Robust high-permeance PTMSP composite membranes for CO2 membrane gas desorption at elevated temperatures and pressures

Journal of Membrane Science 470 (2014) 439–450

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Robust high-permeance PTMSP composite membranes for CO2 membrane gas desorption at elevated temperatures and pressures G.A. Dibrov a, V.V. Volkov a,c, V.P. Vasilevsky a, A.A. Shutova a, S.D. Bazhenov a, V.S. Khotimsky a, A. van de Runstraat b, E.L.V. Goetheer b, A.V. Volkov a,n a

A.V. Topchiev Institute of Petrochemical Synthesis, Moscow, Russian Federation TNO, Delft, The Netherlands c National Research Nuclear University MEPhI, Moscow, Russian Federation b

art ic l e i nf o

a b s t r a c t

Article history: Received 27 February 2014 Received in revised form 22 July 2014 Accepted 26 July 2014 Available online 4 August 2014

This work covers the development of robust and stable in time poly[1-(trimethylsilyl)-1-propyne] (PTMSP) thin film composite (TFC) membranes with high CO2 permeance for its application in high pressure/temperature gas–liquid membrane contactors used for amine-based solvents regeneration. For the first time, a novel technique of two-layers coating was proposed and successfully implemented to apply thin top-layer on microfiltration support by adjusting PTMSP solubility in organic solvents. By using different catalytic systems such as NbCl5 and TaCl5/TIBA, it was possible to synthesize two polymer samples (“PTMSP-Nb” and “PTMSP-Ta”), which are insoluble and soluble in hexane, respectively. The first intermediate layer made of “PTMSP-Nb” was cast on the microfiltration porous support from a toluene solution in order to form a PTMSP-coated porous support. The solution of “PTMSP-Ta” in n-hexane was used for subsequent formation of a thin defect-free selective layer (second layer). By using two commercial polymeric (MFFK-1) and metal-ceramic (MC) supports with high surface porosity of 85% and 60%, respectively, it was possible to fabricate the tailor-made TFC membranes “PTMSP/MFFK” (toplayer – 0.5 mm) and “PTMSP/MC” (top-layer – 1.2 mm) with an initial CO2 permeance of 50 and 36.3 m3(STP)/(m2 h bar), respectively, and CO2/N2 selectivity of 3.5. A preliminary conditioning of the PTMSP/MC membrane at 100 1C for 100 h in air was applied for the accelerated relaxation of PTMSP top layer. The resulting robust PTMSP/MC TFC membranes still provided a high CO2 permeance of 1.6 m3(STP)/(m2 h bar) (
600 GPU) without a noticeable decline of membrane performance within the long-term gas permeation testing (at least 250 h) at 100 1C. The PTMSP/MC membranes were further successfully utilized in HPT MGD for regeneration of 50 wt% methyldiethanolamine (MDEA) at elevated temperature (100 1C) and pressures (up to 30 bar). A stable TFC membrane performance was demonstrated and no solvent leakage through the membranes was observed. & 2014 Elsevier B.V. All rights reserved.

Keywords: PTMSP Thin-film composite membrane Membrane contactor СО2 Chemical solvent regeneration

1. Introduction The most mature technology for CO2 capture is absorption by ethanolamine-based solvents. Absorber–stripper units represent a proven, well-accepted technology. However, the major drawbacks of this process are the considerably high heat and energy penalties required for releasing the chemically bonded carbon dioxide and the capital expenditure. One of the potential routes for further development in this area is the introduction of gas–liquid membrane contactors [1–6]. The application of membrane gas–liquid contactors, among other benefits, offers a 10-fold reduction in volume and weight compared to a conventional packed tower [7,8]. Membrane n

Corresponding author. Tel.: þ 7 495 955 48 93. E-mail address: [email protected] (A.V. Volkov).

http://dx.doi.org/10.1016/j.memsci.2014.07.056 0376-7388/& 2014 Elsevier B.V. All rights reserved.

contactors have already shown good performance in pilot-scale trials for the treatment of exhaust and natural gas [9]. For natural gas sweetening, the typical operating pressures for conventional amine packed towers are in the range of 50–70 bar, whereas the stripping/regeneration unit usually operates at 1.3–1.5 bar [5]. The application of membrane gas/liquid contactors is a promising approach for the regeneration of CO2 rich solvents [10–12]. It allows for maintaining the liquid side at elevated pressure, while the driving force is additionally provided by the CO2 pressure difference across the membrane. At the same time, there are a number of requirements for the membrane material in such a case: (i) chemical and mechanical stability in strong base media (pH411) at elevated temperatures (100 1С) and pressures (40 bar), (ii) the absence of liquid permeation through the membrane at working conditions, (iii) high CO2 flux and (iv) stable performance of the membrane.

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Despite a large number of publications on membrane contactors for CO2 removal, only a limited part of it is focusing on the use of membrane contactors during solvent regeneration by temperature swing (most effective for chemical solvents) and/or decrease of CO2 partial pressure in the gas side by vacuum [11–21]. An important consideration is that the high temperatures needed for thermal regeneration can have an impact on the traditional microporous membranes due to higher solvent–membrane interaction and dramatic changes in membrane surface morphology resulting in wetting of membrane pores [13,16]. The membrane wetting problem can be eliminated by the introduction of a thin dense layer, which can lead to an additional mass transfer resistance [11–21]. In addition, it was previously reported [12,16] that the replacement of porous PP or PTFE membranes with the membranes having the dense top-layer (PPO or PES) resulted in noticeable decline of liquid evaporation through the membrane. Besides polymer membrane materials, surface modified alumina ceramic hollow fiber membranes have been also implemented for carbon dioxide stripping from monoethanolamine (MEA) solution at high temperature [19]. It should be pointed out that membrane contactors were typically studied at trans-membrane pressures not exceeding 2 bar [12,22] despite the vast majority of the reported research, even for high-pressure applications [23,24]. One of the major issues faced is the pore wetting, which can lead to a significant resistance to mass transfer through the introduction of a stagnant liquid layer in the pores of the membrane [12,22,25–27]. To prevent pore wetting, the use of thin-film composite (TFC) membranes, consisting of a defect-free selective layer on top of a porous support, has been considered [12,24,28]. In this work, hydrophobic glassy poly[1-(trimethylsylil)-1propyne] (PTMSP) was considered as the main candidate for the selective layer preparation, due to its extra high free volume fraction ( 425%) [29,30] and the highest CO2 permeability coefficient (P(CO2)), which can be up to 47,000 barrer [31]). Trusov et al. [11] proved the possibility to use PTMSP in conditions typical for the regeneration of CO2 absorbent liquids, based on aqueous solutions of different ethanolamines, at elevated pressures and temperatures, i.e. 40 bar and 100 1C. Taking the above reasoning into account, the challenge was to develop robust and stable PTMSP-based TFC membranes for high pressure membrane gas desorption (MGD) applications.

