for water to pass the membrane, in another word, more hydrophlic. Under the same pervaporation testing conditions, overall titania tube-supported membranes showed slightly better separation
Fig. 4. Arrhenius plots. Temperature influence on water permeate transport through titania tube supported sPBI membrane (a) and titania-zirconia tube supported sPBI membrane (b). 5
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Fig. 5. Pervaporation dewatering performance with different acetic acid feed concentrations (10, 30, and 50 wt% acetic acid in water). (a) sPBI membrane layer coated on outer wall of titania support tube. (b) sPBI membrane layer coated on outer wall of titania/zirconia ceramic support tube.
[13,22]. In this case, the membrane needed 10 h to reach the steady state. The total flux decreased a little bit since the water content of the feed decreased. After 60 h testing, we washed the membrane tube and repeated pervaporation with feed concentration 30 wt%, the same flux
55 wt% (Fig. 7), the permeate increase from 3 wt% to 10 wt% within first 10 h, and maintain 10 wt% from 10 h to 60 h, separation factor decreased in the first 10 h and then increased accordingly, which is typical swelling and polarization effect on polymer based membrane
Flux effect on processing efficiency Processing time [days]
9 8 7
Range for current acid resistant HiPAS
6 5 4 3 2
State-of-the-art (Literature, free-standing)
1 0 0
0.1
0.2
0.3
0.4
0.5
0.6
Flux [LMH]
Selectivity effect on processing efficiency
9
Processing time [days]
8
Our HIPAS
SF = 10 -15
7 6 5
Higher SF
4 3 2 1 0 0%
2%
4%
6%
8%
10%
12%
Acetic acid conc. of permeate [wt%] Fig. 6. Parametric sensitivity modeling calculations shows where the current HiPAS membrane technology performance is in relative to the literature SOA. It also probes into what materials research needs to be done to address the challenges toward future higher flux (higher process efficiency) membrane dewatering. 6
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Author contributions (Michael Hu) Membrane conceptualization, initial proof-of-principle experiments on polymer coating and pervaporation, obtaining proposal funding, and co-writing the manuscript. (Mi Lu) Conducting membrane preparation and pervaporation experiments, analyzing data, and writing the original draft manuscript. Funding This work is sponsored by the Department of Energy (DOE) in the United States, Bioenergy Technologies Office (BETO), Bioprocessing Separations Consortium program at the Oak Ridge National Laboratory. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgments The authors would like to thank Department of Energy (DOE), Bioenergy Technologies Office (BETO), Bioprocessing Separations Consortium program, for sponsoring Dr. Hu’s membrane research at the Oak Ridge National Laboratory. SEM imaging was conducted at the Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility at ORNL. Authors would also like to thank Dale Hensley (CNMS) for SEM assistance, Mike Gruender (PBI Performance Products) for providing PBI polymers and materials information, and Kensen Hirohata (Sterlitech Corporation) for providing ceramic supports and information. We thank summer intern student Olumide Agboola for assisting contact angle measurements. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.
Fig. 7. Continuous stable cross-flow membrane pervaporation to demonstrate membrane enrichment capability. Feed aqueous solution was concentrated from 30 wt% to 55 wt% acetic acid while permeate maintain around 10 wt% within 60 h (top figure). Flux and separation factors change in 60 h respectively (bottom figure). Pervaporation conditions: 70 °C with vacuum 27 inHg on sPBI coated 50 KD TiO2 membrane tube.
