Chemistry in a spinneret — Sinusoidal-shaped composite hollow fiber membranes

Journal of Membrane Science 585 (2019) 115–125

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Chemistry in a spinneret — Sinusoidal-shaped composite hollow fiber membranes

T

Hannah Rotha,b,1, Michael Aldersb,1, Tobias Luelfa,b,1, Stephan Emondsa,b, Sarah I. Muellerb, Maik Teppera,b, Matthias Wesslinga,b,∗ a b

DWI – Leibniz-Institute for Interactive Materials, Forckenbeckstraße 50, 52074, Aachen, Germany Chemical Process Engineering AVT.CVT, RWTH Aachen University, Forckenbeckstraße 51, 52074, Aachen, Germany

A R T I C LE I N FO

A B S T R A C T

Keywords: Chemistry in a spinneret Composite hollow fiber Membrane dryer Sinusoidal-shaped fibers Secondary flow

Composite membranes are highly permeable and used with great success in nanofiltration, reverse osmosis and gas separation. However, the higher the membrane permeability gets, the more pronounced becomes the role of the laminar boundary layer. Common approaches to overcome boundary layer limitation include the use of spacers or inserts, which induce secondary mixing. In contrast, we use composite hollow fiber membranes with an altered geometry to generate mixing effects on the lumen side without integrating spacers. In the sinusoidalshaped lumen channel of our fibers, secondary flows and vortices evolve. To fabricate the hollow fibers, we combine two technologies. With the chemistry in a spinneret approach, we fabricate composite hollow fibers in a single-step process. Pulsating the bore fluid flow creates the sinusoidal geometry of the fiber. The superposition of a sinusoidal pulsed bore fluid flow and the chemistry in a spinneret approach fabricates sinusoidal-shaped composite hollow fiber membranes in a single step. This geometric feature reduces the boundary layer resistance and we demonstrate that the sinusoidal-shaped fibers excel the straight fibers in their performance of drying compressed air.

1. Introduction Membranes applied in nanofiltration, reverse osmosis and gas separation are dense asymmetric membranes. The dense region of the membrane is the active separation layer and dictates the separation performance depending on characteristics such as thickness or material. Two types of asymmetric membranes exist which differ in their fabrication methods. Integrally skinned asymmetric hollow fiber membranes arise via the phase inversion process. These membranes consist out of a single polymer blend which is transformed into a hollow fiber membrane with a dense skin layer either on the lumen or shell side. This formation is realized during the single step spinning process [1–3]. In contrast, composite membranes consist of a porous support structure which is coated with a thin dense layer of a different material. This technique, therefore, offers more variation and tailoring of the material system concerning the desired application compared to integrally skinned membranes [4–8]. Thin film composite membranes are highly engineered and excel integral asymmetric membranes in filtration performance. Therefore, composite membranes are produced in

large scale and find wide application. Nonetheless, still new process procedures, materials, further tailoring and fundamental understanding of the fabrication of flat sheet thin film composite membranes are focused in recent research [9–17]. On the other hand, the fabrication of composite membranes requires complicated and time-consuming multistep procedures. The execution of these multi-step procedures is easier on flat sheet membranes. Therefore, almost all commercially available composite membranes are limited to flat sheet geometries. To reduce the fabrication time, researchers work on single step processes for fabricating flat sheet composite membranes. The production of spiral wound modules with thin film composite membranes is highly automated due to the large-scale industrial usage. But in contrast to hollow fiber modules, the module design with flat sheet membranes is more complex [18]. These modules cannot be backwashed and have lower packing densities. The transferal of the flat sheet membrane coating procedure to hollow fiber membranes has been published multiple times [19–23]. Especially, the co-extrusion of composite or dual-layer hollow fibers is challenging but highly desired to take advantage of higher packing densities, backwashability and the maximized performance from various material combinations. The challenges such as

∗

Corresponding author. Forckenbeckstraße 51, 52074 Aachen, Germany. E-mail address: [email protected] (M. Wessling). 1 Contributed equally to this work. https://doi.org/10.1016/j.memsci.2019.05.029 Received 23 January 2019; Received in revised form 7 May 2019; Accepted 10 May 2019 Available online 16 May 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