Since the first report of PTMSP synthesis [32], its high gas permeability has given rise to an interest in applying PTMSP as a selective layer for high flux composite membranes for the separation of gases [10,28,33–38] or liquids [39–41]. Table 1 presents an overview of works dedicated to the research on PTMSP TFC production with the focus on the gas transport characteristics and support. As follows from the above examples, PTMSP-based TFC membranes have usually been fabricated using ultrafiltration level porous supports. These supports typically exhibit a N2 permeance below 150 m3(STP)/(m2 h bar), pore size lower than 100 nm and surface porosity less than 15%, from such materials as PAN [35,36,39] or others [33,34,38]. However, calculations with the resistance in series model [34] showed that the considerable resistance of the porous support to the gas transport may result in an overall low flux. Another important issue to be thoroughly considered is the fact that PTMSP shows a drastic decrease in gas permeability upon aging [44–47]. The first systematic studies of PTMSP aging revealed three mechanisms of PTMSP aging [44]: physical aging, chemical aging (i.e. oxidation) and membrane contamination. Physical aging in PTMSP was connected with gradual chain relaxation, decreasing inter-chain spacing, reduction of non-equilibrium free volume and, therefore, gas permeability decline. The permeability and diffusion coefficients decreased by 1–2 orders of magnitude and the density increased by 20–30% for PTMSP samples kept for about 4 years at ambient conditions [45]. The rate of the permeability change of PTMSP can become orders of magnitude faster in films with a thickness of a few microns or less [47,48], which is typical for other amorphous polymers in the glassy state [49–52]. The deviations from bulk behavior in thick films are generally attributed to enhanced mobility at the free surface and attractive substrate–polymer interactions (for the supported films). Moreover, CO2 has a high solubility coefficient in PTMSP [30] and acts as a plasticizing agent, leading to an increase in membrane permeability [48]. Therefore, the CO2 transport in thin PTMSP films is influenced by two phenomena: plasticization and physical aging. There is a certain tradeoff between these two phenomena, but aging dominates over long time scales [48,52]. To the best of our knowledge, there are few works devoted to the application of PTMSP in membrane contactors [11,28,38,53,54]. There is only one full length paper, published by Nguyen et al. [28], devoted

Table 1 Gas transport characteristics of PTMSP TFC reported in literature. Support material P/l in m3(STP)/(m2 h bar)

Support

PTMSP layer thickness

Gas permeance (P/l in m3(STP)/(m2 h bar) and selectivity (α)

Refs.

Porous polysulfone (PSf)

UF

ca. 20–40 nm

[33]

Mesoporous polyetherimide support with surface porosity 5%

UF

3–5 mm

Polyacrylonitrile (PAN)

UF

1.6–3 mm

PAN P/l(N2) ¼85

UF

0.5 mm

PSf, pore diameter: 0.02–0.05 mm, porosity 0.4%

UF

2.5 mm

P/l(N2) ¼26.9 Asymmetric porous support P/l(N2) ¼930 Polypropylene hollow fibers, maximum pore diameter 0.55 mm

MF MF

7 mm –

MF

1.9 mm

P/l(O2)¼ 14.4 α(O2/N2)¼ 2.7 25 1C, single gas P/l(C4H10) ¼35 α(C4H10/CH4) ¼ 27 Mixed gas measurements: 10 bar, 30 1C, 3% C4, 97% C1 P/l(CO2) ¼0.68 α(CO2/CH4) ¼ 4.3 20–22 1C, single gas P/l(CO2) ¼12 α(CO2/CH4) ¼ 2.67 25 1C, single gas P/l(CO2) ¼0.26 α(CO2/N2) ¼3.5 Room temp., single gas Gas mixture: methane, ethane, propane and butane P/l(CO2) ¼19.3 α(CO2/N2) ¼3.3 Single gas P/l(CO2) ¼3.07 Single gas

Polypropylene hollow fibers (Oxyphan) P/l(N2) ¼45 [33]

[34]

[35]

[36]

[38]

[37] [10]

[28]

G.A. Dibrov et al. / Journal of Membrane Science 470 (2014) 439–450

441

et al. [55] reported the difference in solubility of PTMSP with various cis/trans unit ratios and synthesized with different catalysts. As is seen from Table 2, “PTMSP-Ta” is soluble in hexane while “PTMSP-Nb” is not. Two types of flat sheet microfiltration-grade membranes were used as supports: a flexible metal-ceramic (MC) microfilter, supplied by LLC “Nanopor” (Russia), and MFFK-1, supplied by ZAO STC “Vladipor”. The MC microfilter was 215 mm thick and consisted of a porous TiO2 layer atop a macro-porous stainless steel support, which in turn had thicknesses of 15 and 200 mm, respectively. Because of a unique flexibility, the MC microfilter could be rolled and welded, providing a tubular porous support of 10 mm outer diameter (LLC “Nanopor”). The MFFK-1 had a microfiltration layer made from tetrafluoroethylene/vinylidenefluoride copolymer (fluoroplastic F-42L) on a polypropylene non-woven support (further referred to as MFFK/PP) and had a thickness of approximately 170 mm. Prior to the TFC preparation, all the supports were washed in ethanol for 24 h and dried in a Binder airflow oven at 60 1C until a constant weight was achieved.

to the application of PTMSP-based TFC membranes for carbon dioxide capture by means of gas–liquid membrane contactors. Based on a porous polypropylene hollow fiber support, PTMSP composite membranes with a permeance of 3.07–5.58 m3(STP)/(m2 h bar) were prepared and tested for membrane gas absorption applications using monoethanolamine solutions. The goal of this work was focused on the development and exploitation of a novel two-layer coating technique based on the same high permeability glassy polymer PTMSP for the elaboration of robust, highly permeable thin film composite membranes for CO2 membrane gas desorption at elevated temperatures and pressures. The following critical requirements were taken into account: (i) high CO2 permeance, (ii) absence of absorption liquid leakage through the membrane and (iii) stable performance in membrane contactors at elevated pressures and temperatures.

