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Membrane gas dehydration in a pressure-electric coupled field
Author’s Accepted Manuscript Membrane gas dehydration in a pressure-electric coupled field Jennifer Runhong Du, Xinyang Shi, Xianshe Feng, Yanfen Li, Yufeng Zhang, Xi Chen www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(15)30047-8 http://dx.doi.org/10.1016/j.memsci.2015.07.019 MEMSCI13838
To appear in: Journal of Membrane Science Received date: 4 October 2014 Revised date: 8 July 2015 Accepted date: 11 July 2015 Cite this article as: Jennifer Runhong Du, Xinyang Shi, Xianshe Feng, Yanfen Li, Yufeng Zhang and Xi Chen, Membrane gas dehydration in a pressure-electric coupled field, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.07.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Membrane Gas Dehydration in a Pressure-Electric Coupled Field
Jennifer Runhong Dua*, Xinyang Shia, Xianshe Fenga,b, Yanfen Lic, Yufeng Zhanga, Xi Chena
a
State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic
University, Tianjin 300387, China b
c
Department of Chemical Engineering, University of Waterloo, Ontario N2L 3G1, Canada
Tianjin University of Technology and Education, Tianjin 300222, China
*Corresponding author: 86-15022718286; [email protected]
Abstract A novel membrane gas dehydration process in a pressure-electric coupled field was proposed to enhance water vapor permeation and water/gas separation. An electric potential gradient generated by an asymmetric electric field across the membrane was utilized as an additional driving force for polar molecules (e.g., water) to transport through the membrane. The enhancement in water permeation was evaluated and the improvement in water/gas separation was confirmed. The effects of operating parameters (including voltage gradient, pressure gradient, and operating temperature) on permeation were investigated. The results also showed that the activation energy for water permeation was reduced by the electric field.
1
Keywords: electric field; electric potential gradient; membrane gas separation; dehydration; water vapor permeation
1. Introduction Membrane gas separation, based on the difference in the permeation rates of different components in the feed mixture through a membrane, has become a commercially viable process for various applications in the gas industry.[1,2] Commercial polymeric membranes for gas separation are based on the solution-diffusion transport in dense membranes. However, the well-known trade-off between permeability and selectivity with polymeric membranes (that is, a high permeability is often associated with a low selectivity, and vice versa) usually leads to a separation upper-bound for the current generation of membranes. In general, it is desirable to maximize the productivity so as to minimize the membrane area required; therefore, to develop membrane systems with improved separation performance has been a subject of study with extensive efforts in the field of membrane gas separation.[3-6] A great deal of interest has been focused on optimizing physicochemical properties and structures of membranes to improve membrane performance. Facilitated transport using certain active carriers for specific and reversible reactions with the targeted molecules can enhance the permeation of the targeted species to achieve a high permeability and selectivity. However, facilitated transport membranes have a major potential drawback of saturation of carriers under an elevated pressure.[7-10] Immobilization of suitable room-temperature ionic liquids in a porous support to form supported liquid membranes may increase the solubility of the fast permeating gas and thus 2
increase the permselectivity. However, the long term stability of supported liquid membranes remains a challenge for practical applications.[11,12] An alternative technique is to introduce molecular sieving mechanism. Polymers of intrinsic microporosity possess highly rigid and contorted molecular structures which dictate a low packing density and thus a high free volume. These polymers behave as molecular sieves and generally have a high permeability, but their selectivity is often limited by the broad distribution of pore sizes.[13,14] Mixed matrix membranes, prepared by dispersing inorganic particles (e.g., zeolites, metal-organic frameworks, and nanotubes) into a continuous polymeric matrix, have also been attempted to improve the permselectivity of the membrane by taking advantage of the favorable sorption characteristics and molecular sieving effect of the inorganic fillers. However, to improve the dispersion and orientation of ultrafine particles within the polymeric matrix as well as the interfacial contact between the polymeric and inorganic phases are still an important issue for practical use of mixed matrix membranes.[15-17] Some other research groups have been engaged on improving membrane formation techniques. For instance, interfacial polymerization and layer-by-layer assembly can produce an ultrathin selective layer with a high permeation rate. However, it is difficult to fabricate a defect-free ultrathin film on a large scale for gas separations.[18-20] There have been advances on development of novel membrane materials, membrane structures and membrane formation processes to increase the separation performance of membranes, but much more work is still needed to apply them in practice. Currently, commercial gas separation membranes are still limited to only a few traditional membranes and materials. 3
