Shouliang
Yi
a,
Xiaohua
Ma
b,
Ingo
Pinnau
*b and
William J.
Koros
*a
aSchool of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311Ferst Drive, Atlanta, GA 30332, USA. E-mail: bill.koros@chbe.gatech.edu
bAdvanced Membranes and Porous Materials Center, Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia. E-mail: ingo.pinnau@kaust.edu.sa
First published on 24th September 2015
Acid gases carbon dioxide (CO2) and hydrogen sulfide (H2S) are important and highly undesirable contaminants in natural gas, and membrane-based removal of these contaminants is environmentally attractive. Although removal of CO2 from natural gas using membranes is well established in industry, there is limited research on H2S removal, mainly due to its toxic nature. In actual field operations, wellhead pressures can exceed 50 bar with H2S concentrations up to 20%. Membrane plasticization and competitive mixed-gas sorption, which can both lead to a loss of separation efficiency, are likely to occur under these aggressive feed conditions, and this is almost always accompanied by a significant decrease in membrane selectivity. In this paper, permeation and separation properties of a hydroxyl-functionalized polymer with intrinsic microporosity (PIM-6FDA-OH) are reported for mixed-gas feeds containing CO2, H2S or the combined pair with CH4. The pure-gas permeation results show no H2S-induced plasticization of the PIM-6FDA-OH film in a pure H2S feed at 35 °C up to 4.5 bar, and revealed only a slight plasticization up to 8 bar of pure H2S. The hydroxyl-functionalized PIM membrane exhibited a significant pure-gas CO2 plasticization resistance up to 28 bar feed pressure. Mixed-gas (15% H2S/15% CO2/70% CH4) permeation results showed that the hydroxyl-functionalized PIM membrane maintained excellent separation performance even under exceedingly challenging feed conditions. The CO2 and H2S permeability isotherms indicated minimal CO2-induced plasticization; however, H2S-induced plasticization effects were evident at the highest mixed gas feed pressure of 48 bar. Under this extremely aggressive mixed gas feed, the binary CO2/CH4 and H2S/CH4 permselectivities, and the combined CO2 and H2S acid gas selectivity were 25, 30 and 55, respectively. Our results indicate that OH-functionalized PIM materials are very promising candidate membrane materials for simultaneous removal of CO2 and H2S from aggressive natural gas feeds, which makes membrane-based gas separation technology an attractive option for clean energy production and reducing greenhouse gas emissions.
Commercially available polymers for gas separation membranes (polysulfone, cellulose acetate, polyimide, etc.) are limited by their moderate permeability and selectivity.4,11–13 The most promising materials, glassy polyimides, especially 6FDA-based polyimides offer: (i) solubility in common processing solvents, (ii) thermal and chemical stability, and (iii) robust mechanical properties under high-pressure natural gas feeds.10 Moreover, some polyimides also offer excellent intrinsic CO2/CH4 separation properties, i.e., high productivity and selectivity for feeds with low CO2 partial pressures, and are processable into thin-skinned hollow-fibers by standard commercial fabrication processes.3,8,10,14–29 While many membrane materials have been developed for the separation of CO2/CH4 over the past several decades, only a few studies have focused on the development of materials for removal of all natural sour gas components due to the high toxicity of H2S.3,4,11,21,22,24 To safely handle H2S, additional strict safety measures are required, so the few literature studies addressing H2S removal using membranes have generally focused on relatively low H2S concentration and feed pressure.11,30 Such studies show that polymers with high H2S/CH4 selectivities usually show CO2/CH4 selectivities of less than 10–15.11 Chatterjee et al. investigated the sour gas permeation properties for several rubbery polymers with selectivities as high as 74 for H2S/CH4 (1.3% H2S feed) for a feed pressure of 14 bar with H2S molar concentrations of 1.3% and 12.5%. Mohammadi et al. studied the acid gas permeation behavior of poly(ester urethane urea) membranes. Although H2S/CH4 selectivities of 34 and 43 were achieved for 0.6% H2S and 3% H2S feed, respectively, the corresponding moderate CO2/CH4 selectivity was only 14. Furthermore, their studies were performed for feed pressures less than 30 bar.30 These conditions are unlikely to reflect the most aggressive operating conditions in natural gas applications. Because many gas well pressures can reach 50 bar or higher, more aggressive feeds need to be considered, and this is the focus of our work. Feed streams containing both CO2 and H2S with relatively high total acid gas concentration and partial pressures require more robust materials capable of removing both acid gases, with stable properties.24
Recently, a number of advanced polymers, including 6FDA-DAM:
DABA (3
:
2), and 6FDA-based poly-amide-imides, have been developed for aggressive sour gas separations.3,21,22 The goal of the present work was to develop another novel membrane material for the simultaneous removal of H2S and CO2 from natural gas under aggressive feed conditions. To the best of our knowledge, intrinsically microporous polyimides have never been applied for sour gas separations. In this paper, we report, for the first time, a novel thermally annealed hydroxyl-functionalized polyimide with intrinsic microporosity (PIM-6FDA-OH) for simultaneous removal of CO2 and H2S from natural gas under aggressive feed conditions. Specifically, the performance of PIM-6FDA-OH was tested with high concentrations of H2S and CO2 feeds as well as high feed pressures.
