Scale-up of amine-containing thin-film composite membranes for CO2 capture from flue gas

Journal of Membrane Science 555 (2018) 379–387

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Scale-up of amine-containing thin-film composite membranes for CO2 capture from flue gas

T

Varun Vakharia, Witopo Salim, Dongzhu Wu, Yang Han, Yuanxin Chen, Lin Zhao, ⁎ W.S. Winston Ho William G. Lowrie Department of Chemical and Biomolecular Engineering and Department of Materials Science and Engineering, The Ohio State University, 151 West Woodruff Avenue, Columbus, OH 43210-1350, United States

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2-selective amine-containing membrane Membrane scale-up Continuous roll-to-roll fabrication Pilot-scale thin-film coating

Membrane technology is a cost-effective and energy-efficient separation process for CO2 removal from flue gas. Recently, there have been remarkable advances in the development of polyvinylamine (PVAm)/piperazine glycinate (PG)-containing facilitated transport membranes for CO2 removal. The advanced PVAm/PG-containing membranes have demonstrated desirably high CO2/N2 selectivity of greater than 140 and good CO2 permeance of > 700 GPU (1 GPU = 10−6 cm3 (STP)/(s cm2 cmHg)) at the typical flue gas temperature of 57 °C. This work investigated the scale-up fabrication of the PVAm/PG-containing membranes and demonstrated it successfully. Effective and efficient continuous fabrication of the membranes in a roll-to-roll manner was developed using the thin-film coating (TFC) assembly of the pilot-scale machine at Ohio State. The design of the TFC assembly was improved and several operating parameters were optimized for the fabrication with desirable membrane thickness and gas transport performance. A total of > 2000 feet long and 14-in. wide membranes with a selective layer thickness of < 200 nm was successfully fabricated. In addition, small representative samples taken from the scale-up membranes were tested, showing the CO2 permeance and CO2/N2 selectivity at 57 °C in reasonably good agreement with those obtained from the lab-synthesized membranes.

1. Introduction Recently, the electricity generation in USA using natural gas as the fuel surpassed the quantity of electricity generated from the coal-fired power plants [1–3]. This major transformation was believed to be due to two factors. First, the significant drop in price of natural gas (in the last 8 years) has made it economically feasible to use natural gas as the fuel for power generation [4,5]. Second, a less amount of CO2 is released into the atmosphere using natural gas as the fuel for power generation as compared to coal. This aids to significantly mitigate the global warming concerns. However, greater than 30% of electricity in the United States still comes from the coal-fired power plants. Therefore, it is imperative to capture the CO2 from the flue gas in coal-fired power plants in addition to that from the power plants using natural gas. There are several processes that can be considered for CO2 removal from flue gas. But, the state-of-the-art absorption processes using amines (e.g., monoethanolamine) for carbon capture from flue gas are costly and energy-intensive. This is mainly due to high capital costs, numerous operational concerns, and high energy penalty [6–8]. Since

⁎

the last decade, there have been significant developments in the field of membrane technology for gas separation. Membrane technology is a cost-effective and energy-efficient separation process for CO2 removal due to low capital cost, low energy penalty, and operational simplicity relative to other competitive technologies [9–11]. Polymer membranes have been studied extensively for CO2 removal both at low and high temperatures. The state-of-the-art Polaris™ membranes from Membrane Technology and Research Inc. (MTR) have demonstrated impressive CO2 permeances (1000–3000 GPU, 1 GPU = 10−6 cm3(STP)/(s cm2 cmHg)) with a moderate CO2/N2 selectivity (20–50) [12,13]. These membranes separate the CO2 molecules via the solution-diffusion transport mechanism. On the other hand, facilitated transport membranes can provide significantly higher CO2/N2 selectivity (100–1500) and high CO2 permeance via the reaction-diffusion transport mechanism [13–18]. Recently, there have been remarkable advances using amine-containing facilitated transport membranes, e.g., polyvinylamine (PVAm)/piperazine glycinate (PG), at 57 °C. These membranes have demonstrated very high CO2/N2 selectivity of greater than 140 and good CO2 permeance of > 700 GPU in the laboratory scale at ~ 1 bar feed pressure and 57 °C [13–18].

Corresponding author. E-mail address: [email protected] (W.S.W. Ho).

https://doi.org/10.1016/j.memsci.2018.03.074 Received 11 December 2017; Received in revised form 23 March 2018; Accepted 25 March 2018 Available online 27 March 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.

