One of the important parameter in migration of SMM from the polymer dope to the surface of the membrane is the time between casting or spinning the polymer solution and immersion in the coagulation bath. For the flat sheet membrane, this time can be as long as needed, but in the hollowfibermembrane fabrication process, this time is very limited and depends on the air gap length. In order to study of the air gap effects on the morphology of hollowfiber membranes, several researches have been done. Bakeri et al. (2012a) studied the effect of air gap length of surface modified polyetherimide PEIhollowfibermembrane by application of Response Surface Methodology (RSM). Their regression models could provide some statistically meaningful results. For example, their model for membrane pore radius predicted that plot of membrane pore radius versus air gap has a minimum point. Khulbe et al. (2007) fabricated PES hollowfibermembrane with blending 1.5 %wt. of SMM in spinning dope. Their results showed the contact angle of the outer surface of the fabricated membranes increased significantly when the air gap had increased from 10 to 30 cm, but the pores diameter were almost constant in that range of air gap.
Bakeri et al.  added low molecular weight organic compounds to the spinning dope as phase inversion promoters and studied their effects on the structure of polyetherimide (PEI) hollow fibers. They employed water, methanol, ethanol, glycerol and acetic acid as additives in the spinning dope and fabricated hollowfiber membranes via the wet spinning method. Their results showed that the solution containing water as additive had the lowest thermodynamic stability and highest viscosity, which yielded a hollowfiber with a thin skin layer of high porosity and a sublayer with sponge-like structure. The four other polymer solutions were more stable thermodynamically and less viscous. Among all their fellow fiber membranes, adding methanol resulted in the highest absorption flux.
Since dry jet-wet phase inversion method was used for the fabrication of membrane #M1, the VIPS (Vapor Induced Phase Inversion) process makes a nascent skin layer on the outer surface of the membrane that can hinder the penetration of coagulant; the lower length of macrovoids originating from the outer surface of the membrane #M1 can be related to this phenomenon (see Figure 3). Furthermore, the polymer solution #2 is more unstable thermodynamically than polymer solution #1. However, the characteristics tests results for the fabricated membranes (Table 5) show that membrane #M1 has bigger mean pore size (191% higher) and higher gas permeation rate (566% higher), i.e. the skin layer of the membrane #M1 is more porous that may be related to the higher hydrophilicity of PES solution that makes more intrusion of coagulant into the structure of nascent fiber. On the other hand, membrane #M2 has higher porosity that can be related to the lower concentration of polymer at the cloud point. The higher porosity of membrane #M2 provides lower tortuosity that reduces the effective diffusion length of the vapor through the membrane. The LEPw test results show that membrane #M1 has lower wettability, i.e. the size of the biggest pores on the surface of membrane #M1 is smaller than membrane #M2 and the pore size distribution of membrane #M2 is wider than membrane #M1.
A newly developed of corn cob ash (CCA) as an alternative material for ceramic hollowfibermembrane was successfully reported. Interestingly, from the preliminary characterization study, CCA has a great potential to practically used as raw main substrate with low sintering temperature (1000 °C) which indirectly able to reduce energy consumption. From the findings, due to remaining ash starch that cannot easily eliminated is able to create gelanization into membrane suspension which put some membrane fabrication constraints in facilitating the spinnability of suspension. Thus, CCA treatment needs to be attempted and will be reported in the next futher detail investigation that will be carried out to ensure the applicability and suitability of this proposed materials towards membrane applications. Thus, by utilising corn cob ash as the low cost ceramic material, it could successfully overcome the issue of significantly higher production cost of ceramic membrane. Knowing that, this corn cob waste has transformed into valuable and useful material, especially in membrane applications.
economically feasible process and is a creative method [9-16]. The advantages of using hollowfiber membranes contactors are the membrane fibers have a large area per unit volume compared to traditional distillation column [17, 18]. Liquid membrane contactor module using silver nitrate solution successfully for separation of Olefin/paraffin but not to the extent of commercial achievement due to the high cost of the silver nitrate solution . Other than silver are copper salts can from complexation with olefins in the same manner that silver does, by contrast, copper carriers also suffer from stability problems and not easy to tackle, membrane was used as an air-sweep vacuum membrane distillation using fine silicon, rubber, hollow-fiber membranes . Gas-liquid membrane contactors are a new and effective strategy for gas absorption that integrates membrane separation and liquid absorption. Compared with traditional absorption process, membrane contactor combines the advantages of both liquid absorption; low cost and high selectivity and membrane separation; high surface area to volume ratio and compact equipment. Some researchers studied various, operating parameters on the membrane separation performance, the concluded that the silver nitrate concentration and trans-membrane pressure does effect separation performance significantly [21, 23]. The effect of pressure ethylene separation using gas-liquid hollowfibermembrane contactor was not fully covered; also, pervious mathematical models did not include the effect of pressure and silver nitrate concentration in their models .
