Several spinning sessions were performed (D1–D5), the studied spinning parameter range, as well as separation performance of hollow fibers of each spinning sessions are presented in Table 3
. For each spinning session a dope composition was established, and the spinning conditions were varied in order to find the optimal combination of spinning parameters for achieving the best performance for gas permeation. As-spun fiber separation performance was evaluated. These results are compared with gas permeation through a 20 µm thick dense flat film of P84® at similar operational conditions (1.28 Barrer CO2
; 36.6 CO2
selectivity at 35 °C) [32
]. Asymmetric membranes are defined to be “defect-free” if the ideal selectivity is greater than 80% of the intrinsic selectivity of dense films [45
]. In the case of asymmetric P84® hollow fibers this value has been fixed at a CO2
selectivity of ~29.3 at 35 °C.
3.2.2. P84®/NMP/THF/EtOH Systems
The addition of THF to the spinning dope results on the obtention of less defective fibers. Figure 5
shows that an increase in ideal CO2
selectivity is observed by decreasing the NMP/THF ratio (THF content increases). Dope D3, with an NMP/THF ratio of 6, gives fibers with CO2
permeances lower than 193 GPU and CO2
selectivity ranging from 3.28 to 21.4. A further increase in THF content in D4 (NMP/THF ratio of 2.4) promotes the formation of a tighter skin layer and therefore, less permeable fibers (CO2
permeance < 35 GPU). However, the obtained selectivity value ranging from 15.2 to 17.4, almost half of the dense film selectivity value, denoted the presence of small defects in the selective layer of the fibers. The THF content was further increased to NMP/THF ratio of 1 for the D5 spinning session. Also, in order to accelerate phase separation LiNO3
was introduced in the system [43
]. For all prepared fibers, an ideal CO2
selectivity higher than 20 was obtained. Even more, a selectivity of 40.4 was obtained for one of the states, higher than the dense film selectivity values. This phenomenon has been reported before for 6FDA-based polymer spinning [46
]. It was hypothesized, that it was due to the uniaxial orientation of polymer chains resulting from the high shear rate in the spinneret. This results in a tighter packing of the polymer chains, leading to an increase in selectivity over the unaligned state of polymer chains of the dense film. This demonstrates that as-spun fibers with a small number of defects or even defect-free fibers can be obtained by fine tuning of the dope composition and spinning parameters.
The performance of the four different fibers states obtained from dope D5 are analyzed in detail. Spinning conditions, separation performance and selective layer thickness of the four states of hollow fiber membranes are presented in Table 4
. The selective layer thickness was estimated from the intrinsic CO2
permeability of the dense film and the permeance value of the asymmetric hollow fiber membrane. The influence of two spinning parameters was studied: The air gap height and the spinneret temperature.
Selective layer thickness, CO2
permeance and ideal CO2
selectivity as a function of the air gap for two spinneret temperatures (25 and 40 °C) are presented in Figure 6
. The selective layer thickness is increased by the increase of the air gap (from 2 to 10 cm) for both spinneret temperatures, from 56 to 262 nm and from 107 to 363 nm for a spinneret temperature of 25 and 40 °C, respectively. Correspondingly, a decrease in CO2
permeance is observed by the increased air gap height. The dense selective layer is formed by the evaporation of the solvent in the air gap, mostly volatile THF. As the air gap height increases, the residence time of the fiber in the air gap increases, and hence, the evaporated solvent amount, increasing the polymer concentration at the outermost region of the fiber. Therefore, an increase in selective layer thickness and a decrease in gas permeance is expected by increasing the air gap.
also shows that an increased selective layer thickness and a decreased CO2
permeance is observed for the higher spinneret temperature. The increased spinneret temperature may induce a larger amount of evaporated solvent and therefore an increased selective layer thickness and hence a decreased CO2
SEM images of the cross-section of the overall fiber, fiber wall and the selective layer as well as the outer surface of hollow fiber states prepared at the lowest air gap and spinneret temperature (ST-1) and highest air gap and spinneret temperature (ST-4) are presented in Figure 7
. Both fibers present an outer diameter of ~430 µm and a similar sponge-like substructure. The trend observed in the estimation of the selective layer thickness from the CO2
permeance value (i.e., 56 nm for ST-1 and 363 nm for ST-4) is somehow confirmed by the SEM images, showing a thicker selective layer of ~500 nm for the latter. The selective layer thickness measured by SEM is higher than the one given by CO2
permeance due to bending of the outer layer produced during the freeze-fracturing of the sample in liquid nitrogen. Although a cleaner cut could improve the estimation of the selective layer thickness by SEM, in most cases would not be easy to identify the borderline between selective layer/transition layer/porous support. In addition, SEM analysis is local and did not give an average. Although the calculation of selective layer thickness from CO2
permeance value is also a simple estimation—it does not consider substrate resistance to the overall membrane resistance—it is useful for evaluating if the produced fibers have the thinnest achievable selective layer (i.e., ~100 nm) or if there is still room for optimization.
On the other hand, Figure 6
c shows that the threshold selectivity value to consider P84® fibers as defect-free (CO2
~29.3) is attained only for the lowest air gap height of 2 cm for both spinneret temperatures. This result is in contradiction with the expected increase in selectivity with the increase in solvent evaporation rate at larger air gaps. We speculate that, during the residence time of the spun fiber in the air gap, an early phase separation on the outer surface of the fiber could be induced by water absorption from the ambient humidity. This may cause formation of small defects in the outer dense selective layer. A short air gap helps to minimize the effect of the water vapor within the air gap, and therefore, the number of defects. Nevertheless, the obtained selectivity value of 20–25 for the maximum air gap of 10 cm suggests that the defects could be easily healed by the conventional silicone rubber coatings.
The highest permeability and ideal selectivity were obtained for fibers spun with the smallest air gap (2 cm) and lowest spinneret temperature (25 °C). An ultra-thin selective layer of ~56 nm was obtained, almost ten times lower than the 500 nm thick selective layer obtained by Barsema et al. [32
]. These hollow fiber membranes present an ideal CO2
selectivity of 40.4, and the highest permeance reported in literature for P84® hollow fibers, 23 GPU of CO2
at 35 °C, against 2.2 GPU at 25 °C reported by Barsema et al.
A new spinning process has been performed at a larger scale (~5000 meters of fibers) using the optimal spinning parameters from D5, ST1 with the difference that we used a flow of dry N2
in the air gap with the objective of eliminating humidity influence. Five modules containing 10 fibers each, taken at different production times have been characterized and the average value is provided in Table 5
. For comparison purposes, the hollow fiber performance for the reference spinning process is included. The ideal selectivity was reproduced for the scaled up fibers, while the CO2
permeance was lowered. We assume there was a combination of factors that gave lower CO2
permeance, like faster evaporation of solvent induced by forced N2
flow in the air gap, or small variation in the other spinning parameters like room temperature (15 °C for spinning D5_ST1 versus 25 °C for up-scale D5_ST-1). SEM images of up-scaled fibers are presented in Figure 8
. Two fibers were subject to SEM analysis and several areas of the selective layer were checked. A selective layer thickness of around 200 nm could be estimated from the clearest cut of fiber 2 area B, slightly higher than the one estimated from CO2