Journal of Membrane Science 555 (2018) 379–387

V. Vakharia et al.

Nomenclature b c DH f h K ℓ ℓgap n

P Q ReH S tw v w Z Δhf ρ ρdry τw

opening (gap) of the channel for the coating solution flow total solids concentration of the coating solution (by weight) effective hydraulic diameter [(4*flow channel area)/(flow channel perimeter)] friction factor for laminar flow flow channel height (height of the insert) consistency constant for the power law model selective-layer membrane thickness gap setting of the coating knife power law index

(PSf) and polyethersulfone (PES). Moreover, the adhesion of the coating solution was improved by incorporating a calculated amount of surfactant sodium dodecyl sulfate (SDS) to the PVAm/PG blend. The PVAm/PG-containing membrane is shown schematically in Fig. 2. Ultra-thin membranes with selective layer thickness < 200 nm were successfully synthesized in the laboratory. These membranes were coated in the laboratory using the knife coating technique (Fig. 3). The PVAm/PG coating solution with a known total solids concentration was coated on top of the substrate using a GARDCO adjustable micrometer film applicator (Paul N. Gardner Company, Pompano Beach, FL). The clearance between the tip of the knife and the top of the substrate (gap setting) was controlled precisely to obtain a wet-film thickness, which after drying resulted in a dry selective layer of the membrane. The empirical equation (Eq. (3)) was used to determine the gap setting of the coating knife in order to control the final thickness of the dry membrane.

This work is an extension of the previous work by Chen et al. that reported the synthesis and transport characterization of the advanced PVAm/PG-containing nanocomposite membranes in the laboratory scale [16]. However, this work focusses on the scale-up fabrication of the PVAm/PG-containing membranes. Effective and efficient continuous fabrication of the membranes in a roll-to-roll manner was developed using the pilot-scale machine at The Ohio State University (OSU). This is a first-of-kind work that systematically illustrates the pilot-scale technique for the continuous fabrication of flat-sheet membranes for gas separation. Various design parameters were investigated and several operating conditions were optimized to develop an effective coating process for the pilot-scale continuous membrane fabrication.

2. PVAm/PG-containing facilitated transport membranes In recent years, Ho and his coworkers have extensively investigated the use of amines as CO2 carries for facilitated transport membranes [14–26]. These membranes have provided remarkably high CO2/N2 selectivity and reasonably high CO2 permeance. This is mainly due to the reaction-diffusion transport mechanism with the presence of amines as the CO2 carriers. This mechanism for gas transport is described below and schematically shown in Fig. 1. CO2 molecules in the feed gas react reversibly with the carrier molecules (mobile and fixed-site carriers) that are well dispersed in the membrane matrix. The CO2-carrier reaction product diffuses from the feed side to the sweep side of the membrane driven by the concentration gradient. The CO2 molecules are released on the sweep side via the reversible reactions (Eqs. (1) and (2)) due to a low partial pressure of CO2 on the sweep side. This mechanism helps to significantly enhance the CO2 solubility in the selective layer of the membrane and thereby obtain a high flux for CO2 transport [25–27]. The non-reactive gases (e.g., N2, H2, CH4, and others) can only transport through the membrane via the solution-diffusion transport mechanism. CO2 + 2 R-NH2 ⇌ R-NH3+ + R-NH-COO− CO2 + R-NH2 + H2O ⇌

R-NH3+

+

HCO3−

pressure delivery rate of the coating solution modified Reynolds number coating speed of the substrate wet film thickness of the membrane average velocity of the coating solution width of the channel elevation of the point above reference frictional head loss along the height of the insert density of the coating solution density of the dry membrane average wall shear stress

ℓρdry = 0.5cρ ℓgap

(3)

where ℓ (nm) is the dry PVAm/PG membrane thickness, ρdry (g/cm3) is the density of the dry PVAm/PG membrane, ρ (g/cm3) is the density of the coating solution, c (% by weight) is the total solids concentration of the coating solution, and ℓgap (nm) is the gap setting of the coating knife. The transport performances of the PVAm/PG-containing membranes obtained by Chen et al. along with the test conditions are summarized in Table 1 [16]. These membranes were prepared from a coating solution with a viscosity of 1180 cp, 719,000 MW PVAm, the PG/PVAm weight ratio of 65/35, the amine-containing layer thickness

(1) (2)

In the previous work, Chen et al. investigated the use of PVAm as the fixed-site carrier and the amino acid salt PG as the mobile carrier for amine-containing facilitated transport membranes [15,16]. The use of PG as the mobile carrier was believed to enhance the CO2 transport by increasing the mobility of the CO2-carrier reaction product. Moreover, the presence of two amino groups per unit carrier molecule (2 amino groups for every PG molecule) helped enhance the CO2 facilitated transport in the membrane, resulting in a higher CO2 permeance. Different molecular weights of PVAm were synthesized via the free radical polymerization reaction. The amino acid salt PG (mobile carrier) was incorporated into the PVAm solution (fixed-site carrier) to form the coating solution for membrane synthesis. The PVAm/PG solution was coated on top of different nanoporous substrates, including polysulfone