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4. Conclusion In this study, we have developed large dimension porous tubular ceramic supported polymer layer membranes for pervaporation dewatering of acetic acid aqueous solutions, by permeating water vapor while rejecting the dissolved molecular species of acetic acid. The crack defect-free ceramic tube-supported polymer layer membranes are synthesized by the rate-controlled dip-coating method followed by appropriate drying, sulfonation, and heat treatment. It was found critical that the drying of dip coated PBI polymer liquid on the ceramic wall should avoid air humidity exposure and using vacuum drying (instead of regular oven drying) to make crack-free, intimately adhered PBI solid coating layer onto the ceramic wall. Ceramic supports with optimal pore size as well as polymer coating thickness significantly improved the adherence as well as membrane performance (high water flux, separation factor, and stability) for dewatering/dehydration of acetic acid aqueous solutions. Future study will include adding crosslinkers to the polymer solution to make thinner coating layer to achieve better separation factor while maintain high flux. 7
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Novel porous ceramic tube-supported polymer layer membranes for acetic acid/water separation by pervaporation dewatering Mi Lu, Michael Z. Hu
⁎
Energy and Transportation Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane material synthesis Dewatering membrane Acetic acid dehydration Polybenzimidazole polymer coating membrane Polymer coating on ceramics High flux membranes
New polymer-ceramic hybrid membranes consisting of sulfonated polybenzimidazole (sPBI) coating layer on porous ceramic tube supports have been successfully developed for acetic acid/water separation by selective pervaporation removal of water from the liquid mixtures. The enabling preparation procedure that were critical to form crack/defect-free polymer layer coated intimately on the wall surface of ceramic support tubes is reported. Air humidity during dip coating, the dying method, and pre-mixing of polymer solution were found critical to the intimate adherence, crack-free, and uniformity of the coating membrane layer on the wall of porous ceramic tube. Our hybrid membranes have shown a high-flux selective separation performance data: 2.5–10 times higher flux than previously reported state-of-the-art value (~0.15 LMH) for free-standing sPBI polymer membranes. Also, the ceramic tube-supported membranes provide better mechanical ruggedness, pervaporation stability, and a unique platform for large scale multi-tube membrane module applications.
1. Introduction Acetic acid is one of the most important intermediates in the chemical and the food industries, and as well, an essential intermediate in various aqueous streams generated during biofuel processing via thermochemical (hydrothermal liquefaction and pyrolysis) pathways or biochemical pathways [23,29]. Membrane pervaporation technology for the molecular separation of organic/organic mixtures with close boiling points (especially for azeotropic mixtures) has attracted significant attention since it only requires mild temperatures and pressures [8,15,17,24]. Most recently reported work prepared free-standing polymeric film membranes or small polymeric disc type membranes for pervaporation dehydration of acetic acid-water mixtures studies [1,19,25]. To date, the commercially available polymeric membranes and small disc type polymer-film membranes evaluated in academia have not attained widespread industrial acceptance due to material defects and instability nature of the thin layer membrane in organic/ water mixtures that often cause swelling, corrosion, degradation of membrane performance, and poor mechanical ruggedness [14,23]. An alternate approach, as reported in this paper, is to develop a new type of inorganic supported polymer hybrid nanostructured membranes that may overcome above-mentioned issues [16,21,26]. In the academic testing of a polymer film type of membranes, the application typically requires a porous solid frit flat disc to support the
⁎
fragile free-standing polymer film membrane to reach better mechanical ruggedness or higher-pressure tolerance. It would be better for easy use and long-term membrane performance stability if we could prepare a ceramic-polymer membrane that intrinsically coat the polymer layer onto a rugged porous ceramic support tube. The emergence of robust inorganic membranes of controlled nano pore size has also motivated the development of ceramic-supported polymer membranes in which the coated polymer layer governs the separation performance (e.g., selectivity) of the membrane [22]. In our previous study for ethanol/ water separation, we called the surface-modified porous inorganic membranes as high-performance architecture surface selective (HiPAS) membranes [9]. The polymer surface chains are expected to have greater mobility than fully cross-linked chains, yet the polymer phase is stable even when contacted by liquid mixtures, while the rugged porous inorganic support provides the desired mechanical integrity for the coated polymer layer. The ceramic-polymer hybrid membranes can be suitable not only for the separation of chemical mixtures, but also for catalysis, nanoscience, and electrolyte membrane fuel cells [6,11,20]. For dehydration of acetic acid-water solution mixtures, the hybrid membranes also need to provide sufficient chemical corrosion resistance and thermal stability as well as good mechanical strength to withstand acidic conditions, high pressures, and higher temperatures [5,27,28]. Furthermore, it offers more opportunities to enable large scale membrane fabrication via mature ceramic membrane fabrication,
Corresponding author. E-mail address: [email protected] (M.Z. Hu).
