Effect of bore fluid flow rate on formation and properties of hollow fibers
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In this work, for high performance and wide range of ultrafiltration applications, the effects of the most widely used values of internal coagulant flow rates (ICFR) (i.e., 2.6, 3.6, 4, 5, 7, 9, 11, and 13 ml/min) on the different features of the polyvinylchloride hollow fiber have been investigated. Both the idealized straight and the cylindrical pore with small effect of tortuosity were approximately obtained through the effect of ICFR. Atomic force microscope (AFM), scanning electron microscope (SEM), and ultrafiltration measurements were utilized to characterize the hollow fibers. The SEM and AFM results indicated that the cross-sectional morphology of the fibers is changed significantly with various ICFR. The structure of the inner surface was also changed from an open cellular structure to a porous structure by means of high pore density and small pore diameter. In addition, the membrane thickness was reduced by 314% with an increase in the ICFR from 2.6 to 13 ml/min. The pure water permeation flux was improved 17 times when ICFR was increased to 13 ml/min, while the BSA rejection remained within the acceptable range (from 93.4 to 90.4) when the ICFR was increased from 2.6 to 9 ml/min.
KeywordsHollow fiber Ultrafiltration PVC Internal coagulant Separation performance
One of the main goals of membrane technology is to control both membrane pore size and distribution during the configuration, as they have a large impact on the separation performance of any membrane. Several factors affect membrane pore size and distribution such as composition of the polymer, composition and type of the additives, air-gap length, type and temperatures of inner and outer coagulants, flow rate of the polymer solution, flow rate of the internal coagulant, and the design of the spinneret. In fact, the flow rate of the internal clotting is the most significant factor influencing the membrane pore size and distribution (Jack et al. 2006). Therefore, one of the main goals of this effort is to investigate this hypothesis. The literature review indicated that the effect of the flow rate of the internal coagulant on the characteristics and performance of the fiber has not been extensively studied (Jack et al. 2006; Chung et al. 1977; Aptel et al. 1985; Mok et al. 1995; Miao et al. 1996; Qin and Chung 2004; Wan and Chung 2015; Cheng et al. 2017; Peng et al. 2012). For example, Chung et al. (1977) studied the fabrication of polyethersulfone (PES) membrane with ultrathin skin layer membranes of 50 nm skin layer thickness controlled by the lumen liquid flow rate and lumen liquid chemistry. Aptel et al. (1985) investigated the influence of the internal coagulant flow rate on characteristics of polysulfone (PSF) ultrafiltration. They reported that, as the non-solvent flux in the lumen side of the nascent fiber increased from 4 × 10−2 to 6 × 10−2 cm3/s, the hydraulic permeability also increased from 2 × 10−8 to 6 × 10−8 cm/s-Pa. Whereas, rejection for polyvinylpyrrolidone [PVP, molecular weight (MW) = 10,000] decreased from 80 to 40%. The influence of the internal coagulant flow rate, the temperature, and the composition of casting solution on the separation performance of PES membranes was investigated by Mok et al. (1995). These authors found that the outer to inner radius of the hollow fiber seems to decrease with the internal coagulant flow rate.
The influence of internal coagulant flow rates (7.5 and 5 ml/min) on the properties of PES membranes made using dope spinning method was investigated by Miao et al. (1996). They reported that, at a constant air-gap length, by increasing the flow rate of the internal coagulant, both inner diameter (ID) and outer diameter (OD) can be enhanced with the reduction of the thickness of the wall for the formed membranes. The separation factor and permeability of UF membrane for polyethylene glycol (PEG) solutes reduced with the increase in the flow rate of the lumen-side coagulant due to the decrease in the skin layer thickness and the size of the skin layer pores, and narrow the pore size distribution on the skin layer. Moreover, Qin and Chung (2004) found that, as the flow rate of the internal coagulant diminished, the diameter of the inner PES membrane surface declined, while the diameter of the outer surface remained the same. Besides, as internal coagulant flow rate increases, the mass transport at the outer surface increases, while that at the inner surface is minimized, resulting in an increase in the macro-void length.
In fact, extensive work has been dedicated to the fabrication of PVC hollow fiber membranes for different separation applications. However, the effect of spinning parameters on either the fabrication of PVC fibers or the properties of the PVC fibers was not studied extensively. Membrane properties, especially permeability, can be affected by different resistances, which are a function of pore-size distribution, porosity, membrane barrier thickness, and the solvent properties, whilst both the idealized straight and the cylindrical pore with small effect of tortuosity are relevant to the influence of ICFR. Consequently, this study was focused on the impact of ICFR on the structural properties and membrane performance, as it exerts the greatest influence on the membrane thickness, size, and distribution of the pores. Inner and outer surface structures, as well as the fiber cross-section were characterized by SEM, and the ultrafiltration measurements were carried out using bovine serum albumin (BSA) with MW = 67 kg/mol as a solute.