In addition to tailoring membranes for improved separation performance, process design is also important. Conventional membrane gas separation is a pressure driven process where a partial pressure gradient across the membrane acts as the driving force for mass transport. Although an increase in the pressure gradient can increase the permeation flux, it requires the membrane and the equipment to be robust enough to withstand the high pressure or vacuum. Alternatively, when an additional driving force is applied to enhance the permeation of the fast permeating gas, the separation efficiency can be increased. This motivates us to look into coupled pressure-electric fields to enhance gas separation. In our previous study, an electric potential gradient generated by an asymmetric electric field worked as an additional driving force for water vapor transport in membrane distillation process, based on dielectrophoresis of polar molecules.[21-23] Mixtures of polar/non-polar molecules are commonly encountered in gas separations (e.g., dehydration of air and natural gas, removal of hydrogen sulfide from natural gas), where the polar molecules usually have a higher permeability in the membranes.[24-27] This study proposes to utilize an asymmetric electric field to provide an additional driving force to enhance the permeation of polar gases, thereby improving the separation of the polar/non-polar gas mixtures. For purpose of demonstration, the dehydration of nitrogen was selected in this study as a simple system to prove the concept of permeation enhancement of polar gases by applying an electric field. The effects of operating parameters (including voltage gradient, pressure gradient, and operating temperature) on the overall separation performance were investigated.
2. Experimental 4
2.1 Materials Water vapor can interact strongly with hydrophilic polymers via hydrogen bonding, and it tends to have a much higher solubility and diffusivity in hydrophilic polymers than permanent gases do. Poly(N,N-dimethylaminoethyl methacrylate) (PDM) contains hydrophilic amino groups and appears to be effective in selective removal of water vapor from N2.[28] Therefore, PDM was used in this study as the membrane material for N2 dehydration. PDM was synthesized by free radical bulk polymerization, which has been described previously.[29] Polysulfone (PSF) ultrafiltration membrane (molecular weight cut-off of ca. 100 kDa, provided by Vontron China) was thoroughly rinsed with pure water before use as a substrate. Research grade N2 (99.99% pure) was used for the gas permeation experiments. Pure water used in this study had a conductivity of 14.6 μs/cm. 2.2 Membrane preparation PDM/PSF composite membrane was prepared by interfacial crosslinking of a PDM coating layer on the PSF substrate [30]. A solution of PDM in ethanol (10 g/L) was coated onto the surface of the PSF substrate for 15 min. The excess coating solution was removed and the membrane was air dried. Then the membrane surface was allowed to contact with p-xylylene dichloride (XD) dissolved in heptane (10 g/L) for 4 h, during which period PDM was crosslinked with XD at the solid-liquid interface. The crosslinking led to a quaternary ammonium network through alkylation reaction between the tertiary amino groups in the side chains of PDM and the chloromethyl groups in XD. Then the excess crosslinking solution was removed and the membrane surface was washed with heptane to remove the unreacted residual XD on the membrane surface. After the membrane was air dried, the above coating-crosslinking procedure was repeated one more time to form a double-coated 5
composite membrane, which was shown to be defect-free on the membrane surface. 2.3 Design of membrane permeation cell suitable for use in an asymmetric electric field Two stainless steel electrodes connected to a DC power supply (maximum 20 kV and 1 mA, Dongwen, China) were located on the two sides of the permeation cell. An asymmetric electric field was formed by placing a disk electrode on the feed side (parallel to the membrane surface) and a needle electrode at the center of the permeate side (perpendicular to the disk electrode). The local electric field strength in such a needle⊥plate electric field increases from the plate electrode to the needle electrode. As a comparison, a uniform electric field was generated as well by placing a second disk electrode on the permeate side, parallel to the disk electrode on the feed side. The detailed design has been described in our previous study.[23] 2.4 Permeation tests A schematic diagram of the bench-scale permeation system is shown in Fig.1. The membrane was mounted on a sintered porous polytetrafluoroethylene plate embedded in the permeation cell. The effective membrane area for permeation was the same as the disk electrode size (15.9 cm2). The permeation fluxes of water vapor and N2 were measured at different voltages, pressures and temperatures. All the experiments were carried out with the same membrane with a PDM skin layer thickness of a few micrometers. The membrane was tested with pure gas of CO2 and N2, and the ideal separation factor of CO2/N2 reached 50, indicating that the membrane was defect-free [30]. 1) Measurement of water vapor permeation A stream of N2 gas was bubbled through a humidifier, and the humidified gas was essentially saturated with water. The flow rate of the gas stream was controlled at 50 ml(STP)/min by a mass 6
flow controller (S49-32B/MT, Huibolong, China). The operating temperature was controlled using a thermal bath for the humidifier. A paraffin oil seal was used to prevent water vapor in air from entering the system through the retentate line. The feed side was kept constant at atmospheric pressure, and the permeate side was controlled at a low pressure in the range of 10~30 kPa. When varying the permeate pressure, the purging air entering the vacuum pump was dried by two-stage desiccators: anhydrous calcium chloride granules were used as desiccant in the first stage, and silica gel particles were used in the second stage. The water contents were monitored by a dew point meter (DM70, Vaisala, Finland). The permeated water vapor was collected in a cold trap immersed in liquid nitrogen, and the permeation flux of water vapor was measured by weighing the permeate water collected in the condenser.