![]() | (1) |
Permeability is commonly expressed in units of Barrer, where 1 Barrer = 1 × 10−10 (cm3 STP cm cm−2 s−1 cmHg−1).
In the case of nonideal gas mixtures, an alternative definition of permeability is used to describe the permeation driving force in terms of a fugacity difference rather than a partial pressure difference. For dense films, the fugacity-based permeability, which is used throughout this study, is defined as the flux (ni), normalized by the transmembrane fugacity driving force of component i (Δfi) and membrane thickness (l), as shown in eqn (2).
![]() | (2) |
The fugacity coefficients of pure H2S, CO2 and CH4 as well as their state in gas mixtures can be calculated using the Peng–Robinson equation-of-state and the SUPERTRAPP program developed by NIST.22
Permeability can also be expressed as the product of the effective diffusion coefficient, D, and sorption coefficient, S, of a given gas i within the membrane, as described in eqn (3).
Pi = DiSi | (3) |
The diffusion coefficient characterizes the kinetic contribution to transport, and the apparent diffusion coefficient D (cm2 s−1) of the polymer membrane can be calculated by D = l2/6θ, where l is the membrane thickness and θ is the time lag as deduced from pure-gas permeability measurements. The sorption coefficient S (cm3 (STP) cm−3 cmHg−1) can then be obtained from the relationship S = P/D.
The sorption coefficient represents the thermodynamic contribution to transport, and it can also be measured independently by pressure-decay sorption to allow D to be calculated from D = P/S. As shown in eqn (4), for cases with negligible downstream pressure, the sorption coefficient can be expressed as:
![]() | (4) |
![]() | (5) |
Koros et al. extended the dual-mode sorption model represented by eqn (5) to account for competition in binary gas mixtures at concentrations below which swelling induced complications occur.33,34 Recently, this dual-mode model was used for ternary mixed gas feed cases by Kraftschik et al.24 The ternary mixed gas dual-mode sorption model for components A, B and C can be given in eqn (6)–(8).
![]() | (6) |
![]() | (7) |
![]() | (8) |
The ideal selectivity between the fast gas (i) and slow gas (j), defined as the ratio of permeabilities (eqn (9)), equals the mixed-gas “separation factor” (eqn (10)) when the downstream pressure is negligible compared to the feed pressure, as valid in the current study.