The PVC resins and N, N-dimethylacetamide (DMAc) solvent were supplied by Sigma-Aldrich, while Chemical Co was used for membrane preparation. PVC with Mw of 65 kg/mol was obtained from Georgia Gulf Company (Georgia, USA). It was used as a membrane material in this study due to its excellent physical and chemical properties, stiffness, low cost, superior mechanical properties, and good solvent resistance.
Polymer spinning solution
PVC dope solution was prepared from 16 wt% PVC polymer and 84 wt% DMAc solvent. Dried PVC resin was gradually added into the covered container containing DMAc solvent and then mixed by a magnetic stirrer until the solution became clear and homogeneous.
PVC/DMAc hollow fiber spinning procedure
The PVC/DMAc homogenous solution was kept for at least 24 h to eliminate air bubbles. The PVC/DMAc solution was then transferred to a vertical column with 6 cm inner diameter. Throughout the entire spinning process, the temperature was maintained at 26 °C. Hollow fibers were prepared using the dry/wet spinning method with various internal coagulant flow rates, as described elsewhere (Alsalhy et al. 2011, 2013; Alsalhy 2013). The dope polymer solution was forced via pressurized nitrogen to the spinneret of 0.5 and 0.9 mm inner and outer diameter, respectively. The PVC/DMAc ratio was 16/84 wt%, and the fabrication conditions of the prepared PVC membranes were as follows: 1.75 bar extrusion pressure, 3 cm air-gap length, 19 °C external coagulation bath temperature with tap water, and bore fluid with various flow rates (i.e., 2.6, 3.6, 4, 5, 7, 9, 11, and 13 ml/min).
The experimental apparatus and the process procedure were described elsewhere (Alsalhy et al. 2011, 2013, 2014; Alsalhy 2013). It is well known that water is always used as an internal and external coagulant in hollow fiber preparation and other membrane configurations due to the low cost of solvent and its good affinity and solubility with all of the polymer solvents. Therefore, in this work, water was used as the internal and external coagulant liquid. The prepared fibers were wetted and kept in a tap water bath at ambient temperature for 24 h to eliminate the residual DMAc solvent. PVC fibers were kept for 72 h in a container containing 40 wt% glycerol aqueous solution to avoid the fiber structure collapse. Prior to testing, the PVC fibers were dried in air at room temperature.
SEM and AFM observations
The internal and the external membrane structures were tested by means of scanning electron microscopy (SEM) at University of Technology/Nanotechnology and Advanced Material Research Center. The hollow fiber (HF) membranes were dried and then immersed in liquid nitrogen for 15 s and thereafter cut to reveal the cross-sectional structure.
AFM device (Angstrom Advanced Inc., Braintree, Boston, USA) model AA3000 was used to obtain 2D and 3D images of the membrane surface. The fiber was subjected to a wide surface analysis using an AFM in contact mode with a tip made from silicon. A statistical distribution of the pore size was estimated for the PVC surfaces of each membrane using IMAGER 4.31 software.
Diameter, thickness, and porosity of PVC membranes
The diameter and thickness of HF membranes were determined using optical microscope Model B600.POL-I S/N: 0016243, Italy.
Results and discussion
Under lumen liquid rate of 5 ml/min, the layer in the center of the cross-section toward the external surface is seen to be merged with the layer located near the edge of the outer surface, and their final shape is transformed into a finger-like structure shown in Fig. 2c. However, the layer in the center of the cross-section toward the inner surface has been combined with the layer located near the inner edge of the fiber, whereby the final structure takes ellipsoidal shape, as shown in Fig. 2c. Further increase in the internal coagulant flow rate to 7 ml/min results in the two layers adopting the finger-like structure depicted in Fig. 2d. Moreover, due to the increase in the internal coagulant flow rate to 9 ml/min, two layers with different structures are formed. The first small one is located near the outer surface and takes a finger-like form, while the second layer is found near the inner surface and forms an ellipsoidal-like structure, as exhibited in Fig. 2e. Further, increasing the flow rate of the internal coagulant can be guided to create two layers of large finger-like structure with the same thickness such as that shown in Fig. 2f, even as the increase of the flow rate may be resulted to construct two layers of the finger-like structure, as shown in Fig. 2g. Figure 3 confirms the effect of internal coagulant flow rate on the inner surface structure of the hollow fiber membranes. It can be noticed that, as the internal coagulant flow rate increases, the obtained inner surface seems to be a skinless porous structure of the fiber membrane made under lumen liquid rate of internal coagulant of 3.6 ml/min, as depicted in Fig. 3a. Increasing the flow rate of the internal coagulant to 5 and 7 ml/min has also led to the formation of the same structure, along with diminishing in the pore size at the surfaces, as shown in Fig. 3b, c.