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fabrication of composite or surface modified hollow fiber membranes in a single-step process [49–52]. The first chemistry in a spinneret concept used a reactive polymer, which creates the porous support structure via the phase inversion process and at the same time reacts with an additive from the bore fluid [53]. The limiting factor of this concept is the need of a polymer which precipitates in a porous way and is reactive at the same time. Therefore, we generalized the concept: the reaction of a crosslinking agent and an amine reactant superimposes the conventional phase inversion of an inert polymer during dry-wet spinning. While the inert polymer precipitates to become the porous support structure, the crosslinking reaction forms the desired dense separation layer on the lumen-side. Gherasim et al. [52] transferred the layer-bylayer (LbL) nanofiltration membrane concept to the chemistry in a spinneret by provoking ionic crosslinking of polyelectrolytes from the bore fluid and polymer solution during spinning of hollow fiber membranes. In this work, we use polyethyleneimine (PEI), a hyperbranched amine reactant, as an additive in the polymer solution and dose glutaraldehyde (GA) as a crosslinker to the bore solution. A solvent stable separation layer with red color forms on the lumen channel surface and a porous support structure out of polyethersulfone (PES) is created through the phase inversion process. The resulting membranes have been studied for nanofiltration applications previously. The pure water permeance of the membranes is low with ∼ 0.4 LMH/bar, but at the same time, the molecular weight cut off (MWCO) is very reproducible at 1000 Da [49]. Using a tailor-made pulsation device which is easily included in the traditional spinning setup to vary the bore fluid volume flow, Luelf et al. [48] spun sinusoidal-shaped hollow fibers made of polyvinylidene fluoride (PVDF). Different pulsation parameters (frequency, amplitude) create unique sinusoidal diameter changes. Through the alternating smaller and larger diameters, the feed flow velocity changes as well. In the areas with larger diameters, vortices are induced, and the boundary layer is interrupted. The sinusoidal-shaped PVDF membranes show increased oxygen flux compared to the straight reference fibers in gasliquid contacting experiments. In this work, we show the successful fabrication of sinusoidalshaped composite hollow fiber membranes with a separation layer consisting of PEI crosslinked with GA. This robust reaction system suits to study the effects of the superposition of the chemistry in a spinneret with the pulsation of the bore fluid flow. We optically analyze the membrane geometry and prove that the built up of the selective layer is not influenced by the bore fluid flow pulsation. The dense separation layer makes these membranes attractive for drying of compressed air which is a process prone to concentration polarization effects. Fig. 1 visualizes the proposed flow patterns in a straight and sinusoidalshaped fiber. The membrane is depicted simplified as the red separation layer. In the sinusoidal lumen channel, vortices evolve, and we assume back-mixing of the feed flow into the bulk taking place. The chemistry in a spinneret concept proves to be versatile and robust, even with a pulsating bore fluid flow. We envision the transferability of other reactions into the spinneret as well as other geometry changes to the concept. Therefore, manifold new membranes with novel shapes and surface chemistry are about to evolve.

delamination in fabricating dual-layer membranes and resolutions for those challenges have been thoroughly investigated and summarized in the following works [24–32]. As just summarized, fabrication processes and membrane materials have been subject of excessive studies and optimization for many years now. However, due to the improvements made in this area, the mass transfer resistance in membrane applications often shifts from the membrane itself to the adjacent boundary layers. Concentration polarization and deposition of retained components govern the process performance. A process which is especially prone to be governed by concentration polarization is the dehumidification of compressed air [33]. Low concentrations and high permeances cause the built up of a performance reducing boundary layer adjacent to the membrane surface [34,35]. In air drying applications, boundary layer resistances are likely to dominate the overall mass transport [36,37]. Furthermore, the diffusion coefficient in the boundary layer is dependent on the feed 1 pressure (D ∝ p ) [36]. Hence, the higher the feed pressure, the higher is the boundary layer resistance. To overcome boundary layer limitations, a new field arises of inducing mixing in membranes and membrane modules in various ways. All spiral wound modules comprise spacers which in the first place create a flow path for the feed but at the same time depending on their geometry induce mixing to decrease concentration polarization [38]. Hollow fiber modules have been less subject to design optimization since the lumen channel of the fibers functions as feed flow path and no spacers are necessary. Nevertheless, industry and research have come up with novel concepts for hollow fiber modules. For the process of drying compressed air, arranging the hollow fibers like a helix with the fibers crossing each other in a 90° angle in a module is beneficial and increases the drying efficiency [39]. In hemodialysis, the fibers are ondulated to prevent them from coming into full contact with each other. At the same time, the ondulation of the fibers improves the flow conditions of the dialysate around the fibers [40]. In the style of spacer mats of spiral wound modules, Armbruster et al. [41] inserted static mixers into tubular ceramic membranes. 3D printing techniques created different shapes of static mixers. The static mixers induce a secondary flow to reduce fouling and concentration polarization in the tubular membranes. A drawback of the concept is the time-consuming assembly, resulting in infeasibility for hollow fibers with small diameters. During membrane fabrication, X-Flow B.V. includes a static mixer in the geometry of their tubular membranes for ultrafiltration [42]. While applying the top layer to the support, it is shaped such that a helical ridge is created in the lumen channel. Wiese et al. [43] studied the filtration performance of a helix membrane compared to a conventional membrane and were able to visualize the reduced fouling with MRI imaging. This helix membrane concept is only available for tubular membranes. A few years ago, a microstructured insert in the spinneret created hollow fibers with unique shapes [44,45]. The influence of the microstructure on the fouling behavior was studied. The advantage of the concept is its applicability for small fiber diameters. Recently, Luelf et al. [46] published several methods to provoke geometry changes during the spinning process of hollow fiber membranes. Curled hollow fibers evolve by exploiting the rope coil effect. Microstructuring the cross section of the hollow fiber is achieved with 3D printed spinnerets. A self-made device to rotate the spinneret creates innovative membrane geometries when using non-rotationally symmetric spinneret orifices [47]. This work aims to combine two concepts developed in-house. A pulsation module added to the bore fluid flow induces a sinusoidal shape in the lumen channel [48]. The chemistry in a spinneret concept creates a composite hollow fiber in a single step [49]. The combination of those enables us to spin sinusoidal-shaped hollow fiber composite membranes in a single step to exploit synergies of both principles. In the following, both concepts are presented in detail. The chemistry in a spinneret technology base enables direct

Fig. 1. Flow profiles in straight (left) and sinusoidal-shaped (right) hollow fibers. At higher velocity the sinusoidal-shaped fibers induce vortex formation. 116