![]() | (9) |
![]() | (10) |
To measure permeability, the gas was first introduced on the feed side and allowed to fill a ballast volume. The pneumatically actuated valve was then opened on the Labview® program to feed the membrane on the upstream side. As the gas permeated through the membrane to the downstream (permeate) side, the increase in pressure with time was recorded using the Labview® program until steady-state was reached. To ensure attainment of true steady state, the total flux was monitored until a constant value was achieved and then compositional steady state was verified. Specifically, not only was it verified that the total flux was constant, but also that the mole fraction composition of the various components contributing to the total flux was constant. As a practical matter, the attainment of total flux steady state and permeate compositional steady state coincided. This fact notwithstanding the achievement of steady state exceeded the expected time based on sample Fickian transport. Nevertheless, once steady state was achieved, it was stable. The upstream pressure was maintained constant throughout the experiment and also recorded using Labview®. A downstream pressure vs. time plot was then generated using the collected data. A Varian 450-GC was used during mixed gas permeation experiments to determine the permeate gas composition. The feed pressure and retentate flow were maintained by keeping the stage cut below 1% using an ISCO syringe pump and a metering valve. The syringe pump was maintained at constant pressure and the gas was fed at the same rate as the retentate vented through the metering valve. A low stage cut was used to: (i) prevent concentration polarization on the upstream side and (ii) maintain essentially a constant feed and residue concentration in order to provide a constant driving force across the membrane during the experiment. A high-temperature epoxy, Duralco™ 4525 (Cotronics Corp.), was used on all samples to seal the membrane-tape masking interface. This was done to prevent delamination of the masked films, which has been observed with less durable sealing epoxies when using highly sorbing species such as CO2 and, in particular, H2S.22
![]() | ||
Fig. 2 CO2/CH4 permeability–selectivity trade-off curve12,13 comparison of thermally annealed PIM-6FDA-OH to other polymers materials3,35,44,45 in pure-gas feeds at 35 °C. |
![]() | ||
Fig. 3 H2S/CH4 permeability–selectivity trade-off comparison of thermally annealed PIM-6FDA-OH to other polymers materials3,4,22,24 in pure-gas feeds at 4.5 bar and 35 °C. |
![]() | ||
Fig. 4 Pure gas (H2S, CO2, and CH4) sorption isotherms and dual-mode model fit of thermally annealed PIM-6FDA-OH at 35 °C. |
Polymer | H2S | CO2 | CH4 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
k D | C′H | b | k D | C′H | b | k D | C′H | b | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
a k D [cm3 (STP) cm−3 bar−1], C′H [cm3 (STP) cm−3], b [bar−1]. b Data from ref. 22. c Data from ref. 24. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
PIM-PMDA-OH | 5.28 | 45.9 | 2.19 | 1.52 | 45.7 | 0.40 | 0.81 | 9.63 | 0.72 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
6FDA-DAM![]() ![]() ![]() ![]() |
9.55 | 38.1 | 3.81 | 2.39 | 44.0 | 0.72 | 0.06 | 27.2 | 0.19 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
TEGMCc | 7.16 | 38.7 | 2.87 | 2.34 | 37.0 | 0.87 | 0.27 | 27.9 | 0.13 |
As shown in Table 1, the Henry's law sorption coefficients of H2S and CO2, kD, for the thermally annealed PIM-6FDA-OH film are significantly lower than those of 6FDA-DAM:
DABA (3
:
2) and crosslinked TEGMC. This may be due to the higher polarity of H2S, whose sorption depends more on physiochemical interactions with the polymer than simply free volume within the matrix. However, PIM-6FDA-OH gives a significantly higher Henry's law parameter, kD, for CH4, indicating that non-polar CH4 sorption depends more on free volume within the matrix rather than physiochemical interactions with the polymer. Table 1 also shows that the total sorption capacity of the lower-density regions (unrelaxed gaps), characterized by the Langmuir capacity constants, C′H, of both H2S and CO2 in the PIM-6FDA-OH are slightly higher than those of the other polymers, which is consistent with higher fractional free volume of PIM-6FDA-OH compared to the other materials.
To obtain insight into gas sorption levels under mixed gas feed conditions that correspond to the mixed gas permeation tests performed in this work, mixed gas sorption values were estimated using the dual-mode sorption parameters for H2S, CO2 and CH4 recorded in Table 1 according to eqn (6)–(8). Fig. 5 shows the predicted mixed gas sorption values for H2S, CO2 and CH4 at 35 °C with a 15% H2S, 15% CO2, and 70% CH4 feed mixture in the thermally annealed PIM-6FDA-OH material. Calculations were also performed with the crosslinked TEG-modified PEGMC material for comparison, and the corresponding dual-mode sorption parameters were taken from ref. 24. It can be seen that the gas sorption values for H2S, CO2 and CH4 are predicted to show a decrease in solubility in the mixture. Compared with the crosslinked PEGMC material, both H2S and CO2 sorption in the thermally annealed PIM-6FDA-OH material experienced a significant reduction under mixed gas feed conditions due to lower Langmuir affinity constant values. This affinity coefficient is an equilibrium constant equal to the ratio of solute sorption and desorption rate constants from the Langmuir sites,32 associated with H2S and CO2. However, the difference of CH4 sorption between the pure- and mixed-gas sorption isotherms in the thermally annealed PIM-6FDA-OH material was much smaller than that in the crosslinked PEGMC material due to a relatively high CH4 Langmuir affinity constant value in the PIM-6FDA-OH. This is currently not well understood, but based on the mixed gas sorption predictions, it is very clear that strong competitive sorption effects should exist in these sour gas separation systems. Although the mixed gas sorption calculations in this work are a good starting point, further research on mixed gas sorption experiments corresponding to mixed gas permeation tests would be a more useful approach for evaluating membrane performance for realistic sour gas separations. This approach may provide direct insight into how the material structure relates to competitive sorption under practically relevant operating conditions. Unfortunately, these tests were beyond the scope of the current work and we are not currently able to make direct mixed gas sorption measurements.