Fig. 1. Schematic representation of the gas transport mechanism using the facilitated transport membrane. 380

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Fig. 2. Schematic of the PVAm/PG-containing facilitated transport membrane coated on top of a porous substrate.

the samples can be rolled and stored in controlled humidity and temperature conditions. 3. Experimental 3.1. Materials The materials used for the synthesis of the coating solution for the pilot-scale membrane fabrication are given here. Glycine (98.5%), piperazine (99%), N-vinylformamide (NVF, 98%), α,α′-azodiisobutyramidine dihydrochloride (AIBA, 97%), and sodium dodecyl sulfate (SDS, 99%) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, Certificated ACS Plus) and ethanol (99.5%) were bought from Fisher Scientific. Strong base anion-exchange resin (Purolite® A600OH) was kindly donated by Purolite Corporation (Bala Cynwyd, PA, USA). All the chemicals were used as received without further purification. Polyethersulfone (PES) substrates (14 in. wide) prepared in-house using the pilot-scale casting machine were used for the pilot-scale membrane fabrication. The information related to the substrate fabrication process and the substrate characterization is published in the work by Wu et al. [34].

Fig. 3. Schematic of the knife coating technique to coat a thin film of polymer solution on top of a porous substrate (taped over a glass plate). Table 1 The transport performances of lab-synthesized PVAm/PG-containing membranes coated on lab-fabricated PES substrates [16]. Temperature (°C)

Feed water vapor content (%)

Sweep Water vapor content (%)

CO2 permeance (GPU)

CO2/N2 selectivity

57 57 57

17 17 17

17 17 17

734 750 753

136 145 148

3.2. Coating solution preparation The same procedure was followed for the preparation of the PVAm/ PG-containing coating solution as developed by Chen et al. in the previous work [16]. The PVAm (fixed-site carrier) was synthesized via the free radical polymerization reaction. NVF was used as the monomer and AIBA as the initiator. The polymerization reaction was carried out under nitrogen atmosphere at 50 °C for 3 h. After the polymerization reaction, the acid-catalyzed hydrolysis step was performed by transferring the polymer solution to a 2 M HCl aqueous solution at 70 °C for 5 h. Subsequently, the polymer was precipitated out by pouring the polymer solution into a large amount of ethanol (ethanol:polymer solution volume ratio = 4:1). The obtained polymer was dried in a vacuum oven (at 60 °C for 48 h), and the dried polymer was dissolved in water to obtain a 3 wt% PVAm solution. The amino acid salt PG solution was prepared by adding a stoichiometric amount of piperazine to a solution of glycine (17.5 wt% in water). The reaction was carried out at room temperature for 2 h to obtain a homogenous solution of PG. The amino acid salt PG solution was blended with the PVAm solution (fixed-site carrier) at a PG/PVAm weight ratio of 65/35 to prepare the PVAm/PG coating solution with desirable viscosity. The viscosity of the blend was enhanced by subsequently incorporating 5 wt% SDS into the PVAm/PG coating solution.

of about 185 nm, and 5 wt% SDS in the total solid composition, and they were coated on lab-fabricated PES substrates. Chen et al. demonstrated the PVAm/PG-containing facilitated transport membranes with impressive transport performances [16]. Here, we describe the development of a pilot-scale coating process for continuous roll-to-roll fabrication of the PVAm/PG-containing membranes. This coating process incorporated a thin-film coating (TFC) assembly for the continuous roll-to-roll fabrication [28,29]. The design of the TFC assembly was improved, and the operating conditions of coating process were optimized for the effective pilot-scale fabrication of ultra-thin PVAm/PG-containing membranes. There are several publications in the literature related to the pilotscale membrane testing for CO2 removal [30–33]. But, none of these publications put an emphasis on the continuous pilot-scale membrane fabrication process and/or highlight the challenges faced during the fabrication of pilot-scale flat-sheet membranes in the roll-to-roll manner. This work illustrates the same, but puts an emphasis on the development of an effective process for the continuous pilot-scale membrane fabrication. The pilot-scale membrane fabrication process can essentially be categorized into 3 steps: (1) coating solution preparation, (2) the operation of the continuous membrane fabrication machine, and (3) posttreatment (if applicable). The coating solution was prepared using the same procedure as developed in the previous work [16]. Moreover, the scale of the synthesis of PVAm/PG coating solution was increased for the pilot-scale membrane fabrication. The pilot-scale membranes were fabricated using the TFC assembly of the continuous membrane fabrication machine at Ohio State. The operating parameters were established based on the required final dry film thickness. After fabrication, the final membranes were rolled and stored for further use. If needed,