2. Experimental 2.1. Materials

2.2. Dense membranes

Hexane, toluene and n-methyldiethanolamine (MDEA), 499.5% purity, were supplied by Sigma Aldrich Chemie Gmbh and were used as received. Distilled water was used to prepare the 50 wt% solution of MDEA. Two PTMSP samples, i.e. “PTMSP-Nb” and “PTMSP-Ta”, were synthesized according to the techniques developed at TIPS RAS [55]. PTMSP-Nb and PTMSP-Ta were synthesized in a toluene solution using the catalysts NbCl5 and TaCl5 with the co-catalyst triisobutylaluminum (TIBA), respectively. Polymers were precipitated and washed in methanol. The intrinsic viscosities [η] of the polymers were determined with an Ostwald–Ubbelohde-type viscometer using solutions of toluene at 25 1C (Table 2). The value of the weighted-average molecular mass (Mw) was measured by gel permeation chromatography, as described elsewhere [55]. The polymers were dissolved in hexane or toluene (0.1–0.8 wt%). The solutions were filtered under a pressure of nitrogen using double filter paper with continuous concentration control by measuring the solid residual. Khotimsky

Dense (homogeneous) membranes of 30 μm thick were prepared by casting the 0.1–0.8 wt% solutions of the corresponding polymer (“PTMSP-Nb” or “PTMSP-Ta”) in the appropriate solvent onto cellophane at room temperature. The cast solution was covered with a Petri dish and left for slow solvent evaporation for several days (about 7 days), followed by drying in the vacuum oven at 40 1C for 12 h to remove any residual solvent. The membrane thickness was measured by an electronic micrometer (Mitutoyos). Further treatment of all of the membranes was according to standard protocol of the membrane preparation [56], which included soaking of the membrane samples in n-butanol (2 days) and ethanol aqueous solutions, with stepwise decreases of the alcohol concentration from 96% to 0% (2 days), followed by drying in vacuum for 1 day at a temperature of 40 1C. The absence of residual solvent was proven by IR-spectroscopy.

Table 2 Characteristics of PTMSP. Polymer

“PTMSP-Nb” “PTMSP-Ta”

Catalytic system

NbCl5 TaCl5/TIBA

[η], dl/g

0.75 7.1

Selective layer of PTMSP-Ta (from hexane solution)

Mw, g/mole

5

3.2  10 1.1  106

Cis/trans unit ratio

65/35 45/55

Solubility Toluene

Hexane

þ þ

– þ

Ethanolamine + CO2

Intermediate layer of PTMSP-Nb (from toluene solution) Porous TiO2

1.2 μm 15 μm 200 μm

Porous stainless steel

CO2 (gas)

Fig. 1. The general idea of two-layer PTMSP-based TFC membrane on a MC support for CO2 capture in a gas–liquid membrane contactor.

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2.3. Thin film composite membranes In this work, TFC membranes were prepared by using the twolayer PTMSP solution coating technique developed at TIPS RAS. The general idea behind this approach is presented in Fig. 1 for the case of the MC porous support. The two layer coating with two different PTMSP samples provided the opportunity to form asymmetric PTMSP defect-free layers on top of the porous microfiltration support by using a simple two step dip coating technique. The main functions of the PTMSP-Nb intermediate layer (Fig. 1) were (i) to narrow the surface pore size distribution of the porous polymeric or inorganic support; (ii) to act, in view of the high gas permeability coefficients of PTMSP [57], as a high efficiency intermediate porous layer, i.e. to provide fast lateral diffusion of the penetrating CO2 molecules to the inlet of the pores in the support; (iii) to smoothen the roughness of the porous support surface and (iv) to provide a surface of the obtained “PTMSP-Nb”/ porous support with enhanced adhesion/affinity towards the thin ”PTMSP-Ta” selective layer. These functions will be shown in Sections 3.2 and 3.3. In contrast to “PTMSP-Ta”, the “PTMSP-Nb” sample is insoluble in hexane (Table 2). With this in mind, the following new procedure was used for the formation of PTMSP-based TFC membranes. In the case of the MC support, the first intermediate layer was deposited from a toluene solution of “PTMSP-Nb”. Then the selective layer was deposited from a hexane solution of “PTMSP-Ta”. The thicknesses of the intermediate and selective layers were controlled by the concentration of the polymers in the coating solutions. The contact time of the coating solutions with the porous support was set to 10 s. The concentration was 0.5 wt% for both the coating solutions of “PTMSP-Nb” and “PTMSP-Ta”. These concentrations were determined experimentally using the criteria of maximal permeance and surface porosity and most narrow pore size distribution for the gutter “PTMSP-Nb” layer and maximal permeance and intrinsic selectivity of PTMSP-Ta for the thin selective layer (the results are presented in Section 3.3). TFC membranes on polymeric porous supports (MFFK/PP) were prepared by a kiss-coating technique, which is a variation of the dip-coating technique (see Fig. 2) and ensures the coating of only one surface of the support. The membranes were prepared using either a laboratory coating setup (semi-continuous, 0.1 m wide) or a pilot-scale (semi-continuous, 0.25 m wide) coating device. The optimal coating speed in the preparation was 0.25 m/min, which

Porous support

Thin top-layer (PTMSP)

Casting solution (PTMSP)

was determined from the set of gas permeation experiments. Higher coating speeds did not produce a uniform coating, whereas lower speeds provided lower CO2 permeance with the same selectivity α(CO2/N2), due to a higher intrusion of the casting solution into the pores of the support layer. The intermediate layer was deposited from the toluene solution of “PTMSP-Nb” (0.8 wt%), whereas the selective layer was then deposited from the hexane solution of “PTMSP-Ta” (0.1 wt%) using the same criteria as for the MC support (results presented in Section 3.2). The difference in the coating solution concentration is attributed to the initial support properties: a broader pore size distribution requires a higher polymer concentration. The membranes were cut into discs with diameters of 60 mm for lab-scale tests. All of the TFC membranes were dried for 16–24 h at ambient conditions and another 2 h in a vacuum oven at 40 1C. Finally, the membranes were purged with a nitrogen stream at 10 bar to remove any residual solvent. 2.4. Gas permeability Pure nitrogen and carbon dioxide permeability tests were carried out at pressures up to 40 bar and at both room temperature (23 7 2 1С) and 100 1C. The gas flux, J, at constant pressure, p, was determined by the volumetric method according to the following equation: J¼