![]() | ||
Fig. 5 Sorption isotherms at 35 °C for the thermally annealed PIM-6FDA-OH and cross-linked PEGMC films with TEG as the cross-linking agent. Pure gas dual-mode sorption model fits are plotted along with mixed gas sorption predictions for a 15% H2S, 15% CO2, and 70% CH4 feed mixture. The dual-mode sorption model best fit parameters of cross-linked PEGMC are adapted from ref. 24 and used for calculations. |
The pure H2S, CO2, and CH4 permeation isotherms for the PIM-6FDA-OH dense films at 35 °C are shown in Fig. 6–8. It can be seen that no H2S-induced plasticization of the PIM-6FDA-OH film occurs in a pure H2S feed at 35 °C up to 4.5 bar feed pressure, and only a slight plasticization is observed above 4.5 bar of pure H2S. This can be explained by accelerated chain relaxation and reduced free volume in the annealed samples. In our previous studies on the annealing effect on the 6FDA-DAM:
DABA (3
:
2) polymer, the results showed that the annealed films exhibit non-negligible plasticization pressures.22 Increasing the annealing temperature from 180 °C to 230 °C caused an increase in the H2S plasticization pressure of approximately 25% to a value of 2.5 bar.
Similarly, the hydroxyl-functionalized PIM films indicated significant improvement in CO2 plasticization resistance up to 28 bar feed pressure (Fig. 7). Carbon dioxide showed a higher resistance to swelling due to its lower sorption, caused by its lower critical temperature compared to H2S. In comparison, for the unannealed PIM-6FDA-OH films, CO2-induced plasticization occurred at ∼6.9 bar.35 In addition to the increased plasticization resistance in the annealed films, the CO2 permeability of annealed PIM-6FDA-OH films decreased. Presumably, this is again due to accelerated chain relaxation and lower free volume effects caused by sub-Tg annealing.22 Swelling-induced plasticization is a common phenomenon for polymer membranes in gas separations involving aggressive feed streams, such as CO2 and H2S in sour gas.4 Plasticization occurs when a penetrant significantly increases the mobility of polymer chain segments, thereby increasing the diffusion coefficients of all penetrants in the membrane. Penetrant molecules with higher critical temperatures like H2S and CO2 are more capable of inducing swelling, because they have a considerably higher sorption capacity, particularly in glassy polymers.4,8,22,24 As shown in Fig. 8, methane does not plasticize at any of the pressures investigated.
As discussed in the theory section, the permeability coefficient is defined as the product of the diffusion coefficient and the sorption coefficient; therefore, the selectivity can be decoupled into diffusion and sorption selectivity. To better understand the role of hydroxyl groups in H2S/CH4 and CO2/CH4 selectivity, diffusion coefficients (D) and solubility coefficients (S) were measured using both the time-lag permeation method and pressure-decay sorption method (Fig. 4). Using the permeation and sorption results, the kinetic (diffusion) and thermodynamic (sorption) individual contributions of the thermally annealed PIM-6FDA-OH and the comparison to other polymers were calculated and are shown in Table 2. It can be seen that the D and S values determined from the two methods are in good agreement. Despite the lower diffusion coefficients of functionalized PIM-6FDA-OH, it shows much higher CO2/CH4 diffusion selectivity in comparison with PIM-1 and PIM-PI-3, which is more typical for a glassy polymer, with diffusion selectivity as the major contributor to overall permselectivity.22 Benefitting from the PIM segments in the molecular structure, the OH-functionalized polymers reported in this study showed increased CO2/CH4 selectivity while maintaining high permeability in comparison with traditional PIMs and PIM-PIs. It can also be seen from Table 2, the H2S/CH4 selectivity of the thermally annealed PIM-6FDA-OH is primarily derived from the solubility contribution despite the fact that it is a glassy material. Because H2S has a smaller kinetic diameter than CH4 and should be diffusion-favored, this result suggests that strong polymer–H2S interactions decrease the diffusion coefficient of H2S in the membrane. Similar results were observed in our previous research on 6F-PAI and 6FDA-DAM–DABA (3:
2) materials.21,22
Polymer | D (10−8 cm2 s−1) | S (10−2 cm3 (STP) cm−3 cmHg−1) | α D | α S | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
CH4 | CO2 | H2S | CH4 | CO2 | H2S | CO2/CH4 | H2S/CH4 | CO2/CH4 | H2S/CH4 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
a D is determined by the constant volume time-lag method; S is deduced based on the equation P = DS. b S is measured by pressure-decay sorption; D is calculated from P = DS. c Data from ref. 35. d Data from ref. 44. e Data from ref. 45. f Data from ref. 4. g Data from ref. 38. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
PIMs-6FDA-OHa | 0.844 | 8.92 | 0.637 | 4.98 | 16.7 | 46.4 | 10.6 | 0.75 | 3.35 | 9.3 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
PIMs-6FDA-OHb | 0.868 | 9.61 | 0.599 | 4.84 | 15.5 | 49.3 | 11.1 | 0.69 | 3.20 | 10.2 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
PIMs-6FDA-OHc | 2.02 | 9.88 | — | 4.51 | 26.7 | — | 4.89 | — | 5.92 | — | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
PIM-1d | 6.8 | 26 | — | 18 | 88 | — | 3.82 | — | 4.89 | — | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
PIM-PI-3e | 3 | 12 | — | 9.3 | 44 | — | 4.0 | — | 4.73 | — | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
CAf | 0.21 | 0.86 | 0.34 | 0.62 | 4.93 | 14.0 | 4.09 | 1.63 | 7.99 | 22.7 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Crosslinked PDMCg | 0.82 | 7.18 | 1.07 | 2.79 | 12.2 | 20.9 | 8.70 | 1.29 | 4.37 | 7.49 |
As shown in Fig. 9, it is very clear from the CH4 and CO2 permeability isotherms for PIM-6FDA-OH films that plasticization did not occur even at high acid gas partial pressures. However, severe H2S-induced plasticization effects were evident at the highest feed pressure of 48 bar. This is expected due to the very high condensability of H2S and its capacity to form polymer–penetrant interactions with the polymer backbone. It is somewhat surprising that despite the plasticization apparent for H2S, CO2 and CH4 as co-penetrants do not show apparent increases in permeabilities for the same mixed gas feed. Similar results were observed in our previous studies using 6FDA-DAM–DABA (3:
2) and CA materials.4,22,24 Again, as shown in Fig. 6, in pure gas feed, H2S-induced plasticization of the thermally annealed PIM-6FDA-OH film did not occur until the H2S feed pressure was higher than 4.5 bar (only a slight change even at 8 bar), which means the annealed PIM-6FDA-OH film has a much less pronounced plasticization response. Clearly, the plasticization/swelling effects under aggressive conditions that caused a reduction of CO2/CH4 selectivity to about 15 (unpublished data) in non-annealed films were significantly minimized in the films annealed at 250 °C. Interestingly, Fig. 10 shows that when the CO2/CH4 selectivity decreases, the H2S/CH4 selectivity increases with increasing feed pressure. This is because in a ternary system, the highly condensable gases, CO2 and H2S, now have to compete for the Langmuir sorption sites.4 Because H2S has a higher affinity for these sorption sites, the sorption capacity of CO2 is presumably greatly reduced. This leads to a decrease of CO2 permeability whereas an increase of H2S permeability is observed. Methane is not greatly affected in this case because it already has lower affinity for the Langmuir sorption sites than both CO2 and H2S, which shows a similar trend to cellulose acetate and thermally annealed 6FDA-DAM–DABA (3
:
2) films.4,22 It should be noted that at the high end of the pressure range (48 bar), the H2S/CH4 selectivity reached nearly 30 in the thermally annealed PIM-6FDA-OH film, which is much higher than most other glassy polymers (∼15) and is very competitive compared with rubbery polymers. More interestingly, the CO2/CH4 selectivity (∼25) suffers less significant loss versus the non-annealed sample, indicating the thermally annealed OH-functionalized spiro-polyimides in this study plasticized significantly less as compared to conventional non-crosslinked 6FDA-based polyimides. This unique performance at high pressures is very impressive, since most literature reports on rubbery polymers, which generally show very high H2S/CH4 selectivity, were determined only at low H2S concentrations and low pressures.