3.3. Pilot-scale membrane fabrication The principles for the lab-scale knife casting technique (Fig. 3) were adopted for the development of the continuous pilot-scale roll-to-roll coating process for the fabrication of flat-sheet membranes. The schematic of the continuous membrane fabrication process developed for this coating process is shown in Fig. 4. In addition, Fig. 5 shows the continuous pilot-scale membrane fabrication machine installed at The Ohio State University. The machine is designed with a capability to fabricate 21 in. wide flat-sheet membranes. The commercial membrane 381

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tension for operation was 5–8 lb-force, and a web tension of 5 lb-force was used for the pilot-scale experiments reported in this work. The motion of the substrate/web was controlled via motors that were operated using a separate control panel for the coating machine. The web speed (coating speed) could be adjusted up to 5 ft/min from the minimum of 0.1 ft/min. The coating trough was the volumetric solution hold-up between the coating knife and the back-support plate in the TFC assembly. The coating trough (see Fig. 4) was designed to store a bulk quantity of coating solution for operation. The polypropylene dams were used as barriers and installed on the sides of the coating trough to control the coating width. These dams were used to prevent the coating solution in the trough from escaping through the sides during operation. The TFC assembly consisted of a coating knife with a well maintained, sharp and smooth knife tip. The gap setting (gap between the knife tip and the substrate to be coated) could be calculated and correlated with the final membrane thickness as shown in Eq. (3). The gap setting was adjusted using the micrometer knobs and measured via feeler gauges. The precise gap setting during operation resulted in a desirable wet-film thickness and hence membrane thickness. The continuous membrane fabrication machine was equipped with a convection oven with air inlet and outlet ports. The wet-film coated on the nanoporous substrate was subsequently directed to the convection oven for the drying/curing process, resulting in a dry membrane film on top of the substrate. The temperature and flow rate of the hot air could be controlled by a separate control panel for the coating machine. The convection oven was designed with a web length of 8 feet. During the operation of this machine, a dry membrane film on top of the substrate was obtained as the web passed through the convection oven. The membrane was subsequently directed from the oven outlet to the rewind end of the machine for the final rolling step. Both the room and coating solution temperatures were maintained the same at 23 °C for each pilot-scale run reported in this work. Efforts were made to make sure that the coating solution temperature remained consistent for the pilot-scale experiments. Finally, the rolled membrane obtained was stored and eventually used for the fabrication of spiral-wound (and/or plate-and-frame) membrane elements for the pilot-scale gas separation performance. The main objective of this work was to fabricate PVAm/PG-containing membranes in pilot scale with a selective layer thickness of less than 200 nm. This thickness was needed to achieve the desirable gas transport performance obtained in the previous work by Chen et al. for the lab-synthesized membranes [16].

Fig. 4. Schematic of the continuous pilot-scale machine with a thin-film coating (TFC) assembly for fabrication of thin-film composite membranes.

Fig. 5. Continuous pilot-scale membrane fabrication machine with the TFC assembly at The Ohio State University.

machines have a width of 42 in. for operation [9]. The coating machine illustrated in this work was custom designed by OSU and built by Vortex Engineering (Santee, CA, USA). This machine was designed for pilot-scale demonstration of a fabrication technique for thin-film composite membranes with up to 21 in. in width. A fabrication capacity of 21 in. wide membrane was used since the main objective of the work was to develop the fabrication technique in pilot scale. Once the fabrication process is successfully established in the pilot scale, one can consider scaling it up for the commercial width of 42 in. The overall design shown in Figs. 4 and 5 consists of: (1) a web path for the substrate motion from the unwind roll to the rewind roll, (2) the TFC assembly for the thin-film coating process to replicate the lab-scale knife coating technique, and (3) an air convection oven for the drying/ curing of the membrane. The nanoporous substrate was connected from the unwind roll to the rewind roll according to the specified threading path. The rollers equipped with tension measurement sensors were called load cell rollers. The load cell rollers were attached at the unwind and rewind ends for web tension measurement and control. The web tension was an important operating parameter during fabrication. It needed to be calibrated regularly after fabrication of every 500 feet long membrane using a known 10 lb weight. A higher web tension or a significantly lower web tension could result in a non-flat surface that is undesirable during membrane fabrication. The recommended web