https://doi.org/10.1016/j.seppur.2019.116312 Received 10 September 2019; Received in revised form 8 November 2019; Accepted 8 November 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Mi Lu and Michael Z. Hu, Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116312
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(overnight), then take out of the oven and let it cool down to room temperature in a desiccator to avoid humidity uptake from air. (5) Second coating (to balance the coating thickness on the top and the bottom portion of the tube): Reverse the tube upside down, repeat above dip coating (step 3 and 4). (6) Sulfonation: Soak the entire coated tube in a 5 wt% sulfuric acid aqueous solution at 50 °C for overnight. The sulfuric acid solution is contained in a top-covered volumetric cylinder to avoid acid solution evaporation during sulfonation. After the sulfonation step, there is no observed significant amount of superficial acid drops left on the surface of the tube. (7) Thermal treatment: Insert the sulfonated tube with an inverted T shaped stainless-steel rack, the rack allows the tube stand vertically without contacting the outside coated layer. Place the tube and rack into a furnace (pre-heated at 450 °C) for 90 s. After cooling down to room temperature, the coated tube is soaked in about 1-L roomtemperature water overnight to wash out unreacted sulfuric acid from the sPBI coating. Then, typically the sPBI polymer-coated membrane tube is stored by soaking in water prior to pervaporation testing.
more polymer selection, and even new 3D printing technologies for more membrane module configurations. The common approach proposed in literatures has been to utilize cross-linked and/or blended polymeric membrane materials that can be optimized for specific chemical mixtures and operating conditions [3,5,12,18,22,26,28]. In this work, we prepared a new polymer-ceramic combination of supported polymer membranes for molecular level separations (pervaporation), by permeating (water) and rejecting the dissolved molecular species (acetic acid). The ceramic tube-supported polymer layer membranes were made by dip-coating method, followed by room temperature air drying, vacuum oven-drying, sulfonation, and heat treatment. A thin layer of polybenzimidazole (PBI) polymer solid were coated onto the surface of different porous ceramic supports after the entire coating and treatment procedure. The surface chemistry of the polymer could be tailored, for example, by sulfonation of polybenzimidazole (PBI) to provide hydrophilic property of sPBI that favors more stable higher flux performance for dehydration or dewatering applications with less polymer swelling. Controlling air humidity exposure after dip coating and vacuum drying (instead of regular convective oven drying) were found critical to enable the desirable adhesion between the crack/defect-free polymer layer and the surface of ceramic support.
The graphic summary of synthesis can be found in Fig. 1. Different pore sizes of tubular supports (1, 10, 50, 100 KD titania and 50 KD titania-zirconia) , coating compositions (7 wt% PBI, 10 wt% PBI and 15 wt% PBI in DMAc), and processing conditions (drying in the air and vacuum oven) have been studied for improvement of adherence.
2. Materials and methods 2.1. Materials The commercially formulated viscous liquid of 10 wt% Polybenzimidazole (PBI) in n,n-dimethylacetimide (DMAc) was purchased from PBI Performance Products Inc. Two types of ceramic tubular supports (titania and titania/zirconia based, 250-mm in length, OD 10 mm. with different pore sizes (molecular weight cutoff of 1, 10, 50, 100 KD for titania, and 50 KD for titania/zirconia) are purchased from Sterlitech Corporation. Acetic acid (99.9%), acetone, and ethanol are purchased from Sigma Aldrich.
2.3. Membrane pervaporation evaluation and sample analysis Membrane pervaporation experiments were conducted in a crossflow mode. The typical outer wall sPBI-coated ceramic tubular membrane was assembled into a concentric stainless-steel tube holder. The membrane-holder assembly contains shell side (feed flow) and membrane tube inner lumen side (permeate side), which was connected with an air vaccum pump that can supply vacuum in the range of 1–29 inHg. The temperature-controlled feed (acetic acid aqueous solution) in a reservoir (300-mL three-neck round bottom glass flask) were pumped out by a peristaltic pump and recirculated flowing through the shell side of the membrane-holder assembly so that the flowing feed solution contacts the outer wall of the membrane tube. The acetic acid concentrations of aqueous samples, collected either from the permeate side or the feed side, were determined by a high-performance liquid chromatography (HPLC) analysis using a Shimadzu HPLC system equipped with a UV–VIS Detector (Shimadzu SPD 10-AV) at 208 nm. For the analysis of acetic acid, the samples were separated in an phenomenex column (Rezex ROA organic acid LC column), using 5 mM H2SO4 as the mobile phase at 0.5 mL/min flow and a column temperature of 30 °C. The separation factor of the membrane was calculated using the formula:
2.2. Synthesis methods for polymer-ceramic hybrid membranes The material synthesis method and condition for the sulfonated polybenzimidazole (sPBI) coating on tubular ceramic support were modified according to those for free-standing sPBI polymer membranes [25]. sPBI on ceramic tubular membrane were developed on the different nano porous ceramic tube supports. The sulfonation involves soaking the entire PBI-coated ceramic tube in a 5 wt% H2SO4 aqueous solution. The procedure for preparing sPBI-coated ceramic tubular membranes, in contrast to the free standing PBI membrane fabrication procedure [25], is described as follows: (1) Cleaning of support tube: Wash and dry the porous ceramic tube and heat up to 700 °C to remove any pre-existing organic or polymeric residues. Wash the tube with soap water, ethanol, acetone, and dry in air at room temperature for overnight. (2) Pre-mixing of polymer coating solution: Take a bottle of 10% PBIDMAc liquid out of a refrigerator and prepare it by homogenizing on a horizontal roller for overnight, and then degas by sitting still the bottle for another overnight. (3) Dip coating: Dip coat a dry ceramic support tube (either outer or inner wall) with the homogenized and degassed 10 wt% PBI-DMAc liquid contained in a volumetric cylinder. a) First time dip coating: submerge the ceramic tube vertically into the polymer liquid, soak for 10 min, pull out upward at a controlled speed at 200 mm/mins, let the liquid flowing down the wall until no obvious liquid dripping from the bottom of the tube. (4) Drying treatment after dip coating: a) Air-dry the coated ceramic tube in a chemical fume hood for 10–30 mins until no obvious liquid flow down the surface of the vertically held ceramic tube. b) Dry the coated tube at 125 °C in a vacuum oven for ~16 h
α = (XH2O/X acetic
acid ) perm /(XH2O/X acetic acid ) feed
in which XH2O is the fraction (in wt%) of water and Xacetic acid is the fraction (in wt%) of acetic acid. The concentration of acetic acid in the samples was determined by a HPLC analysis. 2.4. Membrane characterization 2.4.1. SEM SEM (Zeiss Merlin VP) were used for imaging of the polymer film on the ceramic supports. The membrane samples were cut into small pieces with gold sputtering to be able to conduct SEM measurements on the cross-sections. 2.4.2. Contact angle measurement The membrane surface wetting properties were checked by measuring the contact angle by tantec contact angle meter after applying a 2
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Fig. 1. Membrane tube synthesis summary and pervaporation setup.
vacuum oven drying had produced intimate adherence of the PBI solid layer to the ceramic wall. In contrast, the regular oven dried PBI layer detached from the ceramic wall with a big gap. Titania supported sPBI has better adherence with vacuum oven heat treatment than the regular oven. As seen in Fig. 3 (SEM image on the right side), there is no gap between the ceramic porous support and polymer coating layer (~40μm thickness). This drying method study indicates that the moisture in the air during the air drying and regular oven drying process affected the microstructure and density of the dried PBI coating solid. The longer humid air exposure tends to produce a pale whitish colored (less dense) dry PBI layer material in contrast to the dense dark brown colored PBI coating material. This humid-air-drying phenomenon has not been reported elsewhere to our best knowledge, but it is critical to ensure intimate PBI layer adherence to the porous ceramic wall. Second, the thickness of the thin polymer film needs to be controlled very well. We use relatively low concentration 10 wt% PBI-DMAc solution and a large-dimension commercial dip coater with well-controlled submerging and lifting rate at 200 mm/mins. The consistently prepared polymer-ceramic tube membranes were evaluated for their pervaporation dewatering performance (flux and selectivity) on aqueous acidic acid solution feed. All the permeate/feed samples were analyzed by HPLC in determining the acetic acid (AA) concentration for permeate or feed samples generated during the evaluation of various prepared membranes. Membrane pervaporation for 1 h evaluation results on different pore sizes of Titania supports are summarized in Table 1. Major findings from the pervaporation evaluation include: Both 10 KD and 50 KD titania tube supported sPBI layer appeared to be crackfree, intimately adhered coatings to the ceramic tube wall surface. No liquid permeate samples can be obtained at 20 °C and 29 inHg, indicating that the dense polymer coating has little crack or defect pinholes to leak feed solution undesirably. At 60 °C, the sPBI coated 10 KD (Cera10) tubular membranes processed a high acid concentration feed (30 wt% acetic acid) and produced a lower concentration (~10.1 wt%) permeate sample, with high flux ~6.95 LMH. The crack-free sPBI coating on the wall of 50 KD titanic tube is uniform and strongly bonded to form a reflective membrane, normally means the polymers are highly crosslinked, so the acetic acid concentration for the permeate sample was reduced down to 3.26% (separation factor = 12.7) at 60 °C with a flux of ~0.3 LMH. This indicates that 50 KD titania provides the best support for coating a defect-free sPBI polymer layer. The 10 wt% sPBI coating on 1 KD and 100 KD ceramic tubes tend to peel off easily.