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water. A motor-driven wheel stretches the fiber and transports it into the final collecting bath. To further enhance the crosslinking, the collecting bath is preheated to 50 °C which we let cool down to room temperature after spinning. Glutaraledyde and PEI are exposed to each other until drying of the fibers. After 48 h and multiple water bath changes, the membranes are stored in DI water with 5 wt% glycerol for 24 h. Finally, the wet fibers are dried in a well-ventilated environment at room temperature. For the fabrication of sinusoidal-shaped hollow fibers, a pulsation module is operated simultaneously to the syringe pump giving a constant bore volume flow. This module is connected to the tubing of the bore fluid (cf. Fig. 2(b)). Luelf et al. [48] previously introduced the pulsation module. This module consists of a motor which rotates a turning plate. The plunger of a syringe can be fixed to different distances from the turning plates center. In this work, a syringe of 250 μL volume was used with a plunger area of 4.2 mm2. When starting the motor, the rotational movement is translated into a lateral movement which results in a sinusoidal variation of the bore fluid volume flow. The rest of the fiber fabrication is conducted just as for the straight fibers. We discuss seven batches in this work and Table 1 lists the corresponding spinning parameters. Five batches (HzC-0Hz, C-1.6Hz, C-4Hz, C-8Hz, C-16Hz) are for characterization to investigate the influence of different frequencies. These batches are produced with a small variable spinning setup. Two batches (A-straight, A-sine) are fabricated for the application in drying compressed air. To fabricate these batches a larger spinning setup with two coagulation baths and a final bath is used.

2. Experimental The first subsections describe the materials and methods for membrane fiber fabrication, the optical characterization of those and the determination of typical membrane parameters. The later subsections present the particular application of the developed fibers in drying compressed air and the corresponding materials and methods. 2.1. Fiber spinning and characterization 2.1.1. Materials BASF provided polyethersulfone (PES) granulate (PES Ultrason 6020P). Polyvinyl-pyrrolidone with an average molecular weight (MW) of 40 kDa (PVP K30) and 360 kDa (PVP K90) are obtained from Carl Roth. N-methylpyrrolidone (NMP) is purchased with 99% purity from Fisher Scientific (ACROS Organics). Branched PEI with an average MW of 800 Da (by LS) is purchased pure at Sigma Aldrich. Grade II glutaraldehyde (GA) is purchased as 25 wt% solution in water at pH 3 from Sigma Aldrich. Glycerol is obtained at Carl Roth with a purity above 99%. For the characterization of the molecular weight cut-off, polyethylene glycol (PEG) with various molecular weights (400 Da, 1000 Da, 6000 Da, 10,000 Da, 20,000 Da and 35,000 Da) are purchased from Carl Roth (Rotipuran). Calcium chloride (CaCl2) is purchased with ACS reagent grade for salt retention measurements from Sigma Aldrich. 2.1.2. Spinning system and fiber fabrication The composition and preparation of the polymer solution, bore fluids and shell fluid has been previously published by Roth et al. [49]. The polymer solution contains 19 wt% PES, 4 wt% PVP K30, 4 wt% PVP K90, 5 wt% PEI with MW of 800 Da and is dissolved in 68 wt% NMP. For the bore fluid, 5 wt% of glutaraldehyde are diluted with 95 wt% deionized (DI) water. To keep the fibers shell side openly porous, a shell fluid with 80 wt% NMP and 20 wt% deionized (DI) water is used. The same triple orifice spinneret as stated in Roth et al. [49] extrudes the here presented fibers. The dimensions are 0.4 mm diameter of the bore needle, 1.12 mm outer diameter of the polymer solution orifice and 1.85 mm outer diameter of the shell fluid orifice. Fig. 2(a) shows a traditional spinning setup with which straight fibers are fabricated. The polymer solution, bore and shell fluid are coextruded through the spinneret. Typically, a constant bore and shell volume flow is applied with a syringe pump. After falling through the air gap, the fibers immerse into the coagulation bath filled with DI

2.1.3. Optical analysis The created fibers are imaged in various ways. Photos are taken with a single-lens reflex camera. For scanning electron microscopy (SEM) imaging, fibers are fractured in liquid nitrogen to prepare membrane samples for imaging cross sections. With a razor blade, membrane fibers are cut lengthwise to image the inner surface. SEM imaging is performed with a Hitachi Table Top TM3030 plus. Sputtering with tungsten coats the samples for field emission scanning electron microscopy (FESEM). A Hitachi S-4800 Scanning Electron Microscope is used for the imaging. Non-destructive imaging of whole fibers is possible with μ-CT imaging. A Bruker SkyScan 1272 device is used. Luelf et al. [48] established a refractive index matching method to see through the porous membrane material of their integral asymmetric hollow fibers and to visualize the lumen channel geometry. As

Fig. 2. (a) Traditional spinning setup with syringe pumps transporting bore and shell fluid, (b) pulsating device connected to bore fluid flow tubing creates sinusoidal variation in bore fluid flow. 117

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Table 1 Spinning parameters. Membrane type

C-0Hz C-1.6Hz C-4Hz C-8Hz C-16Hz A-straight A-sine

Polymer flow rate

Bore flow rate

Frequency

Distance from turning plate center rplate

Area of syringe plunger 2

Shell flow rate

Pulling speed

Air gap

[mL/min]

[mL/min]

[Hz]

[mm]

[mm ]

[mL/min]

[m/min]

[cm]

3.5 3.5 3.5 3.5 3.5 2.7 2.7

1.2 1.2 1.2 1.2 1.2 1.2 1.2

– 1.6 4 8 16 – 6.3

– 7.5 7.5 7.5 7.5 – 7.5

– 4.2 4.2 4.2 4.2 – 4.2

0.3 0.3 0.3 0.3 0.3 0.3 0.3

2.0 2.0 2.0 2.0 2.0 3.2 3.2

5 5 5 5 5 3 3

Fig. 3. Method for lumen channel geometry analysis shown for a straight composite hollow fiber. A) Scheme and photo of composite fibers. B) Dissolution of support structure made of PES. C) Photo of isolated crosslinked layer as a basis for imaging (middle) and isolated fiber image (bottom) resulting from MATLAB image analysis.