Based on hydrodynamics study, as expected, an increase in water flow rate, or fiber packing density increased energy losses in the contactor as shown in Fig. 5. Since the fiber packing densities of the modules were close, differing by less than 10%, application of the friction factor correlation to packing densities varying greatly from 18.8 to 28.1% of the pipe cross- sectional area should be made with care. Therefore, the application of the fiber friction factor correlation to other fiber types and contactor designs may require
membrane. For pristine membrane, the surface roughness was 93.174 nm and was decreased to 74.408 nm for optimum membrane. The decrease in the roughness can be due to the surface enrichment of PES by well dispersed APTES-SiO 2 . The reduced surface structure could be as a result of the interaction between APTES-SiO 2 particles in the casting solution and the phase inversion kinetics. The incorporation of APTES-SiO 2 particles in the PES casting solution will reduce the activity of polymer and DMAc, increase thermodynamic stability, thereby decreasing the driving force for DMAc outflow of fiber precipitation in external coagulation bath and accordingly produce membrane with smoother surface roughness. This result is in tandem with observation by Vatanpour et al. (2012) who found that the mean roughness decreases with increase in TiO 2 coated CNTs. This is different from the result by Dutreilh- Colas et al. (2008) who found that the roughness parameters of an hybrid silica film increases due to the presence of small drops of silica on the surface. Yang et al. (2015) also reported that the roughness of acrylate polyurethane film increases when coated with SiO 2 film. High roughness might translate to high porosity (see Table 4.8) and the nano-structured architecture of membrane (Zhang and Wang, 2014). Thus, in this work, the decrease in roughness of composite membrane may be associated due to the impregnation of well dispersed APTES-SiO 2 in PES matrix.
structure of membranes to achieve optimum separation performance (Ismail and Lai, 2003). Figure-2 displays the SEM images of PEIhollow fibre membranes using various polymer concentrations. Figure-2a shows the morphological structure of 10 wt. % PEI in NMP. The lower concentration of PEI in dope solution leds to loosely packed structure, thereby the non-solvent water penetrated through the inner surface of hollow fibre membrane and created pores on the membrane surface. Prior to entering the water coagulation bath, the inner water diffused rapidly through the membrane structure and created pores on the surface largely due to the loosely packed polymer chains. The phase separation process was so instantaneous that it created finger-like pores that extended all the way to the outer surface. This morphology will lead to poor separation performance and low mechanical strength (Benjamin and Li. 2009). Similar morphology was found for 15 wt.% PEI/NMP dope solution (Figure-2b). This indicated that 10 and 15 wt. % PEI concentration in NMP solvent is too low to form mechanically strong membranes which may achieve good separation performance. For 20 wt. % PEI concentration, finger-like pores existed but the length and width of the pores have been greatly enhanced forming macrovoids. Also, thin-skinned layer was observed and finger-like pore structure dominated the outer skin layer. The presence of outer skinned layer ensures its ability to perform in separatioin applications but the presence of macrovoids in the surface morphology suggested a membrane with higher permeance but lower selectivity.
The present work describes CFD simulations that couple the Navies-Stokes equations with the energy conservation equation in a two-dimensional domain to describe the hydrodynamic and thermal conditions in a single hollowfiber module with laminar flow for DCMD process. A newly developed heat transfer model, which allows the latent heat transfer due to the evaporation/condensation processes during the MD process, but ignores the transmembrane mass flux itself, has been used to estimate the heat transfer coefficients at different fluid conditions, temperature profiles, temperature polarization coefficients (TPC), mass flux distribution, heat loss and MD thermal efficiency. The aim of this work is to provide a deeper insight into the heat and mass transfer phenomena in the DCMD process and to guide further optimization of MD operation for performance enhancement.