3.4. Gas transport performance measurements The membranes fabricated using the continuous membrane fabrication machine were characterized for transport performance. A small representative sample (with an effective area of 2.7 cm2) from the scaleup membrane was tested for gas transport performance. Gas permeation measurements were conducted by using the gas permeation apparatus described in previous publications [15,16]. In the gas permeation measurement, the PVAm/PG-containing membrane was loaded into a stainless-steel rectangular gas permeation cell. The membrane cell was placed in a temperature-controlled oven (Bemco Inc., Simi Valley, CA, USA). The oven temperature was set at 57 °C, which is the typical flue gas temperature in coal-fired power plants. A mixed gas containing 20% CO2 and 80% N2 (dry basis) was used for testing. The dry feed and sweep gas (argon) flow rates were maintained at 60 cc/min and 30 cc/min, respectively. Both the feed gas and sweep gas were humidified each with a 2-L stainless-steel vessel (Swagelok, Westerville, OH) filled with 150 ml water and 60% (by volume) packing of glass Raschig rings. After leaving each of the humidifiers, 17% (by volume) water content was achieved at 57 °C for each of the feed and sweep gases. The membrane cell was tested using a countercurrent flow configuration for the feed and sweep streams to 382

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4.1. Effect of coating speed on membrane thickness for pilot-scale fabrication

obtain the maximum driving force for gas transport. The water vapor in both outlet streams (feed and sweep outlets) were knocked out in two respective condensers. The testing pressures of the feed and sweep streams were adjusted by using two near ambient pressure regulators, respectively. The feed side pressure was set at 1.5 psig, and the sweep side pressure was set at 1.0 psig. The outlet gas compositions of both retentate and permeate streams were analyzed by using a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) (Agilent Technologies, Palo Alto, CA). For the GC analysis, the SUPELCO Carboxen® 1004 micropacked GC column (Sigma-Aldrich, St. Louis, MO) was used. From the GC analysis, CO2 permeance and CO2/N2 selectivity were determined for characterization of the membrane transport performance.

Although the gap setting of the pilot-scale coating knife could not be reduced beyond a certain limit (0.45 mil), there was another operational parameter that could be considered potentially to control the thickness of the wet-film coating. This parameter was the coating speed of the substrate during operation. The principles of fluid dynamics for a fully developed coating flow have been well investigated for knife coating techniques [35–37]. The wet-film thickness (after the TFC assembly) typically approaches the theoretically predicted value per Eq. (3) at high coating speeds. However, as the coating speed is reduced, the metering effect of the knife gap setting is reduced and a thinner wet-film layer is obtained. This was experimentally demonstrated by the work of Wengeler et al. [35]. For a coating solution with 3 wt% polystyrene in xylene, Wengeler et al. experimentally demonstrated that a coating speed of greater than 20 ft/ min resulted in a film coating thickness as per the theoretically predicted value. Moreover, they also demonstrated that the wet-film thickness reduced significantly as the coating speed decreased to be below 20 ft/min. This could presumably be attributed to the viscoelastic nature of the coating solution (non-Newtonian fluid), resulting in thinner coating film thicknesses at lower coating speeds. The gap setting was no longer the primary governing factor for the wet-film thickness at low coating speeds. Based on the above discussion, the concept of reducing the coating speed in order to obtain a thinner coating film was investigated for the PVAm/PG coating solution using the continuous pilot-scale membrane fabrication machine equipped with the TFC assembly (without using an insert). A set of experiments were performed to demonstrate this phenomenon. The results are plotted in Fig. 6. For the experimental results shown in this figure, the gap settings were maintained at the minimum allowable value of 0.45 mil and a higher 0.5 mil for a uniform coating operation. The PVAm/PG coating solution with the same composition, concentration, and viscosity as the one that demonstrated desirable membrane performances in the lab scale (with a coating solution viscosity of 1180 cp, 719,000 MW PVAm, PG/PVAm weight ratio = 65/35, and 5 wt% SDS in the total solid composition) [16] was used for the pilot-scale experiments. The nanoporous PES substrate with 69 nm average pore size and 17% porosity was used for the fabrication of membranes [34]. As seen from this figure, the thickness of the selective layer was observed to reduce significantly as the coating speed was dropped from 1 ft/min to 0.15 ft/min (for a gap setting of 0.45 mil). A similar trend was observed for the gap setting of 0.5 mil. Thus, a selective layer thickness of ~ 200 nm was successfully obtained at a coating speed of 0.15 ft/min using a gap setting of 0.45 mil. These findings were in good agreement with Wengeler et al.’s results for the development of the coating techniques for polymer based solar cells, where a film thickness in range of 30–200 nm was required [35].