presented in Roth et al. [49], the composite hollow fiber membranes have a crosslinked separation layer on the lumen side which is not soluble in NMP. This material characteristic gives the opportunity to isolate the crosslinked layer which is on the lumen channel surface. The method is schematically shown in Fig. 3 and exemplified for a straight composite hollow fiber comprising the porous PES support and crosslinked layer (cf. Fig. 3 A). To remove the porous PES support, the membrane is immersed into NMP (cf. Fig. 3 B). After the dissolution of the support, the unsupported crosslinked hollow channel is transferred to a Petri dish with water. The isolated separation layer is photographed and then analyzed with MATLAB image analysis (cf. Fig. 3C). As a result, the variation of the diameter of the lumen channel is plotted over the length of the lumen channel.

pressure (TMP). Using the conductivities of feed and permeate (σF and σP ) the retention is calculated within the OSMO Inspectors software:

σ Rsalt = ⎛1 − P ⎞⋅100%. σ F⎠ ⎝ ⎜

⎜

(2)

Three fibers of 27 cm length are glued into tubing without leaving a chamber for the permeate open (cf. Fig. 4). These 2-end modules are purely used for pressure drop measurements at different volume flow rates. The pressure sensors in the feed and retentate tubing of the Osmo Inspector provide the data to calculate the pressure drop along the fibers. DI water functions as model fluid in these measurements. The volume flow rates are varied between 0.9 L/h and 7.25 L/h. This corresponds to Reynolds numbers between 200 and 1600 for the straight fibers. To determine the burst pressure, a single membrane fiber is filled with water and pressurized with hydraulic test pump from the inside. The pressure display logs the highest pressure performed. The tubing of the set-up can withstand a maximum pressure of 25 bar. At least four measurements of each fiber type are performed and then the mean value is taken.

2.1.4. Membrane parameters For characterization purposes, modules with three fibers are fabricated. The modules contain 4-ends and are 30 cm long. Dead-end filtration measurements provide the pure water permeance (PWP) in L/ (m2 h bar). The molecular weight cut-off (MWCO) is determined through cross-flow filtration experiments in turbulent flow regime (10 L/h per module). The aqueous feed solution contains PEGs of different molecular weights. Size-exclusion chromatography (SEC) measurements of the feed and permeate samples are used to calculate the retention with the concentrations in the feed cF and permeate cP:

c R = ⎛1 − P ⎞⋅100%. c F⎠ ⎝

⎟

⎟

(1)

The CaCl2 retention is measured in an OSMO Inspector Poseidon built by Convergence Industry B.V. (Netherlands) with internal conductivity meters. Single modules were tested with 10 L/h feed flow (turbulent regime) of 5 mM salt solution at 5 bar transmembrane

Fig. 4. Scheme of module used for pressure drop measurements. 118

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2.2. Application development: Drying compressed air

Table 2 Properties of modules used for mixed gas experiments.

The following subsections cover the materials and methods related to the application in drying compressed air. Only fibers from the batches A-straight and A-sine operate in these setups.

Membrane type

A-straight A-sine

2.2.1. Pure vapor permeance measurement Pure water vapor permeance is measured using a constant volume variable pressure method. Within the setup, an atmosphere of pure water vapor at a defined activity is established. At the beginning of the measurement, the permeate volume of the membrane module is evacuated. A pressure sensor records the subsequent pressure increase over time in this volume due to permeation and we use it to calculate the mass transfer across the membrane. Koester et al. [54] published further details on the setup and measurement method.

Number of Fibers

Open fiber length

Active area

–

[cm]

[m2]

48 47

26 26

0.0271 0.0265

2.2.4. Calculation of permeate water mass flow The saturation pressure of water vapor is calculated using the Antoine equation:

log ps (T ) = A −

B C+T

(3)

Where A, B and C are parameters of the Antoine equation (here 23.462, 3978 and 233.349), T is the temperature (in °C) and ps is the saturation pressure (in Pa). The dew point mirrors measure the temperature of condensation of the gas flow. Inserting the dew point temperature in the Antoine equation yields the vapor pressure of water in the gas flow. The relative humidity is calculated by:

2.2.2. Module preparation The modules used for the application in drying compressed air consist of a cartridge housing system. The cartridge is a metal tube. The hollow fibers are cut, and 2K epoxy glue (UHU Sofortfest) seals the ends of the fibers to prevent glue from entering the lumen in the next step. The sealed fibers fill up the cartridge, which is closed using 3D printed caps. The caps are connected to a syringe filled with low viscous 2K epoxy glue (Araldite 2020, Huntsman GmbH). A rotatory potting centrifuge spins the cartridge at 350 rpm for 48 h. After spinning, the caps are cut off, and the module cartridge is placed into the module housing. Fig. 5 shows a schematic representation of the cartridge. Table 2 lists the properties of the two modules fabricated for mixed gas permeation experiments.

r. H . =

ps (Tdew ) ps (Toven)

(4)

Where r.H. is the relative humidity of the stream, Tdew is the dew point temperature and Toven is the temperature of the measurement. The sweep flow is completely dry, so total permeated water vapor can be calculated with the permeate dew point, pressure and flux.

m˙ w = m˙ g ⋅X

(5)

Where m˙ w is the water vapor mass flux, m˙ g is the gas mass flux and X is the loading of water vapor in the gas flux. The loading is calculated by