V tS

where V is the amount of gas passed through the membrane with an area S for a period of time t. The permeability coefficients are reported in Barrer, where 1 barrer¼10  10 cm3(STP) cm/(cm2 s cmHg). The exposure time of the membrane at a temperature of 100 1C was typically not more than 1.5 h. The ideal selectivity α(CO2/N2), defined as P(CO2)/P(N2), was used to both characterize the membrane materials (dense membranes) and to control the selective layer integrity (absence of defects) for composite membranes. As was demonstrated by Horn and Paul [52], CO2 induces plasticization of thin glassy polymer films. In the present work, the deviations from a linear dependence of CO2 flux on pressure were observed for the freshly cast TFC membranes, which were in qualitative agreement with the results of [48]. Therefore, freshly cast TFC membranes were conditioned under a CO2 pressure of 40 bar for 0.5 h prior to the gas permeability measurements. 2.5. Gas–liquid capillary flow porometry Capillary flow porometry (CFP), based on gas–liquid displacement, was applied to determine the pore size characteristics of the porous supports [58]. Equipment built in-house with a sample area of 13.2 cm2 was used for this purpose. The membrane sample was first wetted with a liquid (Galwick, P.M.I., CF3–[(OCF(CF3)– CF2)n–(O–CF2)m]–O–CF3). This liquid has a low surface tension (γ¼ 15.9 mN/m), low vapor pressure (3 mm Hg at 298 K), low reactivity and can be assumed to fill all the pores, given that it has a zero contact angle with virtually all materials (cos θ¼1). The wetted sample was subjected to increasing nitrogen pressures up to 5 bar, applied at 23 1C. As the pressure of N2 increases, it pushes the liquid out from the pores of diameters dp, as given by the Cantor equation: dp ¼

Fig. 2. Schematic illustration of the kiss-coating method.

4γ Δp

where Δp is the trans-membrane pressure difference. By monitoring the applied pressure and the flow of gas through the sample when liquid is being expelled during the wet run and for the dried

G.A. Dibrov et al. / Journal of Membrane Science 470 (2014) 439–450

membrane thereafter, the limiting pore size (diameter) distribution can be calculated [59]. 2.6. Scanning electron microscopy The surfaces and cross-sections of the porous supports and TFC membranes, respectively, were analyzed by scanning electron microscopy (SEM). A high-resolution scanning electron microscope, Supra 50 VP LEO (Carl Zeiss SMT Ltd, Germany), was used. To study the cross-section, samples were fractured in liquid nitrogen and sputtered under vacuum with a thin (5 nm) layer of chrome. Gwiddion software was used for graphical analysis to determine the surface porosity, surface pore size distribution and selective layer thickness. 2.7. Infrared spectroscopy

CO2-loading, mole CO2/mole MDEA

The possible changes in chemical composition of the membranes after annealing were investigated by FTIR spectroscopy. An IFS-66 v/s vacuum Bruker was used to collect spectra in transmission mode in the range of 4000–400 cm  m. Data processing was done using the software OPUS 6.0 (Bruker). The dense membrane from PTMSP-Ta (30 μm) was annealed in the airflow oven for 350 h. Its chemical structure was compared with the similar virgin (non-annealed) PTMSP-Ta membrane 0.6 0.5 0.4 0.3

y = 0.0406x - 0.0243

0.2 0.1 0.0 0

2

4

6

8

10

12

14

Rich 50% MDEA conductivity, mSm/cm Fig. 3. CO2 loading vs rich 50% MDEA conductivity.

16

443

sample. When the samples were prepared for the measurement, it was important to avoid any contamination by non-volatile impurities from the environment. Therefore, a pre-treatment was used before the IR-spectra measurement, consisting of sample conditioning in ethanol for 1 day followed by 1 day of drying under vacuum at 40 1C. 2.8. 50% MDEA regeneration using high pressure membrane contactors: laboratory tests A 50 wt% MDEA solution with a CO2 loading of 0.44 mol/mol was prepared as follows to be used in the regeneration experiments. First, a 50 wt% MDEA solution in distilled water was prepared. The solution was then pressurized by CO2 in a sealed vessel equipped with a pressure controller and mixed during loading. The MDEA loading was determined using a standard curve (Fig. 3) by solution conductivity monitoring. Conductivity was measured by a Multiline P4 sensor (Germany). It was shown by Vasilevsky et al. [60] that the dependence of CO2 loading on conductivity is linear. A graph showing the loading as a function of conductivity is presented in Fig. 3. For plotting the curve in Fig. 3, the CO2 loading of the rich solution (αrich, mol/mol) was calculated as follows: αrich ¼

ðρrich  ρ0 ÞМ МDEА M CO2 ρ0 wМDEА

where ρ0 and ρrich are the densities [kg/m3] of 50 wt% MDEA (unloaded) and 50 wt% MDEA loaded with CO2, respectively; M CO2 and MMDEA are the CO2 and MDEA molar weights [kg/kmol], respectively; and wMDEA is the mass fraction of MDEA in the initial solution. Fig. 4 shows the experimental setup for the regeneration of the absorption liquid. Carbon dioxide absorption takes place in the absorber 5, which is equipped with a high pressure stirrer. Then, the rich solvent under the same pressure was supplied into the liquid flow line of the membrane desorption module 8, where thermal regeneration of the absorbent takes place at a temperature of 100 1С and pressures up to 30 bar. In the course of the experimental run, СО2 flow leaves the gas flow line of the contactor at 1 bar. The flow rate of the absorbent in the cell is adjusted with a needle valve. The amount of the regenerated liquid collected in the collector 12 with an electronic microbalance was estimated by weighing. The membrane active area in the desorption module is 16.6 cm2. Thickness of the liquid layer in the desorption module is equal to 0.1 mm in order to minimize the

Fig. 4. Membrane gas desorption setup: 1 – lean absorbent tank; 2 – filter; 3 – gas cylinders; 4 – gas tank; 5 – absorber; 6, 7 – thermal ovens; 8 – gas–liquid membrane contactor; 9 – cold trap; 10 – safety flask; 11 – gas flow meter; 12 – regenerated absorbent tank þ microbalance.