Changing die wall thickness impacts the die shear rate and die swell (L/D). Hollowness increases when die inner diameter is increased and die wall thickness is decreased: 10- 15% hollow compared to 5-10% hollow for PET fiber (Oh, 2006). The die capillary, L, provides space for the polymer melt to relax before extrusion. With constant hollow area and capillary length, decreasing wall thickness results in increased L/D. Increasing L/D leads to less variation in hollowness with processing conditions (spinning temp, quench, mass throughput, spinning speed) (Oh, 2006). The die gaps are the spaces between the segments through which no polymer is extruded. Air flows across the gap and into the fiber center and keeps the hollow from collapsing. The hollowfiber is formed by the polymer melt closing gap, “coalescing”. Die swell negatively impacts hollow formation by increasing the distance below the die at which coalescence happens, a.k.a. the “fissure length” (Rwei, 2001).
this eﬀect was diminished at pH 2.5 resulting in the nearly constant permeation rate as observed. This phenomenon might be related to the extractant saturation at feed-liquid membrane interface 16) and to the hydrogen transfer resistance in the boundary layer of feed. 17) The permeation rate with 30 mass% PC-88A was similar to that with 10 mass%. This result agrees with the results obtained by Youn et al. during the separation of Co and Ni. 17) The permeation rate increased with extractant concentration as they form more complexes with metal ions at the feed solution-membrane interface. In the meantime, this could increase the viscosity of liquid membrane that should counteract the permeation above a certain extractant concentrations as observed.
The hollowfiber performance data are summarized in Table 2. As could be observed, the mechanical properties of hollow fibers start to substantially decrease with the decrease of the air gap from 40 mm. When the air gap decreases from 40 mm to 10 mm, the tensile stress and elongation decline by 30% and 63%, respectively. However, the pure water permeability increases by 166%. One possible reason is due to the higher orientation of polymer chain at high air gap length than that at the low air gap length . The second possible reason can be the growth of finger-like macro-voids with the decrease of air gap; specifically at 10 mm of air gap, the macro-voids are almost across the membrane wall. The results may be caused from dry-jet wet-spinning process. During dry-jet wet-spinning process, the nascent fiber experienced a convective internal coagulation in the lumen and a non- convective external coagulation in the air gap region. Then, rapid solvent exchange at external surface in non-solvent coagulation batch; in the air gap region, the moisture-induced phase separation much slower than in coagulation batch slowed the speed of polymer chain contracting and provided a needed time to contracting polymer chains for conformation rearrangement . As a result, when the air gap length decreases, the contracting polymer chains did not have enough time to rearrange conformation before quick phase separation in coagulation bath. This may result in the growth of finger-like macro-voids (see Figure 1). Furthermore, the as-spun fiber immersing in non-solvent coagulation bath was more rapid at low air gap length than high air gap. This leads to much more non-solvent and solvent trapped in the contracted polymer chains . Consequently, more macro-voids are formed in membranes at the low air gap. The phenomenon is similar to the result reported by Chung and Hu  on PES hollowfiber membranes.
The HFSLM system consists of three phases those are feed phase, organic membrane phase and stripping phase. The aqueous feed solution was 152+154 Eu and 90 Sr radioactive waste) and EDTA was used as stripping phase. The aqueous feed phase and stripping phase were in contact with the organic membrane phase. The extractant cya- nex301 (bis(2,4-4,trimethylphenyl)-dithiophosphonic acid in kerosene) is filled in the membrane pores. The aqu- eous feed flows inside the tube and stripping solution flows in the shell of module. The flow between tube and shell sides is in counter current direction. The transport mechanism of 152+154 Eu and 90 Sr are so called couple fa- cilitated counter-transport, as shown Figure 1 in Transport scheme of extraction and stripping in a liquid mem- brane process using cyanex301 as a carrier.
separation process has been extensively investigated and the process has been recommended by many researchers [6-13]. In these systems, the membrane module is comprised of bunches of hollowfiber membranes surrounded by an external metal casing (referred to here as the shell). Usually, the liquid solvent flows through a central distribution tube inside the membrane bundle, while the gas mixture enters from the casing side. However, vice versa is also possible. The large membrane surface area per unit volume is the main advantage of the membrane contactor compared to traditional contacting columns [14-19].