2.2.3. Compressed air drying setup and measurement Fig. 6 depicts a schematic representation of the setup used to measure the drying performance of the membrane modules. Two thermal mass flow controllers (MFC) (EL-FLOW, Bronkhorst Mättig GmbH, Germany) provide defined mass flows for feed and sweep of the membrane. A thermal mass flow controller for liquids (LIQU-FLOW, Bronkhorst Mättig GmbH, Germany) adjusts the amount of water added to the feed stream which evaporates in a controlled evaporation unit (CEM) (Bronkhorst Mättig GmbH, Germany). A bypass heating prevents condensation of the humid stream until it enters the temperature controlled oven (Memmert GmbH, Germany). Proportional valves (Buerkert GmbH, Germany) regulate the pressures in the retentate and permeate streams of the membrane. Dew point mirrors (Optidew, Michell Instruments GmbH, Germany) measure the dew points in the feed, retentate and permeate side. All experiments are conducted at 10 NL/ min feed flow rate with a water vapor activity of 65% but at different feed pressures. For each measurement, the system is given enough time to equilibrate until data is collected. The actual volume flow changes with the feed pressure. It is not possible to change single parameters independently from each other. By increasing the feed pressure, the diffusion coefficient as well as the Reynolds number decrease. While the diffusion coefficient is a physical property, the Reynolds number is a process property. Reynolds numbers are calculated using the actual volume flow and the inner mean diameter of the fiber. All pressures in this work are given as absolute pressures.

X=

ps (Tdew, p) Mw ⋅ . MN2 pp − ps (Tdew, p)

(6)

Here, Tdew, p is the dew point temperature in the permeate stream and pp is the pressure of the permeate stream. Mw and MN2 are the molar weights of water and nitrogen, respectively. 3. Results and discussion The first part of the results and discussion focuses on the optical characterization of the fibers and the performance characterization of small modules of all batches. In the second part, the application of the batches A-straight and A-sine in the drying of compressed air is discussed, and the corresponding results are presented. 3.1. Membrane characterization The successful superposition of the chemistry in a spinneret concept and the simultaneous variation of the bore fluid volume flow resulted in continuously sinusoidal-shaped composite hollow fibers. Fig. 7 shows a straight fiber (left) and two sinusoidal-shaped fibers spun with different frequencies (middle and right). The reaction of GA with PEI results in a red colored crosslinked polymer film, and therefore the created separation layer appears in red on the inside of all three membranes. Fig. 5. Schematic representation of a mixed gas fiber cartridge.

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Fig. 6. Flow sheet of mixed gas permeation setup (simplified).

Fig. 7. Straight (left) and sinusoidal-shaped (middle and right) composite hollow fibers with red separation layer on the lumen side. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Fig. 9. FESEM images of the A-straight and A-sine fibers. Close-up view of lumen side in cross section for A-straight (A) and A-sine (C). Longitudinal cross section of A-straight (B) and A-sine (D). In these images, the sinusoidal diameter variation is not visible due to the high magnification.

Additionally, the sinusoidal variation of the outer diameter turns up along the fiber length. SEM imaging gives the possibility so visualize the inside of the lumen channel. Therefore, imaging the longitudinal cross section reveals the sinusoidal shape (cf. Fig. 8). The shell fluid evens out the effect on the shell side, as the outer diameters of the hollow fibers show less variation in diameter over the length (cf. Fig. 8 in the SEM images). However, when considering only the lumen channel, the sinusoidal geometry is distinctively solidified in the lumen. FESEM images of the A-straight and A-sine fibers allow for evaluating the microscopic morphology. A close-up view of the layers on the lumen sides (cf. Fig. 9 A and C) shows that the layers are well integrated into the porous support structure and are of similar thickness. The layer thickness (visibly darker area of the cross section) at the location of the fracture is 4.5 μm for both fiber types. The morphology of both fibers exhibits the same features, which have been published previously [49]. The fibers have defect-free inner surfaces and the fiber wall contains macrovoids in the center (cf. Fig. 9B and D). Both, the SEM and FESEM images show defect-free and smooth surfaces of the lumen channels. The same can be concluded from the Xray micro-tomography (μ-CT) images. Fig. 10(a) shows a lengthwise cross section executed by the μ-CT imaging software of an A-sine fiber. The selective layer depicted white as dense materials are colored white. The layer is defect-free and well connected to the porous support. When imaging only parts with high material density, it is almost possible to

Fig. 10. (a) Computer generated lengthwise cross section of a micro CT image of an A-sine fiber, (b) image of densest materials to isolate the selective layer and visualize its defect-free nature.

isolate the separation layer in the lumen channel (cf. Fig. 10(b)). As far as assessable with these images, there are no layer thickness variations visible. With the dissolution of the support structure (presented in Fig. 3), the inner layer remains intact. To this point, we have not been able to characterize this free-standing layer regarding its exact thickness. The

Fig. 8. SEM images of the longitudinal cross section of the straight and sinusoidal-shaped hollow fibers. A) C-0Hz, B) C-8Hz and C) C-16Hz. 120

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Table 3 Geometric properties according to Fig. 11 of fabricated fibers. Membrane type

C-0Hz C-1.6Hz C-8Hz C-16Hz A-straight (0 Hz) A-sine (6.3 Hz)

Fig. 11. Sine function for approximation of the geometric parameters mean diameter dm, amplitude a and cycle length f.