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diffusion resistance. The CO2 flow rate was measured by a flow meter 11 and then used to calculate the specific CO2 flux Jnorm. per unit of membrane area (S) at standard conditions using the following equation: J norm: ¼

J exp: p273:16 101:3TS

The condensate flux across the membrane Jcond. was calculated based on the amount of liquid from the cold trap 9 at 0 1C mcond. as follows: J cond: ¼

mcond: tS

3. Results and discussion 3.1. PTMSP dense membranes The gas transport properties of the “PTMSP-Nb” and “PTMSPTa” samples are presented in Table 3. For the dense membranes, the gas flux as a function of trans-membrane pressure was linear for all of the studied polymers, as shown in Fig. 5. The permeability coefficients which were obtained were reproducible and Table 3 Gas transport properties of PTMSP dense membranes at 23 7 2 1C. Experimental error – 7%. Polymer

P(N2), barrer

P(CO2), barrer

α(CO2/N2)

“PTMSP-Nb” “PTMSP-Ta”

6000 9400

27,000 33,000

4.5 3.5

3.2. PTMSP TFC membranes on MFFK/PP support

N2 (PTMSP-Nb)

120

y = 3.014·x

CO2 (PTMSP-Nb) 100

CO2 (PTMSP-Ta) y = 2.466·x

60

3

2

J, m (STP)·(m ·h)

-1

N2 (PTMSP-Ta)

80

y = 0.858·x

40 20

y = 0.548·x 0 0

10

consistent with data reported previously [29,30,61]. According to the earlier studies, the PTMSP samples synthesized on niobium catalytic systems are enriched with the cis-configuration whereas the PTMSP samples synthesized on the tantalum-based catalytic systems are known to be enriched with the trans-configuration [55]. Moreover, in the polymers containing mixed configurations, the cis- and trans-units are able to produce continuous sequences of varying lengths. Since the “PTMSP-Nb” sample is enriched with cis-units and the length of the trans-sequence is lower than that in the samples synthesized on the tantalum catalytic system, the packing density of polymer chains in the disordered polymer regions (i.e. in the lower density regions) in the “PTMSP-Nb” sample should be higher, and for “PTMSP-Ta”, the packing density should be lower. As a consequence, the gas permeability was lower and the selectivity α(CO2/N2) was higher for the PTMSP samples synthesized using niobium catalyst compared to the PTMSP samples synthesized with tantalum based catalyst [55]. It is worth noting that different sorption and organic solvent nanofiltration behaviors of “PTMSP-Nb” and “PTMSP-Ta” samples were also recently reported [62]. As is seen from Table 3, the “PTMSP-Ta” sample did allow for a higher gas permeability than the “PTMSP-Nb” sample. Thus, “PTMSP-Ta” was more attractive for forming the high-flux top layer of the TFC membrane. In addition, the weight-average molecular mass of “PTMSP-Ta” and its intrinsic viscosity was higher (Table 2), which made “PTMSP-Ta” more feasible in terms of prevention of the polymer penetration into the pores of the support, because of the higher hydrodynamic chain diameter [63]. Another distinctive difference of the “PTMSP-Ta” and “PTMSP-Nb” samples is that the latter is insoluble in hexane. This allowed for deposition of the “PTMSP-Ta” selective layer from the hexane solution atop the intermediate layer of “PTMSP-Nb” to give a TFC membrane having an asymmetric structure of the PTMSP separating layer.

20 p, bar

30

40

Fig. 5. Gas flux as a function of trans-membrane pressure for dense membranes made of PTMSP-Ta and PTMSP-Nb (thickness – 30 mm).

Whereas the calculated N2 permeance of the hypothetic PTMSP-Ta dense membrane with a thickness of 1 μm is 25– 30 m3(STP)/(m2 h bar), the MFFK/PP microfilter with a high surface porosity and gas permeance was selected as the porous support (Table 4). As can be seen, the N2 permeance of the MFFK/PP support is about 30-fold higher than that of the ca. 1 μm PTMSP layer, which should ensure minimal resistance from the support itself. The selectivity α(CO2/N2) of 0.94 suggests that the mechanism of gas transport is transitional between Knudsen (αE0.8) and Hagen–Poiseuille (αE 1) flows, with their contributions calculated to be 30% and 70%, respectively. As seen from Fig. 6a and Table 4, the maximum pore diameter for the MFFK/PP support is 0.6 mm. The limiting pore size distribution, obtained by capillary flow porometry (Fig. 6a, curve 1), is narrower than the surface pore size distribution (Fig. 6a, curve 2), obtained by graphical analysis of SEM micrographs (Fig. 6b). The deposition of the intermediate “PTMSP-Nb” layer resulted in a decrease in the

Table 4 Characteristics of the MFFK/PP support, before and after deposition of the intermediate “PTMSP-Nb” layer, and gas transport parameters of the TFC1 membrane at room temperature. Membranes

Initial support

Support after intermediate “PTMSP-Nb” layer deposition

TFC1 membrane

N2 permeance, m3(STP)/(m2 h bar) CO2 permeance, m3(STP)/(m2 h bar) α(CO2/N2) Maximum pore diameter, μm Mean pore diameter, μm Surface porosity by SEM

760 710 0.94 0.60 0.35 85%

280 280 1.00 0.20 0.10 10%

14 50 3.50 – – –

G.A. Dibrov et al. / Journal of Membrane Science 470 (2014) 439–450

445

1

36

2 4

Pore fraction, %

28

3 4

24 20

3

16 12

2

8

1

4 0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Pore diameter, μm Fig. 6. (a) The pore size distribution of the initial MFFK/PP support (1 – measured by capillary flow porometry and 2 – treatment of SEM micrograph (b)) and after intermediate “PTMSP-Nb” layer deposition (3 – measured by capillary flow porometry and 4 – treatment of SEM micrograph (c)). (b) SEM micrograph of the initial support surface. (c) SEM micrograph of the support surface after intermediate “PTMSP-Nb” layer deposition.