paraffin oil seal ball valve silica gel desiccator pressure regulator
mass flow controller
2
permeation cell needle valve
FC
electrode
N2
thermal bath humidifier & demister
DC
dew point meter
power supply
switch valve
vacuum gauge
1 CaCl2 desiccator
vacuum pump
collection tube
bubble flow meter
7
air
Fig.1. Schematic diagram of experimental setup for membrane gas dehydration in a pressure-electric coupled field.
2) Measurement of nitrogen permeation The permeability of N2 was measured with the constant pressure-variable volume method. In the feed side, a water-saturated N2 gas stream was admitted to the permeation cell. The feed gas pressure was controlled at 200 kPa, while the permeate was kept at atmospheric pressure. The permeation flux of nitrogen was measured by a bubble flow meter. The permeation flux J of water vapor or N2 was determined by
J
Q At
(1)
After the permeation experiments in the absence of the electric filed, a voltage up to 5 kV was applied to investigate the effect of the electric field on the permeation. The permeation flux was measured again after removing the electric field to examine if the membrane experienced any permanent change in the electric field.
3. Results and Discussion 3.1 Verification of enhanced water permeation in an asymmetric electric field When a needle⊥plate electric field was applied across the membrane with an increasing field intensity along the direction of permeation, water molecules were subjected to an electric gradient force at the same direction of permeation, causing an enhancement in the mass transport. The enhancement in permeation G was defined as 8
G
Jt J0 100% J0
(2)
It is expected that increasing the electric potential gradient will help align polar molecules more orderly and make them migrate faster. Fig.2 shows that the permeation flux of water vapor was increased with an increase in the voltage gradient applied in the asymmetric electric field, irrespective of the field directions. An enhancement in water permeation of as high as 46% was achieved at a voltage gradient of 213 kV/m. When the electric field was removed, the water permeation flux was found to gradually decline to the initial value before the voltage was applied, demonstrating that the membrane was intact in the electric field under the experimental conditions.
Fig.2. Effect of electric field on permeation of water vapor and N2 gas. (+): anode; (0): grounded; voltage: 0~5 kV; electrode distance: 23.5~63.5 mm. Water vapor permeation test: operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa; J0(H2O): 0.2 mol/(m2.h). N2 gas permeation test: operating temperature: 20oC; feed pressure: 200 kPa; permeate pressure: 100 kPa; J0(N2): 0.008 mol/(m2.h). 9
The experimental data of water permeation presented in Fig.2 follow the equation derived previously to express the permeation of polar molecules in the presence of an asymmetric electric field:
J t J 0 De
U d
(3)
where De is the transmembrane mass-transfer coefficient driven by the electric potential gradient.[23] Eq.3 suggests that polar molecules exhibit a dual mode transport through the membrane under a pressure-electric coupled field: (1) diffusion of polar molecules from the feed side to the permeate side driven by a partial pressure gradient across the membrane, as in a conventional membrane gas separation process, and (2) orientation of polar molecules and migration to the direction of increasing field intensity driven by an electric potential gradient (Fig.3). The overall driving force for the polar molecules to permeate through the membrane derives from the partial pressure gradient and the electric potential gradient, and the permeation flux is thus enhanced by the electric field as compared to conventional permeation process where the transmembrane pressure is the only driving force for permeation. The observed water permeation flux is the sum of both modes of permeation. When two plate electrodes are arranged in parallel, the resulting electric field will be uniform. In such an electric field, the force on the positive end of a polar molecule will balance the force on the negative end, and thus there is no net force on the polar molecule. Therefore, the polar molecules may line up along the field direction in a uniform electric field but they will not migrate under the voltage gradient. When a plate‖plate electric field was applied across the membrane, water permeation was not accelerated by the uniform electric field, and there is no enhancement in water 10
permeation (Fig.2). This result is consistent with our previous findings for membrane distillation[23], confirming that only an asymmetric electric field can provide an additional driving force for mass transport of polar molecules.