NMP solution dissolving the support structure contains no other visible residuals. We conclude that mostly the inner surface of the lumen channel contains crosslinked PEI and glutaraldehyde. If glutaraldehyde manages to diffuse into the porous support structure, it creates no large crosslinked areas and therefore, we assume that it does not add any significant resistance within the porous support. From these images, we conclude that the additional stress by the variation of the bore fluid flow rate during fiber and separation layer formation does not visibly negatively influence the separation layer formation. The uniform thickness of the separation layer at different positions further supports this. After having discussed the morphology of the composite fibers, the next paragraphs focus in detail the mathematical geometry. The lumen channel diameter can be expressed as a sine function with the parameters mean diameter dm, amplitude a and cycle length f (cf. Fig. 11). As investigated by Luelf et al. [48], the frequency, the area of the syringe plunger and the distance of the syringe plunger from the turning plate center influence the resulting sinusoidal geometry. In this work, solely differing frequencies are applied. To study the influence of various frequencies on the resulting lumen channel geometry, the isolated lumen channels are analyzed. The method presented in Fig. 3 is applied to isolated the lumen channel by dissolving the surrounding PES porous support structure. The resulting images of the unsupported lumen channels are displayed in Fig. 12 on the right side. Such images are analyzed via a MATLAB script. The diagram on the left shows the deviation of the fiber diameter from the mean fiber diameter over the axial position. While the straight fibers diameter stays constant over the fiber length, all three fibers spun with varying bore fluid volume flow exhibit sinusoidally changing deviation of the diameters. The frequencies are well transformed into the period length of change in diameter. When doubling the frequency, the period length is divided in half. On the other hand, the amplitude of the deviation is decreasing with increasing frequency due to damping effects in the system with. While the deviation of the fiber diameter is around 0.3 mm for C-1.6Hz and C-8Hz, the deviation for C-16Hz is only around 0.2 mm. For actual performance evaluation and comparison, the calculation

a

Mean diameter

Amplitude

Cycle length

Area per fiber lengtha

dm

a

f

A′

[mm]

[mm]

[1/mm]

[mm2/mm]

0.67 0.76 0.63 0.62 0.653 0.658

– 0.15 0.14 0.07 – 0.099

– 0.05 0.25 0.51 – 0.211

2.10 2.39 1.98 1.95 2.05 2.07

Calculated.

of the inner surface area of the sinusoidal-shaped fibers is important. We consider the thickness of the separation layer neglectable and therefore use the photographed unsupported separation layers and the SEM images to evaluate the channel geometry. With the geometry analysis, the parameters mean diameter dm, amplitude a and cycle length f are determined according to Fig. 11. The measured parameters for the analyzed fibers are summarized in Table 3 and the area per fiber length A’ is calculated. Measuring absolute values for these parameters is error-prone due to the small fiber dimensions. Nevertheless, it can be concluded that the increase in surface area of the sinusoidal-shaped fibers compared to the straight fibers is small. On the other hand, the surface area of the lumen channel depends strongly on the mean diameter which needs to be chosen similar if a comparison between a sinusoidal-shaped and straight fiber is desired. Even though the measured and controlled spinning parameters beside the frequency stay the same, the resulting mean diameters vary for the different batches (C-0Hz - C16Hz) (as Table 3 shows). These variations in the overall mean diameter impact the area per fiber length more than the actual sinusoidal diameter variation. As an intermediate conclusion, the superposition of the chemistry in a spinneret concept with the pulsating bore fluid flow results in sinusoidal-shaped composite hollow fibers. Visual evaluation shows that defect-free separation layers form. At the same time the sinusoidal shape is well solidified in the lumen channel geometry. Before testing the membrane performance in filtration experiments, we monitored the pressure drop along the fibers in the 2-end modules. A higher pressure drop indicates enhanced mixing effects in the lumen channel. Higher flow rates per single fiber generate increasing pressure drops over the fiber length depending on the channel geometry. The pressure drop is plotted for the different batches in Fig. 13. The pressure drop in the straight hollow fiber increases linearly. For the sinusoidalshaped fibers, two regions are identified. The linear pressure drop increase at low flow rates (< 1.2 L/h) results from the cross section reduction. At higher flow rates, the pressure drop increases exponentially. The more pronounced the sinusoidal structure is, the higher the pressure drop over the fiber length becomes. This increase in pressure drop is caused by the evolution of vortices. The fiber C-8Hz with the intermediate frequency of 8 Hz shows the highest pressure drop of the tested fibers. This corresponds well to the measured amplitudes. While C-8Hz has an amplitude of 0.14 mm, the amplitude of C-16Hz is 0.07 mm. The pressure drop per fiber length is less pronounced in the C-16Hz fibers. These results indirectly prove the enhanced mixing within the lumen channel. Both, the A-straight and the A-sine fibers exhibit high mechanical strength. With a burst pressure of 24 bar the A-straight fibers resist higher pressures compared to the sinusoidal-shaped fibers. The A-sine fibers withstand pressures up to 20 bar. For the sinusoidal-shape some stability is lost, but this does not effect the chosen application. Generally, burst pressures above 20 bar are excellent for lab-made hollow fibers and additionally, our determination method is not very

Fig. 12. Channel geometry analysis of selected membrane batches: Deviation of fiber diameter from mean fiber diameter over axial position (left) and images of the crosslinked separation layers used for analysis (right). 121