maximum pore diameter from 0.6 to 0.2 mm and in the mean pore diameter from 0.35 to 0.1 mm, as well as a narrowing of the pore size distribution. The deposition of the intermediate layer decreased the surface porosity from 85% to 10% (Fig. 6b and c, respectively) and, according to SEM micrographs (Fig. 6b and c), it also diminished the surface roughness of the porous support. After the deposition of the intermediate “PTMSP-Nb” layer, the SEM (Fig. 6a, curve 4) and CFP (Fig. 6a, curve 3) distributions are in fair agreement. The selectivity α (CO2/N2) increased from 0.94 to 1.00 (Table 4), which indicates the possible contribution of a solution-diffusion mechanism to the total mechanism of the gas transport through the membrane. Despite the decrease of the surface porosity after the deposition of the intermediate layer, the gas permeance of the support with the PTMSP layer remained higher than that of typical UF membranes. The increase in selectivity indicates that lateral diffusion of the penetrating CO2 molecules occurs up to the inlet of the pores in the support. Bearing in mind the high “PTMSP-Nb” gas permeability, it can be concluded that the resistance to the gas transport is lower when compared to conventional UF membranes, which typically have lower porosities. The subsequent application of the selective “PTMSP-Ta” layer resulted in the formation of the highly permeable TFC1 membrane (Table 4). The selectivity α(CO2/N2) increased up to 3.5 and was equal to that of “PTMSP-Ta”, indicating that the TFC1 membrane was defect-free. The selective layer thickness of 500 nm was obtained as measured by SEM (Fig. 7). It is important to note that the applied coating technique prevented the penetration of the polymer into the support pores.

500 nm

PTMSP

MFFK/PP

Fig. 7. SEM micrograph of the cross section of the TFC1 PTMSP membrane on the MFFK/PP support.

micrographs (Fig. 8b and c), it also diminished the surface roughness of the porous support. The selectivity α(CO2/N2) increased from 0.94 to 1.04 (Table 5) after the deposition of the intermediate “PTMSP-Nb” layer. This indicates the possible contribution of a solution-diffusion mechanism to the total mechanism of the gas transport through the membrane. The subsequent application of the selective “PTMSP-Ta” layer resulted in the formation of a highly permeable TFC2 membrane (Table 5). The selectivity α(CO2/N2) increased up to 3.5 and was equal to that of “PTMSP-Ta”, indicating that the TFC2 membrane was defect-free. The selective layer thickness was 1.2 μm, as measured by SEM (Fig. 9).

3.3. PTMSP TFC membranes on MC support As seen from Fig. 8a and Table 5, the maximum pore diameter for the MC support is 0.51 mm. The initial MC support has a selectivity α(CO2/N2) of 0.94 and, consequently, the same calculated contributions of Knudsen and Hagen–Poiseuille flows of 30% and 70%, respectively. The deposition of the intermediate “PTMSP-Nb” layer resulted in a decrease in the maximum pore diameter from 0.51 to 0.31 mm and in the mean pore diameter from 0.15 to 0.13 mm, as well as a narrowing of the pore size distribution. Moreover, the deposition of the intermediate layer decreased the surface porosity from 60% to 15% (Fig. 8b and c, respectively) and, according to SEM

3.4. 50% MDEA regeneration using PTMSP TFC membranes on a MFFK/PP support in high pressure membrane contactors Absorbent regeneration tests were carried out using 50% MDEA with a CO2 loading of 0.44 mol/mol at an absorbent pressure of 10 bar and temperature of 100 1C. The CO2 flux as a function of absorbent linear velocity is presented in Fig. 10. The CO2 flux increases with the gradual increase in absorbent linear velocity, becoming flat at high velocities. It can be seen that the CO2 flux decreases with time, losing an order of magnitude in its value within a week of experimentation and almost vanishes after 2 weeks of experimentation. It should also be noted that the

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35

1 2 3 4

Pore fraction, %

30 25

2

20 15

1

10

3

5

4

0 0.1

0.2

0.3

0.4

0.5

Pore diameter, μm

Fig. 8. (a) Pore size distribution of the initial MC support (1 – measured by capillary flow porometry and 2 – treatment of SEM micrograph (b)) and after intermediate PTMSP-Nb layer deposition (3 – measured by capillary flow porometry and 4 – treatment of SEM micrograph (c)). (b) SEM micrograph of the initial support surface. (c) SEM micrograph of the support surface after intermediate PTMSP-Nb layer deposition.

Table 5 Characteristics of the metal-ceramic microfilter support before and after deposition of the intermediate “PTMSP-Nb” layer and gas transport parameters of the TFC2 membrane at room temperature. Membranes

Initial support

Support after intermediate “PTMSP-Nb” layer deposition

TFC2 membrane

N2 permeance, m3(STP)/(m2 h bar) CO2 permeance, m3(STP)/(m2 h bar) α(CO2/N2) Maximum pore diameter, μm Mean pore diameter, μm Surface porosity by SEM

640 600 0.94 0.51 0.15 60%

250 260 1.04 0.31 0.13 15%

9.8 36.3 3.5 – – –

1.2 μm 1.2 μm

PTMSP

MC

PTMSP

MC 1 μm

1 μm

Fig. 9. SEM micrograph of the cross section of PTMSP TFC2 membranes on MC support.

condensate flux decreased along with the CO2 flux, showing almost no relation with the absorbent linear velocity. The ratio of CO2 flux (in kg(m2 h)  1) to absorbent flux (in kg(m2 h)  1), taken at an absorbent linear velocity 0.029 m s  1, was 4.5–5.1 during the experiment. The membrane was unloaded from the membrane module, washed in distilled water and dried at 40 1C in an airflow oven. The comparison of its properties before and after 50% MDEA regeneration is presented in Table 6. The comparison shows no change in the membrane weight. Therefore, it was concluded that the membrane is chemically stable. However, a drastic decrease in the CO2 membrane permeance was observed and the thickness declined from 170 to 120 μm. The permeance decrease can be attributed to the collapse of the porous support or to PTMSP aging.

On one hand, PTMSP aging, observed in a variety of works such as [47,64,65], can have only a limited impact on the total permeance decline, as will be shown later. On the other hand, the decrease in thickness proves that the porous structure of the support collapsed under the harsh conditions of the experiment, namely, elevated pressure and temperature in a MDEA environment. Thus, the reason for this was presumably the simultaneous impact of a temperature of 100 1C and a pressure of 10 bar in addition to the possible plasticization effect of the absorbent on the membrane materials. It remained ambiguous whether the permeance decline was due to the collapse of the PTFE/PVDF porous layer or of the PP non-woven support. However, it was obvious that a support with superior mechanical and thermal stability, such as a MC support should be used.