11
Fig.3. Schematic diagram of membrane gas dehydration. (a) conventional system and (b) pressureelectric coupled field system.
Non-polar molecules (e.g., N2) do not possess permanent dipole moments. Although non-polar molecules may obtain a slight temporary dipole moment induced by an external electric field, they usually respond to the electric field quite weakly. Therefore, non-polar molecules are not subjected to a significant net force and do not exhibit preferred orientation and directional migration in an asymmetric electric field. While the mass transport of polar molecules was intensified by the asymmetric electric field, there is little effect on the permeation of non-polar molecules, resulting in an increased separation of gas mixtures comprising of polar and non-polar molecules in the pressure-electric coupled field. As shown in Fig. 2, the permeation of water vapor was enhanced, but there is no enhancement in nitrogen permeation in the experimental range of field intensities tested. 3.2 Effect of gas pressure gradient on permeation The appropriate operating conditions often depend on the specific applications. For example, in natural gas dehydration, the feed gas is already at a high pressure and the permeate can thus be withdrawn at a low pressure (close to 1 atm) or preferably at a subatmospheric pressure in order to reduce methane loss
[31,32]
. An industrial-scale membrane process for natural gas dehydration was
reported to be operated at a feed pressure of 5.0 MPa and a permeate pressure of 10~90 kPa
[33]
. In
air or flue gas dehydration, a low partial pressure of water vapor on the permeate side is often maintained by using a vacuum pump or a sweeping gas. In a field test for flue gas dehydration, a feed pressure of 0.1 MPa and a permeate pressure of 2~2.5 kPa were used 12
[34]
. In the present work,
the feed side was kept at atmospheric pressure (i.e., 0.1 MPa) while the permeate pressure was varied in the range of 10~30 kPa in order to evaluate the significance of permeation enhancement by the electric field at different permeate pressures. The driving force for mass transport of a conventional membrane gas separation process is the partial pressure gradient of the gas across the membrane, and the permeation flux J0 may be expressed by [35] J 0 Dp pH 2O
(4)
pH 2O p f x p p y
(5)
where Dp is the transmembrane mass-transfer coefficient, and ∆pH2O is the partial pressure gradient of water vapor across the membrane. The transmembrane pressure gradient of water vapor was increased by decreasing the permeate pressure in this study. The relationship between water flux and water vapor pressure gradient is shown in Fig.4a. The water flux was found to increase with an increase in the water vapor pressure gradient, and so does the corresponding mass-transfer coefficient Dp (Fig.4b). Water vapor is a condensable gas, and it can cause plasticization of hydrophilic membranes. At a higher vapor pressure gradient, water concentration in the membrane matrix increases, and thus the polymer will possess a greater segmental mobility leading to an increase in the value of Dp [36]. When an asymmetric electric field is applied, the water flux contributed by the electric field (Jt-J0) increased linearly with the voltage gradient (Fig.5). These straight lines at different permeate pressures do not overlap, showing that the mass-transfer coefficient De also depends on the water vapor pressure gradient (Fig.6). It is believed that the pressure or concentration dependence of De, 13
similar to that of Dp, is due to the plasticization and/or swelling of the hydrophilic membrane. The increased polymer segmental mobility at an elevated vapor pressure gradient will decrease the membrane resistance and favor the mass transport. It is understandable that De depends upon the properties of the permeant (e.g., size and dielectric constant) and the membrane properties (e.g., resistance).
Fig.4. Water flux contributed by the pressure field (a) and mass transfer coefficient Dp (b) at different 14
partial pressure gradient of water vapor. Operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 10~30 kPa.