~2 mm diameter water droplet. The contact angle degree numbers were averaged by three repeatable measurements. 2.4.3. Parametric sensitivity modeling studies Assume we have initial 1 L of 30 wt% acetic acid in water, and running the continuous pervaporation process, the processing time were calculated by known tested flux and separation factor to see whether selectivity or flux will have more impact on processing time. 3. Results and discussion Modified [25] sPBI coatings on ceramic tubular membrane were developed on the different nano porous ceramic tube supports. Effects of sPBI coating process, such as drying methods, different pore sizes and materials of support on the membrane pervaporation performance were investigated. Ceramic tube materials (titania and/or zirconia) have shown good chemical stability in the sulfuric acid solution during the sulfonation process. The PBI coating is somewhat hydrophilic in nature. The sulfonation showed effective improvement of polymer surface hydrophilicity. This can be verified by contact angle data: 72°for PBI coating surface vs. 43°for sPBI coating surface (Fig. 2). Initial coating trials with the 10 wt% PBI polymer solution to dip coat the wall surfaces of porous ceramic (Titania) tubes proved to be very difficult due to the adherence and crack issues of the polymer coating layer. Thus, the membrane development work was first focused on overcoming the adhesion problem on the corrosion-resistant ceramic support tubes. The adhesion between polymer and porous ceramic surface is always a big challenge to many common coating applications besides membrane fabrication. During air drying of dip coated PBI-DMAc liquid on the tube wall, the polymer tends to absorb water from the humid air and thus produce a dried PBI solid layer with varied porosity, mechanical property, and bonding/detachment behavior. First, we observed that the air exposure and drying method played an important role in determining whether or not a dried PBI coating layer form cracks and peel off from the ceramic tube wall. Herein we compared two different heat drying methods for the same PBI-DMAc coating solution, one with a regular convention oven (connected with room air) and the other one with a vacuum oven. After cutting the 50 KD (kilodalton molecular weight cutoff) Titania tube supported sPBI into smaller sample sizes, the dense 35–45 um thick coated polymer layer can be observed by SEM cross-section imaging (Fig. 3). It was found that the 3
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Fig. 2. (a) Experimental setup for contact angle measurement of polymer coated surface on flat ceramic support material. (b) A water droplet on PBI-coated ceramic plate. (c) A water droplet on sPBI-coated ceramic plate.
zirconia porous support to understand the interaction between the support and polymer. The efficiency of pervaporation with different membranes depends significantly on the pervaporation temperature. The separation performance results of water permeation flux as a function of pervaporation temperatures have been evaluated for both Titania supported sPBI and Zirconia supported sPBI. We found that permeate concentration and the flux of water depends exponentially on the feed temperature. The value of apparent activation energies was calculated based on Arrhenius equation and plotted in Fig. 4:
The average molecular weight of PBI is 35 KD [30], we believe 50 KD pores can selective trap and covered by the highly crosslinked PBI while PBI molecules cannot diffuse into smaller molecular cutoff, such as 1 or 10 KD supports. Note that we also tried 7 wt% and 15 wt% PBI in DMAc as the coating solutions on 50 KD titania, the coating layer with 7 wt% PBI-DMAc (dip coating twice) didn’t seem to crack however had serious leaking when perform pervaporation therefore no selectivity, while the coating layer can be easily peeled off with 15 wt% PBI-DMAc coating. Besides titania support, we also investigated coatings onto titania-
Fig. 3. Cross-section SEM images of tubular ceramic (Titania) supported sPBI layer membrane. The SEM image on the left is for Titania supported sPBI layer dried with a regular convection oven; The SEM image on the right is for Titania supported sPBI layer dried by a vacuum oven after dip coating. The sPBI coating layers in both images are around 40 µm thick.