Actually, this phenomenon is due to the speed of the demixing process of water in internal coagulant with the solvent of polymer solution, followed by domain growth that reaches the point of domain (droplet) coalescence. In this case, both phases keep their liquid character until coalescence occurs. This should prevent the formation of a skin layer at the fiber inner surface, resulting in the formation of a microporous surface (Zeman and Zydney 1996; Alsalhy et al. 2011). This phenomenon occurs when the demixing process of the solvent in the dope solution and water in the internal coagulant is delayed.
Additional increase in the internal coagulant flow rate to 9 ml/min can result in the emergence of a porous structure with small pore size, as shown in Fig. 3d. Increasing the internal coagulant flow rate to 13 ml/min might produce a surface with porous structure and high pore density, as observed in Fig. 3e, in comparison with that given in Fig. 3d. This observation can be attributed to the instantaneous liquid–liquid demixing process between DMAC in polymer solution and the water in internal coagulant due to the short contact time between high amount of water and DMAC in dope solution. The short contact time between bore fluid and dope solution can be explained by the high exchange rate between water in internal coagulant and DMAC solvent in polymer solution. This results in an instantaneous liquid–liquid demixing process, which in turn leads to the removal of high amount of solvent from the dope solution during the formation of the hollow fiber. Consistent with the last finding, the solidification occurred very rapidly within the small porous structure of high pore density. Porter (1990) reported that very high rates of precipitation (corresponding to short precipitation time) result in the formation of a finger-like micro-void structure. The use of a larger volume of bore fluid causes complete polymer precipitation as the non-solvent penetrates from the bore towards the outer surface of the fiber. Fast demixing process (higher precipitation rates) forms finer pores while delayed demixing process (slow precipitation rates) results in rough structures.
It can thus be concluded that internal coagulant flow rate is the main factor in controlling the pore size and density (Jack et al. 2006; Miao et al. 1996). As increasing the internal coagulant rate results in a 17-fold increase in the permeation flux, this parameter has a significant impact on the membrane structure and/or membrane performance.
Membrane thickness and porosity
Effect of ICFR on porosity of the hollow fibers
Internal coagulant flow rate (ml/min)
The inner surface of the hollow fiber prepared from low bore flow rate has large-sized pores, as shown in Fig. 5. Increasing the bore flow rate has resulted in a decrease in the pore size at the inner surface, as demonstrated in Fig. 5. The rate of the reciprocity of solvent and water can play a significant role in the formation of the skin of the PVC inner surface. Thus, a higher rate of reciprocity of solvent and water leads to the formation of skin with a porous structure because of the influence of speed of bore fluid at the lumen side of the fiber.
The distribution of pore size of the PVC hollow fiber was studied according to the cumulative and the volume percentage of pore size, as illustrated in Figs. 8 and 9. As shown in Fig. 8, the cumulative percentage of the pore size at the inner surface at different bore flow rates is shifted to the left due to an increase in the lumen liquid rate. This means that the pore size at the inner surface follows a narrow distribution. Figure 8 also reveals that the volume percent of the inner diameter is shifted to the left as the bore flow rate increases. This observation corroborates the narrow range of the pore size on the inner surface. These results support the findings yielded by the SEM images of the inner surface.
Influence of internal coagulant rate on hollow fiber performance
The internal coagulant flow rate (ICFR) in lumen side of the hollow fiber through the spinning operation is one of the most influential factors determining fiber properties. Therefore, the influences of various ICFR (e.g., 2.6, 3.6, 5, 7, 9, 11, and 13 ml/min) on the structural morphology and ultrafiltration properties have been deduced. From the experimental results, it can be concluded that excessively high ICFR within a constantly flowing PVC dope solution leads to an inner membrane surface with a high pore density, small pore diameter, and small skin layer. Whereas, an overly low ICFR flow can result in the formation of an inner membrane surface with open cellular structure. This is due to the change of the demixing process at different ICFR, as well as the delay in the instantaneous liquid–liquid demixing process of both the solvent in the dope solution and the water in the internal coagulant. Our findings further indicate that the membrane thickness was reduced by 314% when the ICFR was increased from 2.6 to 13 ml/min. In addition, the pure water permeation flux was improved by 17 times with an increase in the ICFR up to 13 ml/min, while the BSA rejection was changed from 93.4 to 90.4 as the ICFR increased from 2.6 and 9 ml/min as a result of high pore density, reduced membrane thickness, and small pore size.
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