3.5. Coating solution viscosity and membrane characterization The aqueous PVAm/PG solution was used to coat a thin selective layer on top of nanoporous substrates. The viscosity of the PVAm/PG containing coating solution and that of 3 wt% PVAm solution were measured using a Brookfield digital viscometer DV-E (Brookfield Engineering Laboratories, Inc., USA). As different batches of polymer solution were prepared for the pilot scale membrane fabrication runs, the solution at 3 wt% polymer concentration was used as a baseline concentration to measure its viscosity for comparison. The cross-section morphology of the synthesized membranes was observed by scanning electron microscopy (SEM) using FEI Nova NanoSEM400 (FEI Company, Hillsboro, OR). 4. Results and discussion The main objective of this work was to develop an effective continuous roll-to-roll process for the pilot-scale fabrication of PVAm/PGcontaining nanocomposite membranes using the continuous membrane fabrication machine. Some of the design parameters were improved and the coating conditions were optimized to obtain a thin-film composite membrane with a desirable thickness and transport performance. These are discussed in this section. During the lab-scale knife coating process, the final dry film thickness was governed by the gap setting of the coating knife and the total solids concentration of the coating solution according to Eq. (3). It was typically straight-forward to obtain < 200 nm membrane thickness in the lab scale by using a very low gap setting and the desirable viscosity and concentration of the coating solution. Moreover, the small piece of the substrate (to be coated) was made flat on the glass plate relatively easily by taping all the edges of the substrate to the glass plate. However, during pilot-scale fabrication, it was challenging to ensure that the substrate (14 in. wide) was held “extremely flat” under tension (while being attached from the unwind end to the rewind end via several rollers). This limited the extent to which one could reduce the gap setting for the pilot-scale operation to obtain a low thickness (< 200 nm dry film thickness) of the membrane. The minimum allowable gap setting was measured to be 0.45 mil (1 mil = 25.4 µm) in the TFC assembly of the pilot-scale machine. In the lab-scale, a gap setting of < 0.2 mil was used to obtain a membrane with < 200 nm dry film thickness. However, a gap setting of < 0.45 mil could not be used in the pilot scale as it resulted in non-uniform coating with several uncoated areas by virtue of improper flatness of the substrate. Hence, alternate approaches were attempted to help obtaining a desirable thin layer during the pilot-scale operation using a gap setting of at least 0.45 mil. On those grounds, the effect of coating speed on the final membrane thickness was investigated during the continuous pilotscale membrane fabrication. Furthermore, the fluid dynamics principles of extrusion coating were incorporated in the pilot-scale thin-film coating process to enhance the overall effectiveness of the fabrication process. These are discussed comprehensively in the following subsections.

Fig. 6. Dry selective layer thickness vs. coating speed for the PVAm/PG coating solution (with a coating solution viscosity of 1180 cp). 383

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A larger flow channel height and a smaller opening of the insert and a higher viscosity of the coating solution result in a higher pressure drop and thereby a lower delivery rate of the coating solution. Inserts with channel openings of 2.5, 5 and 10 mils were used during the initial experiments, and the 2.5-mil opening was identified to be optimum to control the delivery rate of coating solution with 500–1500 cp viscosity using an insert with 1–2 in. channel height at a coating speed of 2–5 ft/ min. The delivery rate of the coating solution and the thickness of the selective layer can be predicted theoretically by the principles of fluid dynamics. A viscous gravitational flow in a thin rectangular open channel has been considered for this analysis. The volumetric flow rate (delivery rate) of the coating solution was predicted by using the generalized Bernoulli equation, as shown in Eq. (4), at the inlet and the outlet of the insert [38]. The head loss due to friction was evaluated by using Eq. (5). A shear thinning non-Newtonian fluid, which was assumed to follow a power law model, was used for this analysis. The modified Reynolds number (Eq. (6)), based on the Herschel-Bulkley model [39,40], was used for evaluating the friction factor (Eq. (7)). The Herschel-Bulkley model is equally applicable for fluids exhibiting power law and Bingham plastic behavior in addition to fluids exhibiting yield-shear thinning behavior. The wet film thickness of the selective layer was obtained by assuming the delivery rate of the coating solution to be equal to the rate of film formation (Eq. (8)) via the coating assembly with the insert. In other words, the coating solution delivered through the insert formed a uniform coating layer on top of the substrate.