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Table 1 Pervaporation (PV) testing on sPBI-coated porous ceramic support tube. Sample ID of ceramic tube
Pore size of ceramic tube
Pervaporation temperature, °C
Vacuum pressure, in Hg
Feed conc., wt% of AA
Permeate conc., wt% of AA [at 1 h]
Flux, LMH
Separation factor
Cera1 Cera10
1KD [~ 1 nm] 10 KD [~ 5 nm]
Cera50
50KD [~ 4–25 nm]
NA 10.1% 11.03% NA 3.26% 4.3%
0 6.95 12.5 0 0.3 0.36
NA 3.8 3.5 NA 12.7 9.5
100KD [~ 50–80 nm]
leaked water. 27 27 27 27 27 27 leaked water.
30% 30% 30% 30% 30% 30%
Cera100
Coating peeled off easily and 20 60 70 20 60 70 Coating peeled off easily and
Note: No flux at 20 °C means the liquid feed didn’t penetrate through the coated membrane tube, indicating sPBI-coated ceramic tube has little defects in the dense coating layer.
J = J0 exp (−Ea/ RT ),
factor than the titania-zirconia ceramic tube-supported membranes, while the latter showed a better flux than the former. With titania-zicronia tube-supported sPBI membrane, the water flux remains high even at high concentration of acetic acid as feed. Based on our experimental data, we conducted parametric sensitivity modeling studies to see which factor (flux or selectivity) could reduce more the processing time in Fig. 6. In both graphs, y axis is processing time. For the x axis, the top one is flux (the volumetric flow rate of the permeate), the bottom one is acetic acid concentration of permeate, the more concentrated acetic acid we get from the permeate, the less selectivity and/or separation factor the membrane has. As we all know the flux and selectivity is always a trade-off for membrane separation, but compare with two figures, we can see by increase flux from 0.1 LMH (literature data) to 0.4 LMH (this study) the processing time reduced from 8 days to 2 days. While higher separation factor or selectivity, even when the permeate is pure water, the processing time didn’t reduce more than 1 day. The comparison indicated that at this stage of the HiPAS membrane development, permeation Flux (instead of Selectivity or Separation Factor) is a more sensitive performance parameter which impacts the HiPAS enrichment processing efficiency (i.e., the time needed for concentrating an initial feed of 30 wt% acetic acid to higher concentration). The simulation and our preliminary data provide the guidance on the need of the next generation-new interfaces and HiPAS material chemistry research to address the challenge for water-transport property enhancement. The long term-stability of the membranes is an important parameter to determine their durability in industrial applications. However, this is rarely reported. We further tested long time continuous pervaporation with the 50 KD titania supported sPBI at 70 °C with 27 inHg for 60 h, during which the feed concentration was enriched from 30 wt% to
where J is for flux, also know as volumetric flow rate, with the unit LMH (Liter/m2/h), R for gas constant, and T for temperature. The calculated apparent activation energy Ea for titania-supported sPBI is 62.5 kJ/mol, while titania-zirconia-supported sPBI membrane is 28.3 kJ/mol. Therefore, titania-supported sPBI membrane is more sensitive to pervaporation temperature effect, in other word, flux can be improved significantly by increasing temperature. The values of apparent activation energy for pervaporation transport is usually in the range of 50–60 kJ/mol, depending on the nature of the membrane [10,2]. In literatures, many studies have evaluated titania and zirconia membranes for nanofiltration [4,7], to the best of our knowledge, none of them explained clearly that why these two ceramic materials have different dewatering performance from organic solvent, however, titania-supported membranes show better flux and selectivity than zirconia supported membranes, which is consistent with our results. Dewatering performance data of prepared membranes were collected with starting high feed concentrations (80–10 wt% acetic acid in water). Several findings can be summarized from the coating process and pervaporation experiments: For the titania ceramic tube-supported sPBI membranes (Fig. 5a), permeation flux tends to decrease slightly when the water content in feed decreases (feed concentration increases). In contrast, the titania-zirconia ceramic tube-supported sPBI membranes (Fig. 5b) showed different trend: water flux reached to 0.72 LMH when the water content in feed is 20 wt% (feed concentration is increased to 80 wt%). This is probably because ZrO2 in the ceramic tube is favorable for water to pass the membrane, in another word, more hydrophlic. Under the same pervaporation testing conditions, overall titania tube-supported membranes showed slightly better separation
Fig. 4. Arrhenius plots. Temperature influence on water permeate transport through titania tube supported sPBI membrane (a) and titania-zirconia tube supported sPBI membrane (b). 5
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Fig. 5. Pervaporation dewatering performance with different acetic acid feed concentrations (10, 30, and 50 wt% acetic acid in water). (a) sPBI membrane layer coated on outer wall of titania support tube. (b) sPBI membrane layer coated on outer wall of titania/zirconia ceramic support tube.