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Before testing the membranes in the compressed air drying setup, the pure water vapor permeance of the membrane fibers is evaluated in smaller modules. For different experimental conditions, we observed the trend of a higher water permeance for the sinusoidal-shaped fibers from batch A-sine compared to the straight fibers. This difference in permeance indicates that the crosslinked layer exhibits slightly altered properties when the additional stress induced from the pulsation module is present during fabrication. The second finding of these measurements is, when increasing the vapor activity of the feed, the crosslinked layers of both fiber geometries permeate more water vapor. No absolute values for the permeance measurements are reported due to the measurement constrains and due to the missing distinct corresponding condition in the drying of compressed air. During the pure water vapor permeation tests, the absolute pressure of the system is below the vapor pressure of water vapor. At such low pressures, the flow of water vapor through the thin lumen channels induces a significant pressure drop. The pressure drop along the fiber length is in the same order of magnitude as the absolute pressure. Hence, the obtained data shows high scattering and the estimation of the effective activity of water vapor during the measurement is vague. We are currently working on publishing this procedure on measuring pure vapor permeance of thin hollow fiber membranes in which we focus the details and constraints of the measurements. Additionally in the air drying application, the conditions in the fibers during the drying of compressed air change. The entering air has a higher water loading than the dryed air that exits the system. This difference in water loading means the activity of the air stream in the fiber changes and therefore, the permeance of the membranes is higher at the inlet than at the outlet. For the following experiments, we considered the observations of the higher permeance of A-sine fibers and the higher permeance at higher vapor activities for both fiber types in our conclusions. To prove evidence of the positive effect on concentration polarization of the sinusoidal-shaped fibers, conditions need to be identified in which the transport limitation is not on the permeate side. Fig. 14 shows the relative humidities of the retentate and permeate side over different sweep flow rates. The feed flow of 10 NL/min and the feed composition with a relative humidity of 65% are kept constant. Two distinct regimes are identified: For sweep flow rates up to 2 NL/min, the relative humidity of the retentate and permeate side are very close to each other. Concluding, the membrane module has enough membrane

Fig. 13. Pressure drop in C-0Hz, C-4Hz, C-8Hz and C-16Hz fibers at varying feed water flow rates. Pressure drop increase results from cross section reduction (< 1.2 L/h) and mixing effects due to the formation of vortices (> 1.2 L/h).

accurate. The pure water permeance of the here presented membrane batches is similar as published in Roth et al. [49] with ∼ 0.3 LMH/bar. The MWCO was determined for the batches A-straight and A-sine and both membrane batches retained more than 90% of the PEG with a molecular weight of 1000 Da. The measured CaCl2 retention is ∼ 75%. These results prove the integrity of the separation layer. On the other hand, as expected there is no difference in the salt retention of the straight and sinusoidal-shaped fibers. With such low permeances, concentration polarization is not possible and can only occur at significantly higher fluxes (Peclet Numbers). Diffusion and convection in the lumen channel even out concentration differences that build up during filtration with low permeate fluxes. As concentration polarization does not occur, the enhanced mixing in the lumen channel shows no effect on the membrane filtration performance in this aqueous application. Concluding this subsection, we successfully produced sinusoidalshaped composite hollow fiber membranes, characterized the geometry and linked it to the fabrication parameters. The optical evaluation and the filtration results show that the separation layer is defect-free and exhibits the expected filtration performances. The pressure drop along the fibers indicates enhanced mixing effects within the lumen channel. Aqueous applications are not prone to exhibit concentration polarization in processes with these fibers due to the low water permeance. 3.2. Application: Drying compressed air This subsection presents results from a gaseous application in which we expect to observe and overcome concentration polarization. For the studies in drying of compressed air, the batches A-straight and A-sine are fabricated. The spinning parameters are shown in Table 1. Batch C8Hz showed good transformation of the spinning parameters frequency and amplitude, which is set by the distance from the turning plate center, into the geometry parameters amplitude a and cycle length f of the lumen channel. Additionally, the largest pressure drops with 4 bar/ min at a flow rate of 2.4 L/h per fiber arise in the fibers of batch C-8Hz. Therefore, this type is selected for the scale up. A larger spinning setup creates the batches A-straight and A-sine. Stable spinning conditions and concentric fiber geometries are reached with the stated spinning parameter (cf. Table 1). Compared to C-8Hz, the parameters are slightly different (eg. the frequency of the motor is 6.3 Hz) and so are the geometric properties (cf. Table 3). Since all studied C-batches show an increase in pressure drop compared to the straight fiber C-0Hz (cf. Fig. 13), we expect the A-sine batch to excel the A-straight batch.

Fig. 14. Relative humidity in the retentate and permeate stream over the sweep flow rate measured in a module with A-straight fibers with a feed flow of 10 NL/min at 65% relative humidity. 122

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Fig. 15. Permeate water mass flow measured over the sweep flow rate at the constant feed mass flow of 10 NL/min with a water vapor activity of 65% for an A-sine and an A-straight module at different pressures. The pressures of 3, 5, and 8 bar correspond to Reynolds numbers of 150, 90 and 57, respectively.