3

0.5

35

P/l(N2)

30

P/l(CO2)

0.0

α (CO2/N2)

25

4.5

20 4.0 15 10

3.5

5 0

3.0 0

0.00

5.0

0.02

0.04

0.06

Fig. 10. CO2 flux as a function of absorbent linear velocity for the PTMSP TFC membrane on a MFFK/PP support.

After test

0.520 170 50

0.519 120 o 0.1

200

250

300

350

2.5

1) PTMSP-Та annealed 2) PTMSP-Та initial

2.0

1.5

1.0

0.5

3.5. Aging and stabilization of PTMSP TFC membranes on a MC support

150

Fig. 11. The time dependencies of CO2 and N2 permeance and selectivity α(CO2/N2) at 100 1C for the TFC2 membrane.

Absorbance

Before test

100

Time of annealing, h

Table 6 The characteristics of the PTMSP TFC membrane on a MFFK/PP support before and after 50% MDEA regeneration in membrane contactors at a temperature of 100 1C and pressure of 10 bar.

Membrane weight, g Membrane thickness, μm CO2 permeance, m3(STP)/(m2 h bar)

50

0.08

Absorbent linear velosity, m/s

α (CO2/N2)

1.0

447

2

-1

1.5

3

2

Flux (CO 2), m (STP)· (m ·h)

2.0

P/l, m (STP)· (m ·h·bar)

day 1 day 2 day 3 day 4 day 5 day 6 day 14

2.5

-1

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C=O

C-O

1 2

0.0 500

1000

1500

2000

2500

3000

3500

-1

It is well known that the annealing of PTMSP films in vacuum, an inert atmosphere and air accelerates aging processes [47,64,65]. Merkel et al. [64] showed that the permeability coefficients in PTMSP decrease rapidly upon annealing above temperatures of 80 1C. Takada et al. [65] showed that when PTMSP films with a thickness of 10–50 mm were heated at 100 1C for 15 h, their P(O2) decreased and became 1/10 of the original value, while α(O2/N2) increased from 2.0 to 2.6. Dorkenoo and Pfromm [47] studied thick (85 mm) and thin (1 and 3 mm) PTMSP films, which were dried under vacuum at 100 1C until a constant weight was observed. The thicknesses of the thin films were determined by mass balances, assuming the same macroscopic density as for the thick films. The time dependence of He and N2 permeability allowed for the claim that the thin films were characterized by accelerated aging while the thick films did not age significantly. It was also shown that aging progressively accelerated with decreasing film thickness. Since the gas permeability of PTMSP decreases with time due to aging and this process can be accelerated by exposure to high temperatures, it was important to fabricate TFC membranes based on PTMSP with a CO2 permeance stable in the long term. Secondly, it was desirable to study the contribution of the thin PTMSP selective layer separate from the support, in order to show that PTMSP aging has a finite value and, in this case, that a robust and stable MC support is helpful. Since a MGD process is carried out at elevated temperatures, the prepared TFC2 membranes were subjected to annealing at 100 1C in an air-flow oven. Moreover, the idea was to develop a method of conditioning at 100 1C to obtain more durable membranes. The time evolution of the TFC2 membrane permeance measured at 100 1C is presented in Fig. 11. The initially high values of composite membrane permeance markedly decreased upon annealing, whereas the selectivity α(CO2/N2) increased. The latter

wavelength, cm

Fig. 12. IR-spectra of PTMSP membranes (1) annealed at 100 1C for 350 h and (2) in the initial state are similar.

observation indicated that no defects appeared during the aging process. An approximately 20-fold decrease in the gas permeance was observed during the first 100 h of annealing at 100 1C. Following this, the composite membrane showed a stable CO2 permeance of 1.6 m3(STP)/(m2 h bar) with a selectivity α(CO2/N2) equal to 4 for the next 250 h. Since the metal-ceramic microfilter is thermo-resistant, such a drastic decrease in the gas permeance was attributed to aging of the PTMSP selective layer. It was necessary to determine if the combined influence of high temperature and air induced any other mechanisms of aging except the physical one. To this end, a FTIR study was performed for PTMSP-Ta dense membranes. The IR-spectra for initial and annealed PTMSP dense membranes are presented in Fig. 12. As can be seen, the IR study revealed the absence of absorption bands corresponding to C–O (1700– 1800 cm  1) and C¼ O (1090–1100 cm  1) for the PTMSP-Ta dense membrane annealed at 100 1C for 350 h in air. It should be noted that all precautions were taken to avoid any contaminants in the PTMSP during the membrane preparation and handling (residuals of solvents and catalysts, different compounds which could be absorbed from the environment, etc.). No other evidence of polymer chemical degradation or oxidation, nor any presence of detectable amounts of contaminants, was found. Thus, it was experimentally confirmed that the 20-fold decrease in permeance observed for the TFC membrane during annealing (Fig. 11) could be predominantly explained by physical aging of the PTMSP (relaxation of the PTMSP non-equilibrium free volume) [45,47,64,65,66].

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The thickness of the selective layer after annealing for 350 h was 1.2 mm and did not change from the initial state, as shown by SEM. Aging is typical for all amorphous polymers in the glassy state and its rate is thickness dependent [49] in the range of thicknesses 0.4–10 mm. Despite the dramatic decrease in gas permeability, the TFC PTMSP membrane showed stable performance for 250 h of experimentation at 100 1C with a CO2 permeance P/l(CO2)¼1.6 m3(STP)/(m2 h bar) (
600 GPU). Thus, the proposed two-layer PTMSP solution coating technique and further conditioning at 100 1C for 100 h allowed for the formation of defect-free PTMSP-based TFC membranes with stable gastransport properties.