[13,22]. In this case, the membrane needed 10 h to reach the steady state. The total flux decreased a little bit since the water content of the feed decreased. After 60 h testing, we washed the membrane tube and repeated pervaporation with feed concentration 30 wt%, the same flux
55 wt% (Fig. 7), the permeate increase from 3 wt% to 10 wt% within first 10 h, and maintain 10 wt% from 10 h to 60 h, separation factor decreased in the first 10 h and then increased accordingly, which is typical swelling and polarization effect on polymer based membrane
Flux effect on processing efficiency Processing time [days]
9 8 7
Range for current acid resistant HiPAS
6 5 4 3 2
State-of-the-art (Literature, free-standing)
1 0 0
0.1
0.2
0.3
0.4
0.5
0.6
Flux [LMH]
Selectivity effect on processing efficiency
9
Processing time [days]
8
Our HIPAS
SF = 10 -15
7 6 5
Higher SF
4 3 2 1 0 0%
2%
4%
6%
8%
10%
12%
Acetic acid conc. of permeate [wt%] Fig. 6. Parametric sensitivity modeling calculations shows where the current HiPAS membrane technology performance is in relative to the literature SOA. It also probes into what materials research needs to be done to address the challenges toward future higher flux (higher process efficiency) membrane dewatering. 6
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Author contributions (Michael Hu) Membrane conceptualization, initial proof-of-principle experiments on polymer coating and pervaporation, obtaining proposal funding, and co-writing the manuscript. (Mi Lu) Conducting membrane preparation and pervaporation experiments, analyzing data, and writing the original draft manuscript. Funding This work is sponsored by the Department of Energy (DOE) in the United States, Bioenergy Technologies Office (BETO), Bioprocessing Separations Consortium program at the Oak Ridge National Laboratory. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgments The authors would like to thank Department of Energy (DOE), Bioenergy Technologies Office (BETO), Bioprocessing Separations Consortium program, for sponsoring Dr. Hu’s membrane research at the Oak Ridge National Laboratory. SEM imaging was conducted at the Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility at ORNL. Authors would also like to thank Dale Hensley (CNMS) for SEM assistance, Mike Gruender (PBI Performance Products) for providing PBI polymers and materials information, and Kensen Hirohata (Sterlitech Corporation) for providing ceramic supports and information. We thank summer intern student Olumide Agboola for assisting contact angle measurements. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.
Fig. 7. Continuous stable cross-flow membrane pervaporation to demonstrate membrane enrichment capability. Feed aqueous solution was concentrated from 30 wt% to 55 wt% acetic acid while permeate maintain around 10 wt% within 60 h (top figure). Flux and separation factors change in 60 h respectively (bottom figure). Pervaporation conditions: 70 °C with vacuum 27 inHg on sPBI coated 50 KD TiO2 membrane tube.
References
and separation factor obtained, which indicates the membrane remained chemically/thermally/mechanically stable with the concentrated feed and increasing process time.
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4. Conclusion In this study, we have developed large dimension porous tubular ceramic supported polymer layer membranes for pervaporation dewatering of acetic acid aqueous solutions, by permeating water vapor while rejecting the dissolved molecular species of acetic acid. The crack defect-free ceramic tube-supported polymer layer membranes are synthesized by the rate-controlled dip-coating method followed by appropriate drying, sulfonation, and heat treatment. It was found critical that the drying of dip coated PBI polymer liquid on the ceramic wall should avoid air humidity exposure and using vacuum drying (instead of regular oven drying) to make crack-free, intimately adhered PBI solid coating layer onto the ceramic wall. Ceramic supports with optimal pore size as well as polymer coating thickness significantly improved the adherence as well as membrane performance (high water flux, separation factor, and stability) for dewatering/dehydration of acetic acid aqueous solutions. Future study will include adding crosslinkers to the polymer solution to make thinner coating layer to achieve better separation factor while maintain high flux. 7
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