![]() | ||
Fig. 11 Partial immobilization model projections for pure H2S, CO2 and CH4 permeability at 35 °C in the thermally annealed PIM-6FDA-OH films. |
Permeation modeling based on dual-mode transport through glassy polymers can also be extended to mixed gases (eqn S5 in the ESI†).34 Because the dual-mode model parameters are calculated based on pure gas permeation and sorption data, deviations between mixed gas experimental permeation data and model predictions can be observed in some cases, especially for mixed gas feeds that lead to significant polymer–penetrant interactions or other effects that are not accounted for in the model. More complex models should be used in these cases. For instance, the so called “frame of reference” (or bulk flow) model can be used to account for bulk (convective) flux through the membrane (eqn S12–S22 in the ESI†). The bulk flux contribution usually becomes non-negligible under mixed gas feed conditions, especially when highly condensable species (like CO2 and H2S) are present. The detailed interpretation of these models and difference for the various penetrants are provided in the ESI.†
The dual-mode and frame of reference models were applied for permeation predictions for a 15% H2S, 15% CO2, and 70% CH4 mixed gas feed with the thermally annealed PIM-6FDA-OH films. The results are shown in Fig. 12 and 13. The predicted permeability and permselectivity values given by both models (especially the more complicated frame of reference model with bulk flux contributions) are in close agreement with the experimental values for some cases for binary mixtures in the literature,34,47–49 For the current case, however, it is clear from the permeability and permselectivity calculations shown in Fig. 12 and 13 that the presence of H2S in the feed mixture adds unexpected complexity to the system. The permeability and permselectivity values given by both models are quite poor in capturing the experimentally observed trends, even when the frame of reference is accounted for in the bulk flow model. The experimental permeability values for CO2 and CH4 are far below the model predictions, whereas H2S experimental values are well above the model predictions at higher feed pressures. Additionally, the predicted H2S/CH4 permselectivities are far below the measured values, while the measured CO2/CH4 permselectivities are well above the predicted value at lower feed pressure, although the frame of reference model can describe the CO2/CH4 permselectivities fairly well at the higher feed pressures. It is interesting to note that the frame of reference model gives significantly different predictions for permeability of H2S and CH4, and CO2/CH4 permselectivity than the simpler dual-mode model, however, the predictions in both cases are rather poor. Although at this time it is not possible to identify the exact transport mechanism leading to the poor agreement between observed permeation results and model projections in ternary mixtures containing H2S, it clearly deserves further investigation. Future work in this area will include considerations of models that may address the two models considered here for sour gas permeation performance of the glassy polymers used in this work.
![]() | ||
Fig. 12 Model predictions for H2S, CO2, and CH4 permeability of a 15% H2S, 15% CO2, and 70% CH4 mixed gas feed with thermally annealed PIM-6FDA-OH films. |
![]() | ||
Fig. 13 H2S/CH4 and CO2/CH4 permselectivity projections for 15% H2S, 15% CO2, and 70% CH4 mixed gas feed at 35 °C. |
Table 3 summarizes most of the mixed gas studies that have been conducted on polymeric dense films for sour gas separations. It is clear that the performance of the thermally annealed PIM-6FDA-OH films is very impressive compared to most other polymers shown in this table. The H2S/CH4 selectivity is 30 at 48 bar, which is much higher than most other glassy polymers, and H2S permeability is also much higher than cellulose acetate, 6FDA-DAM:
DABA (3
:
2) and novel 6FDA-PAI materials.21 Even compared with rubbery polymers, PIM-6FDA-OH still shows competitive H2S/CH4 selectivity and much higher CO2/CH4 selectivity, which indicates that this unique OH-functionalized PIM-PI membrane can be a very promising candidate for aggressive sour gas separations. Although PIM-6FDA-OH performance is similar to that of the crosslinked TEGMC and DEGMC, the avoidance of the need to crosslink the PIM-6FDA-OH may be a practical advantage.