In their work, a similar reduction in thickness of the film was reported at lower coating speeds using the 3 wt% polystyrene in xylene as the coating solution. The metering effect of the gap setting was demonstrated to be insignificant at lower speeds for a non-Newtonian fluid as the coating solution. A higher coating speed resulted in a larger shear stress on the surface of the fluid. This presumably led to an expansion in the polymer chains due to the viscoelastic nature of the fluid. Thus, a larger wet film thickness was obtained at higher coating speeds [35,36]. Similarly, the membrane thickness in this work was demonstrated to be strongly dependent on coating speed using the TFC assembly of the continuous pilot-scale membrane fabrication machine. In summary, at low coating speeds of 0.15–1 ft/min, the thickness of the membrane was experimentally determined to depend less on the gap setting and more on the coating speed. A thin membrane thickness of 205 nm was successfully obtained by using a very low coating speed of 0.15 ft/min during the pilot-scale operation. The membranes shown in Fig. 6 were characterized for thickness using SEM. The SEM thickness characterization of the 205 nm sample is shown in Fig. 7. Although the desirable thickness was obtained via the pilot-scale thin-film casting assembly, the coating speed of 0.15 ft/min (to obtain the 205-nm membrane thickness) was low for practical applications. Thus, alternate approaches were sought to meet the final product quality in an efficient and effective manner. 4.2. Control of the coating solution delivery rate using an insert The objective of this approach was to reduce the dependence of the final dry membrane thickness on the gap setting for continuous roll-toroll membrane fabrication with high productivity. In addition to storing a bulk quantity of coating solution in the coating trough of the TFC assembly, the delivery rate of the coating solution was reduced and controlled via the use of an insert in this coating trough as shown in Fig. 8. The purpose of the insert was to restrict the supply of the coating solution onto the moving substrate, i.e., to control the delivery rate of the coating solution, so that the coating speed could be used primarily to control the coating film thickness. This was obtained by restricting the flow path by reducing the flow channel opening using the insert, as shown in the right diagram of Fig. 8. This increased the flow resistance and reduced the delivery rate of the coating solution onto the top of the moving substrate. Thus, a higher coating speed could be used at a low and precise delivery rate of the coating solution to obtain a desirable thin film thickness in an efficient and effective manner. The insert was custom designed to provide a long and narrow opening for controlling the delivery rate of the coating solution. The coating solution entered the coating trough from the top of the insert. The solution experienced a significant viscous drag as it flowed through the long and narrow opening created by the insert. This resulted in a low and precise flow rate of the coating solution. It is important to ensure a uniform distribution of the coating solution along the width of the insert in order to obtain a uniform delivery rate of the coating solution across the top of the moving substrate. The delivery rate of the coating solution is a function of the insert geometry (the opening and height of the flow channel) and the physical properties of the coating solution responsible for the viscous drag (viscosity). A less viscous solution requires a narrower opening to restrict the flow and control the delivery rate. The inverse is true for a highly viscous solution. Thus, various insert geometries, coating solution viscosities and coating speeds were investigated for the pilot-scale membrane fabrication. The results are summarized in Table 2. At a given delivery rate of the coating solution, a higher coating speed may result in a thinner coating layer at the same solids concentration of the coating solution, and flow channel height and opening of the insert. This was confirmed from Runs # 1–3, 4 and 5, 7–9, 10 and 11, and 12 and 13 in Table 2. Moreover, the thickness of the selective layer of the membrane may be inversely proportional to the flow channel height of the insert (as seen from Runs # 1 and 11 in Table 2).

2 2 ⎛ P1 ⎞⎟ + ⎛⎜ v1 ⎞⎟ + Z1 = ⎛⎜ P2 ⎞⎟ + ⎛⎜ v2 ⎞⎟ + Z2 + Δhf ⎜ ⎝ ρg ⎠ ⎝ 2 g ⎠ ⎝ ρg ⎠ ⎝ 2 g ⎠

(4)

h ⎞ ⎛ v2 ⎞ Δhf = f ⎛ ⎜ ⎟ D ⎝ H ⎠⎝ 2 g ⎠

(5)

⎜

8 v2ρ

ReH = τw

f=

⎟

+(K

2 v n DH

( ))

16 ReH

Q = vbw = Stw w

(6)

(7) (8)

where subscripts 1 and 2 indicate the positions at the inlet and the outlet of the insert, respectively, P is the absolute pressure, Z is the elevation of the point above a reference, Δhf is the frictional head loss

Fig. 7. SEM cross-section image of the scale-up membrane with a 205-nm amine-containing layer on top of the PES substrate (shown in Fig. 6) fabricated with a gap setting of 0.45 mil and a coating speed of 0.15 ft/min. 384

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Fig. 8. The schematic of the TFC assembly with and without an insert; the use of an insert helps in controlling the coating solution delivery rate to enable a higher coating speed for a thinner membrane.