3.6. 50% MDEA regeneration using PTMSP TFC membranes on a MC support in high pressure membrane contactors The detailed description of chemical solvent regeneration in lab-scale and pilot setups will be discussed in a forthcoming publication. Here, only those data which prove the stable performance of aged PTMSP TFC membranes in a high pressure/temperature MGD process are presented. The results of the TFC2 membrane tests using a lab scale apparatus are presented in Fig. 11. The temperature of regeneration was 100 1C, while the pressure of the MDEA solvent was up to 30 bar. The membrane was pressurized by CO2-loaded absorbent over the course of 4 days. Fig. 13 demonstrates that no difference can be observed between the CO2 fluxes across the membrane between day 1 and day 4. This means that the composite PTMSP membrane demonstrated stable performance during 100 h of operation at 100 оС, 30 bar and a рНZ11. After washing the membrane with distilled water, no changes in mass and gas permeance were found. For comparison, Fig. 13 also presents the experimental data for the dense PTMSP membrane. As can be seen, the reduction in the polymer thickness from 29 down to1 mm did not allow to achieve the intensification of process efficiency by a factor greater than 2. Such observation allows to conclude that besides the mass-transfer resistance in the dense layer made of high permeability glassy polymer like PTMSP, other factors such as the rate of reversible reactions play a noticeable role during regeneration of amine-based solvents in gas–liquid membrane contactor system. The benefit of gas–liquid membrane contactors to decrease size of the unit holds predominantly for membrane gas absorption. The drivers to examine the use of membrane contactors to replace conventional strippers for high pressure applications were

1.4

3

2

Flux (CO 2), m (STP)· (m ·h)

-1

1.6

1.2 1.0 0.8 0.6 PTMSP composite membrane - day 1 PTMSP composite membrane - day 4 PTMSP dense membrane (29 μm)

0.4 0.2 0.0 0.000

0.005

0.010

0.015

0.020

0.025

Absorbent linear velosity, m/s Fig. 13. Relationship between CO2 flux and 50% MDEA linear velocity during its regeneration using PTMSP TFC membranes on a MC support and dense PTMSP membranes with a thickness of 29 mm in high pressure membrane contactors.

different, namely (i) having the absorption liquid during regeneration at high pressure (decreasing pumping energy), (ii) decreasing steam/CO2 ratio (i.e. the steam leaving with the CO2 out of the stripper unit) and (iii) gravity independence. As was shown above, tailor-made thin film composite membranes based on two-layers of PTMSP can be successfully utilized in high pressure gas–liquid membrane contactors at conditions typical for regeneration of amine-based solvents due to its chemical and mechanical stability, barrier properties towards absorption liquids and high CO2 permeance stable in time. Secondly, the condensate flux through the membrane was 1.4 kg  (m2 h)  1, giving a ratio CO2/absorbent flux of 1.7 that indicates preferential transport of carbon dioxide over water vapors and, hence, further potentials for improvement of energy consumption. Absence of pore wetting problems and no leakage of absorption liquid through the membrane at pressure difference of 40 bar and 100 1C provide the ability to maintain the liquid side at desired pressure, gravity independence of the unit and flexibility in the membrane module design.

4. Conclusions In this work, a novel two-layer coating technique based on the two PTMSP samples with different microstructures and high CO2 permeabilities was proposed and successfully implemented for the first time. The generated TFC membranes are robust, stable (in time) and can be used in high pressure/temperature gas–liquid membrane contactors for ethanolamine solvents regeneration. The essence of the proposed approach is based on varying the PTMSP solubility in organic solvents by adjusting cis/trans ratios of the polymeric chains. Such differences in polymer microstructure can be achieved by proper selection of synthesis conditions, mainly by using different catalytic systems, such as NbCl5- or TaCl5-based catalysts. To fabricate tailor-made TFC membranes, two commercial polymeric (MFFK-1) and metal-ceramic (MC) supports (microfiltration membranes) with high surface porosities of 85% and 60%, respectively, were used. The first intermediate layer, made up of a “PTMSP-Nb” sample, was cast atop the virgin porous support from a polymer solution in toluene, providing a PTMSP coated porous support. Since a cis-enriched sample of “PTMSP-Nb” is insoluble in n-hexane, the solution of a cis-depleted sample of “PTMSP-Ta” in n-hexane was used for the subsequent formation of a thin and defect-free second selective layer. By using the proposed techniques, two types of TFC membranes were fabricated. These were “PTMSP/MFFK” (top-layer – 0.5 mm) and “PTMSP/MC” (top-layer – 1.2 mm), with initial CO2 permeances of 50 and 36.3 m3(STP)/ (m2 h bar), respectively, and a CO2/N2 selectivity of 3.5. Further testing of “PTMSP/MFFK” for regeneration of 50 wt% MDEA (CO2 loading of 0.44) at 100 1C and a trans-membrane pressure of 10 bar revealed a drastic decline of the desorbed CO2 flux in time. Due to the chemical stability of the PTMSP material in 50 wt% MDEA at 100 1C and membrane evaluation after the testing, such behavior was attributed to the collapse of the porous structure of the MFFK polymeric support. Therefore, further study was focused on TFC membranes based on the inorganic support, i.e. “PTMSP/ MC”. In order to allow for stable performance of the high pressure/ temperature membrane gas desorption (HPT MGD) process, fabricated TFC membranes were stabilized by accelerated relaxation of the PTMSP layer by conditioning at 100 1C for 100 h in air. Through FTIR analysis, it was shown that no chemical degradation of PTMSP occurred during annealing in air at 100 1C, not even within longer exposure times of up to 350 h. The prepared robust PTMSP/MC TFC membranes still exhibited a high CO2 permeance of 1.6 m3(STP)/(m2 h bar) (
600 GPU), and no noticeable decline of this value was observed within the long-term testing (at least 250 h) at 100 1C. These robust TFC membranes were further

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successfully utilized in a HPT MGD process for the regeneration of 50 wt% methyldiethanolamine (MDEA) at elevated temperature (100 1C) and pressures (up to 30 bar). It should be pointed out that no liquid permeation through composite membranes was experimentally observed. It can be concluded that the novel two-layer coating method using PTMSP samples with different microstructures, followed by high temperature conditioning, enables one to make robust, high-permeance PTMSP composite membranes for CO2 membrane gas desorption at elevated temperatures and pressures.

Acknowledgments The authors gratefully acknowledge Prof. G.N. Bondarenko (TIPS RAS) for the FTIR spectroscopy, Erin Scholes (TNO) for fruitful discussion and the Center of Collective Use “New petrochemical processes, polymeric composites and adhesives” for providing the equipment. This work was partially funded by the FP7 project “DECARBit”. This research cooperation is a part of the Dutch–Russian Centre of Excellence “Gas4S” (NWO-RFBR # 047.018.2006.014/08-08-92890-CE).

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