Polymer | Pressure (bar) | Feed composition (mol%) | Permeability (Barrer) | Selectivity | Reference | ||
---|---|---|---|---|---|---|---|
(H2S/CO2/CH4) | CO2 | H2S | CO2/CH4 | H2S/CH4 | |||
PIM-6FDA-OH | 34.5 | 15/15/70 | 54.7 | 36.0 | 27.8 | 18.3 | This work |
PIM-6FDA-OH | 48.3 | 15/15/70 | 52.6 | 63.0 | 25.0 | 30.0 | This work |
Cross-linked TEGMC | 48.3 | 20/20/60 | 46.2 | 33.5 | 31.2 | 22.5 | 24 |
Cross-linked DEGMC | 48.3 | 20/20/60 | 54.6 | 38.2 | 28.4 | 19.3 | 24 |
Cellulose acetate | 34.5 | 20/20/60 | 8.66 | 8.71 | 29.5 | 29.7 | 4 |
Cellulose acetate | 48.3 | 20/20/60 | 27.5 | 39.7 | 19.1 | 27.4 | 4 |
Cellulose acetate | 10.1 | 6/29/65 | 2.43 | 2.13 | 22 | 19 | 11 |
6FDA-DAM![]() ![]() ![]() ![]() |
48.3 | 10/20/70 | 55.6 | 25.4 | 32.1 | 14.7 | 22 |
6FDA-DAM![]() ![]() ![]() ![]() |
48.3 | 10/20/70 | 50.8 | 23.6 | 31.1 | 14.4 | 22 |
Pebax 1074 | 10.1 | 12.5/18.1/69.4 | 155 | 695 | 11 | 50 | 11 |
PU1 | 10.1 | 12.5/18.1/69.4 | 55.8 | 183 | 6.9 | 23 | 11 |
PU2 | 10.1 | 12.5/18.1/69.4 | 195 | 618 | 5.6 | 18 | 11 |
PU3 | 10.1 | 12.5/18.1/69.4 | 62.2 | 280 | 12 | 55 | 11 |
PU4 | 10.1 | 12.5/18.1/69.4 | 50.8 | 223 | 15 | 66 | 11 |
6F-PAI-1 | 63.3 | 10/20/70 | 8.1 | 4.2 | 32 | 11 | 21 |
Based on the mixed gas permeation results, the performance of thermally annealed PIM-6FDA-OH films for aggressive sour gas separations appears quite promising. However, due to the inherent differences between rubbery and glassy polymers in terms of CO2/CH4 and H2S/CH4 separations, it is difficult to compare the efficiency of these two different material types for the overall sour gas separation. Therefore, it is helpful to use a separation efficiency term referred to as “combined acid gas selectivity (αCAG)” that takes both of the separations into account, giving a measure of overall performance of different polymer materials. The so-called “combined acid gas selectivity (αCAG)” is defined as the ratio of combined acid gas permeability (PH2S + PCO2) and methane permeability (PCH4). Using this parameter, the results of this work are compared to the literature data in Table 3 through a combined acid gas productivity (PH2S + PCO2)-efficiency (αCAG) trade-off plot (Fig. 14). It can be seen that the location of PIM-6FDA-OH near the upper right quadrant indicates that this material performs significantly better than CA and 6F-PAI based on the combined acid gas metric. It also appears to be very competitive with the majority of the rubbery polymers. Moreover, the sour gas testing conditions considered here are more aggressive than those examined with these rubbery materials. An additional factor to mention is the ultimate form which practical membranes will take for acid gas treatment. Specifically, asymmetric hollow fibers with very high surface to volume packing efficiency are desired. Both the glassy 6FDA-DAM–DABA and PIM-6FDA-OH have the practical ability to be formed into ultrathin selective layers (≤0.1 μm). On the other hand, forming defect-free rubbery selective layers less than one micron is very challenging. Furthermore, the P/l = nc/Δfci, the true measure of productivity of the two glassy polymers may exceed that of corresponding rubbery materials such as those shown in Fig. 14. Formation of such high performance asymmetric glassy polymer membranes based on either 6FDA-DAM–DABA or PIM-6FDA-OH is beyond the scope of the current work. Nevertheless, addressing the relative ease of forming such membranes will be important objectives in our work. Alternative PIM materials are currently explored in our groups to further improve the performance for aggressive sour gas separations.50–53
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Fig. 14 Productivity-efficiency trade-off for combined acid gas separations. Ternary gas data are shown for rubbers (black symbols) as well as PIM-6FDA-OH and some other glassy polymers, 6F-PAI-1, CA and 6FDA-DAM![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Footnote |
† Electronic supplementary information (ESI) available: Pure and mixed gas permeation modelling. See DOI: 10.1039/c5ta05928c |
This journal is © The Royal Society of Chemistry 2015 |