Table 2 Selective amine-containing layer thicknesses for different coating knife assembly configurations, polymer concentrations and coating speeds. Coating knife assembly configuration

Polymer conc. (wt%)

Viscosity of coating solution (cp)

Run #

Web speed (ft/min)

Experimental dry membrane thickness (nm)

Theoretical delivery rate, S (cc/min)

Theoretical dry membrane thickness (nm)

Insert with 1-inch flow channel heighta

1.8

870

2.4

940

3 1.5

1180 690

1.8

870

3.4

1430

1 2 3 4 5 6 7 8 9 10 11 12 13

3 4 5 3.25 3.5 3.5 1 2 4 2 3 1.6 2

175 135 115 210 195 185 230 165 135 220 155 210 178

5.07 5.35 5.71 6.43 6.42 6.65 2.31 3.32 5.38 4.70 4.77 3.34 3.53

234 185 158 274 254 263 320 230 186 325 220 289 244

Insert with 2-inch flow channel heighta

a

2.5-mil of flow channel opening.

along the height of the insert, f is the friction factor for laminar flow in a rectangular channel, h is the flow channel height (height of the insert), ρ is the density of the coating solution, v is the average velocity of the coating solution, ReH is the modified Reynolds number, DH is the effective hydraulic diameter (4 × flow channel area/(flow channel perimeter) = 4 b w/(2 b + 2 w) = 2 b since b/w = 2.5 mil/(14″ × 1000 mil) = 1.786 × 10−4 « 1), K is the consistency constant for the power law model, n is the power law index (0.27 in this case [41]), τw is the average wall shear stress, Q is the delivery rate of the coating solution, b is the opening (gap) of the channel for the coating solution flow, w is the width of the channel, S is the coating speed of the substrate, and tw is the wet film thickness of the membrane. The predicted theoretical membrane thicknesses are also shown in Table 2. Although the theoretical thickness of the selective layer was not accurately predicted (Table 2), but the trend was predicted reasonably well. The coating solution was assumed to follow a power law model for the shear thinning behavior. In addition, a power law index of 0.27 was assumed for this analysis [41]. Thus, the accuracy of the predicted thickness may potentially be improved significantly by using an appropriate rheological model that can accurately represent the PVAm/ PG coating solution. In summary, as seen from Table 2, the membrane thickness of < 200 nm was successfully obtained experimentally at a higher coating

Fig. 9. SEM cross-section image of the scale-up membrane with a 185-nm amine-containing layer on top of the PES substrate fabricated using the insert with 1-in. channel height and 2.5-mil channel opening at a coating speed of 3.5 ft/min.

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fabrication machine at OSU. However, this machine is capable of fabricating membranes with 21 in. in width. Thus, the process used for fabrication of 14-in. wide membranes can be extended for the fabrication of 21-in. wide membranes using the OSU machine or wider membranes using a wider machine. This work has illustrated the pilot-scale membrane fabrication technique specifically for the system of PVAm/PG-containing coating solution. However, the operating conditions and the system design can be adopted and extended to match with the rheological properties of a different category of coating solutions. In other words, this coating technique has a broader scope for application and can be used potentially to coat various types of dense polymer membranes on top of porous substrates.

Table 3 Transport performances of lab-scale [16] and pilot-scale PVAm/PG-containing membranes with the selective layer thickness of about 185 nm at 57 °C. Membrane type

CO2 permeance (GPU)

CO2/N2 selectivity

Lab-scale Lab-scale Pilot-scale Pilot-scale Pilot-scale

734 753 735 748 741

136 148 148 135 151

speed of > 3 ft/min using the insert with a 2.5-mil flow channel opening at two different flow channel heights of 1 in. and 2 in. The efficiency of the pilot-scale fabrication process to obtain a membrane with < 200-nm thickness was significantly improved by increasing the coating speed from 0.15 ft/min (as discussed in Sections 4.1) to > 3 ft/ min. A total of > 2000 feet long PVAm/PG-containing membrane with 14 in. in width was successfully fabricated with the desirable thickness using the continuous pilot-scale membrane fabrication machine equipped with the TFC assembly with the insert. Representative samples taken from the fabricated scale-up membranes were characterized for thickness determination and gas transport performances. The SEM image of the PVAm/PG-containing selective layer (185 nm with a variation of ± 25 nm) on top of the PES substrate is shown in Fig. 9. This membrane was obtained by using the insert with the 1-in. flow channel height and the 2.5-mil flow channel opening with a coating speed of 3.5 ft/min and a coating solution viscosity of 1180 cp (shown in Table 2). The transport performances of the representative small samples (2.7 cm2) taken from the pilot-scale membranes were determined via the gas permeation measurements using the same experimental mixedgas condition described in Section 3.4 as the lab-scale membrane [16]. Both the pilot-scale and lab-scale membranes were prepared from the coating solution with a viscosity of 1180 cp, PG/PVAm weight ratio = 65/35, and 5 wt% SDS in the total solid composition. The results are summarized in Table 3. As shown in this table, the pilot-scale membranes resulted in an average CO2 permeance of ~ 740 GPU and an average CO2/N2 selectivity of 140. These results compare reasonably well with those for the laboratory synthesized membranes, which are shown in Tables 1 and 3 and reported in the previous publication by Chen et al. [16].

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