To relate the enhanced module performance with the hydrodynamic improvement by employing turbulence promoters of various specifications, Fig. 5 shows the local flow fields and temperature distribution in the modified modules. Since all turbulence promoters are inserted with regular intervals, the velocity profiles along the module length can be seen periodically between every two barriers. Therefore, only local flow fields within a certain range of fiber length (0.105−0.125 m) for modified modules are presented in Fig. 5. The velocity profiles (flow fields) are described by the stream traces and temperature distribution by band colors. These results are consistent with the trends of the heat-transfer coefficients curves shown in Fig. 3 and TPC distributions in Fig. 4 for these modified modules. Clearly, in Fig. 5 (a) the attached round spacers (r=0.5 mm) do not show effective disturbance in the bulk flow. As those round spacers are raised (Ly=1.5 mm) and have a larger diameter, stronger secondary flows form in between the barriers and vortices appear near the membrane surface to reduce the thickness of liquid-boundary layers. Similarly, there is no visible altering effect from those short and wide quad spacers (e.g., ∆x×∆y×Lx=0.5mm×0.5mm×10mm). The secondary flows between spacers become more intense with an increasing ∆y [from 0.5 mm to 2 mm in Fig. 5 (b)]. As the gap between membrane surface and spacers Ly increases from 0 to 1.5 mm till the spacers reach the shell wall (i.e., baffles), more vortices form along the x direction and more intense radial mixing is observed from the schemes of flow fields. The flow tends to be more homogenous when an alternate arrangement of attached spacers and baffles 0.2×2×10 is employed. Combined with the simulation results shown in Figs. 3 and 4, in a liquid-film controlled heat-transfer system, the more intense secondary flows and radial mixing will result in reduced thermal boundary layers, alleviated TP effect and hence enhanced heat transfer.
This paper starts by summarizing the basic types of membrane module used for aqueous separations with a focus on hollowfiber modules and related mass transfer models. Then we discuss passive process enhancement techniques involving module/fiber configuration designs and active process enhancement techniques involving shear enhanced aids (vibrations/oscillation, bubbles and ultrasound etc) [10-12]. The focus is given to the latest developments in hollowfiber module design concepts and principles of mass transfer enhancement, because hollowfibermembrane technology is an attractive platform for many engineering processes. Moreover, by analyzing the working principle of each enhancement mode for practical applications, their benefits, limitations and technical requirements are addressed in terms of economic considerations (fabrication cost and complexity, energy demand) and processing engineering (scale-up potential and niche applications). It is hoped that this review can provide insights and inspire novel module design to enhance system performance of membrane-based processes for liquid separations.
RSM is a collection of mathematical and statistical techniques that are used for modeling and analyzing the applications where a response of interest is influenced by several variables. In fact, the main aim of this technique is to optimize these responses. Previous studies show that the hollowfiber spinning processes were affected by numerous parameters such as air gap and coagulant. These factors increase the occurrence of over fitting when a model is excessively complex. Therefore, it is necessary to select the parameters that had major effects on the responses . Since the selectivity and permeability are the crucial features in the performance of membranes, variables must be determined to improve these features. Surface hydrophobicity and pore size are the two important parameters that influence the selectivity via controlling the amount of LEPw. As mentioned before, high LEPw only allows vapor molecules to pass through the pores and then to condense on the cold side. The relation between the membrane parameters and the MD flux can be written as follows :
Virus production has long been an attractive applica- tion for the use of hollow-fiber bioreactors. Cells grown in high density should provide rapid and uni- form infection kinetics for virus propagation, and the resultant virus should be obtained at very high titers. Effective protocols for the production of different viruses have proven elusive. Many of the first at- tempts at virus production centered on retrovirus production in 3T3 or PA317 packaging cell lines. These cell lines are of fibroblastic origin and are ex- tremely adherent. Cells quickly overgrow the hol- low-fiber cartridge, reducing the harvest volume from 10–15 mL to as little as 0.5 mL. Although the virus harvested was of a high titer, the small total volume of virus recovered made this method ineffec- tive. Additionally, if it were necessary to propagate the virus by infection, only the outer layer of cells would become infected and produce virus.