Fig. 16. Permeate water flow over feed side Reynolds number (in respect to mean fiber diameter). The total feed flow is 10 NL/min with 65% relative humidity and the sweep flow is 3.7 NL/min.

the high permeances for water vapor, the boundary layer in which water vapor has to be transported diffusively is expected to be the main transport resistance. The pressures of 3, 5, and 8 bar at 10 NL/min correspond to Reynolds numbers of 150, 90 and 57, respectively. Reynolds numbers are calculated using the actual volume flow and the inner mean diameter of the fiber. The sinusoidal-shaped fibers show a significantly higher water mass transport. Even when considering the fact, that these fibers exhibit a higher pure water vapor permeance, the positive effect is still visible in the relative permeate water flow difference between the measurements. At 3.7 L/min sweep flow rate, the water transport rate for the straight fibers increases from 60 to 70 and finally 93 g/hm2, which is an overall improvement of around 30 g/hm2. The sinusoidal-shaped fibers on the other hand approach a transport rate of 95 g/hm2 already at Re 57 and increase to 140 g/hm2 at Re 150. This results in an improvement of around 50 g/hm2. This effect is explained due to the decreased concentration polarization on the feed side. When plotting the permeance water mass flow over the feed side Reynolds numbers for a sweep flow rate of 3.7 L/min, the performance increase is even more visible (cf. Fig. 16). Due to the higher permeance of the A-sine fibers, the water mass flow is already higher at lower Reynolds numbers. Imagining similar permeances for both fibers, we would expect the performance to be the same for low Reynolds numbers. The performance improvement due to the sinusoidal shape of the fibers is expressed through the slope of the curves. With increasing Reynolds number the increase in permeate water mass flow is higher for the sinusoidal-shaped fibers compared to the straight fibers. This effect is caused by the additional secondary mixing in the sinusoidal-shaped fibers, which reduces the concentration polarization and therefore laminar boundary layer. As expected, concentration polarization governs the process of drying compressed for high sweep flow rates. In this region, fibers with a sinusoidal-shaped lumen channel outperform the straight reference fibers. With this performance increase, we prove the arising of secondary flow in those channels and its capability of reducing the laminar boundary layer on the feed side.

area to equilibrate the feed and sweep flow in terms of water vapor pressure. For sweep flow rates above 2 NL/min, the permeate humidity further decreases as a higher amount of nitrogen dilutes the water vapor, while the retentate humidity approaches a constant value. Under these conditions, the water transport over the membrane reaches a maximum and the sweep flow causes no transport reduction. Therefore, sweep flow rates above 2 NL/min are suited to measure the influence of the feed side flow conditions on the water transport rate and permeate side effects can be neglected. As a result, the comparison study with a module of A-straight and Asine is conducted for the sweep flow rate range of 0–4 NL/min to map the transition of the conditions. Fig. 15 shows the area normalized water transport rate through the membrane for both modules over the sweep flow rate range. For each module (A-straight and A-sine), three different feed pressures are compared at a constant feed mass flow. The water transport rate is measured at 3, 5 and 8 bar feed pressure. Any nitrogen permeation occurring is small compared to the total flow rates in the feed and sweep flow and is therefore neglected. Improved secondary mixing in sinusoidal-shaped fibers is confirmed, if the overall increase in mass transfer is significantly higher than the increase for straight fibers under the same conditions. Other causes for increased mass transport, other than the boundary layer thickness, such as the reduced diffusion coefficient at higher pressures, will be the same for straight and sinusoidal-shaped fibers. The regimes identified in Fig. 14 appear here as well. For low sweep flow rates (below 1 NL/min), the mass transport is directly proportional to the sweep flow rate. Feed and sweep reach equilibrium. Under this conditions, the driving force becomes small and losses through concentration polarization can be neglected. At higher sweep flow rates (above 3 NL/min), the amount of water vapor transported over the membrane is not enough to equilibrate feed and sweep stream. Hence, the concentration difference increases and boundary layers have to be considered. For both fiber types, the mass transport rate is highest at lower feed pressures. The diffusion coefficient in the boundary layer is lower at low feed pressures. Lower pressures also correspond to a higher actual volume flow rate and vice versa. Additionally, a higher volume flow rate is humidified. Therefore, the total amount of water in the feed is higher at lower pressures. A higher volume flow rate increases the velocity of the feed stream, which rises the feed side Reynolds number and reduces the thickness of the boundary layer. Due to

4. Conclusion The superposition of the spinning of sinusoidal-shaped hollow fibers with the chemistry in a spinneret concept was successful and resulted in 123

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sinusoidal-shaped composite hollow fiber membranes. A home-made pulsation device creates the sinusoidal shape of the fibers. The composite layer forms out of polyethyleneimine (PEI) from the polymer solution and glutaraldehyde (GA) from the bore fluid. The sinusoidal variation of the bore fluid flow is properly solidified in the lumen channel. Within the limitations of the process, the set frequency and amplitude of the bore fluid flow are transferred to the lumen channel geometry. For aqueous applications, the separation layers of the sinusoidal-shaped and straight fibers exhibit the same pure water permeance, salt retention and molecular weight cut-off. These results prove the formation of a defect-free separation layer. Concentration polarization governs the process of drying compressed air and the developed composite fibers are suitable for this application. The sinusoidal-shaped fibers transport significantly more water to the permeate side at higher Reynolds numbers compared to the straight reference fibers. Vortex formation and reduced boundary layer resistances in the feed channel cause the increase in performance. As continuing material innovations make gas permeation and reverse osmosis membranes more and more permeable, measures to reduce concentration polarization will become exceedingly important. We are convinced that the here presented composite hollow fiber membranes with a unique shape will trigger further research and innovation in this area.

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Acknowledgements

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This work was performed in part at the Center for Chemical Polymer Technology CPT, which is supported by the EU and the Federal State of North Rhine-Westphalia (Grant no. EFRE 30 00 883 02). This project has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 694946). The research work (IGF-project 18218 N) of the research association “Forschungskuratorium Textil e.V.” was supported via the AiF within the funding program Industrielle Gemeinschaftsforschung und Entwicklung (IGF) by the Federal Ministry of Economic Affairs and Energy (BMWi) due to a decision of the German Parliament. M. Wessling appreciates the support from the Alexander-von-Humboldt foundation.

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