Fig.5. Water flux contributed by the electric field at different permeate pressures. Needle⊥plate electric field; needle electrode: anode; plate electrode: grounded; electrode distance: 23.5 mm. Operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 10~30 kPa.
15
Fig.6. Effect of partial pressure gradient of water vapor on mass transfer coefficient De. Operating conditions same as specified in Fig.5.
In conventional membrane gas dehydration processes, vacuum is often applied to the permeate side to evacuate the water vapor and to provide the driving force for permeation. The permeation flux of water vapor can be increased by lowering the permeate pressure in a conventional process. Fig. 5 shows that the transport of water vapor through the membrane in the asymmetric electric field is favored by decreasing the permeate pressure. As such, a decrease in the permeate pressure has a positive effect on the permeation of water vapor in both of the pressure field and the asymmetric electric field. Therefore, similar to the conventional process, a further increase in the water flux of the membrane gas dehydration process in the pressure-electric coupled field can be achieved by decreasing the permeate pressure. Introducing an asymmetric electric field in a membrane gas dehydration process is shown to increase the water flux. The power consumption We due to the use of an electric field can be 16
estimated by
We UI
(6)
The water permeation was increased by 0.1 mol/m2·h in a needle⊥plate electric field at 5 kV at a permeate pressure of 20 kPa and operating temperature of 20oC (Fig. 2). The current was found to be below 0.1 μA (which was the minimum value that could be displayed on our instrument). Thus the maximum electric power consumption to achieve the increased flux would be 0.5 mW, which is very insignificant. On the other hand, in a conventional membrane gas dehydration process without the electric field, the permeation flux can be enhanced by decreasing the permeate pressure. The permeate pressure would have to be decreased by 6 kPa to achieve the same increase in the flux (Fig. 5). In view of the vacuum conditions on the permeate side, a further decrease in the permeate pressure by 6 kPa represents a significant increase in the energy consumption of the vacuum pump as the energy consumption of a vacuum pump is determined by the outlet (i.e., 101 kPa) to inlet pressure (i.e., the permeate pressure) ratio to the power of 0.23 based on the following estimation [37]: k W0 F Z k 1
k 1 R p2 k T1 1 MW p1
(7)
When the permeate pressure decreased from 20 kPa to 14 kPa, the power consumption of the vacuum pump would increase by about 27%. 3.3 Effect of operating temperature on permeation The operating temperatures of conventional membrane gas dehydration processes are mainly determined by the feed gas conditions, and they are often in the range of 5~50oC [32-34]. In this study, the operating temperature was varied in the range of 15~30oC. The permeation fluxes of water vapor 17
and N2 gas at various operating temperatures are shown in Fig.7. The water flux through the membrane under the pressure-electric coupled field was higher than that of the conventional process in the range of the operating temperatures tested. That is, an increased water flux can be achieved at a relatively low operating temperature compared to the conventional process. The permeation fluxes increased with temperature, and the temperature dependence of permeation fluxes was found to follow an Arrhenius type of relationship
E J K J exp J RT
(8)
where EJ is the apparent activation energy characterizing the overall temperature dependence of permeation flux.
Fig.7. Permeation flux of water vapor and N2 gas at different temperatures. Needle⊥plate electric field; needle electrode: anode; plate electrode: grounded; electrode distance: 23.5 mm. Feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa.
18
During the permeation experiments, the feed gas was fully saturated with water. Thus, the permeation is affected by operating temperature on two aspects: the membrane permeability and the transmembrane pressure gradient. In order to separate these two contributions of temperature, the membrane permeance without the electric field (P0/l) was estimated from the flux J0 normalized by the partial pressure gradient. P0 J0 l p H 2O
(9)
It is well known that the temperature dependence of membrane permeance (P0/l) generally follows an Arrhenius relationship, P0 E K P exp P 0 l RT
(10)
where EP0 is the activation energy of permeation characterizing the effect of temperature on the membrane permeability in the absence of an electric field. This is shown in Fig.8, where the membrane permeance to water vapor decreased with an increase in temperature. It appears to suggest that the reduction in solubility of water in the hydrophilic membrane prevailed over the increase in diffusivity as the temperature increased. Similar results on the relationship between water permeance in PDM membrane and the operating temperature have been observed in our previous work. [28]
19
Fig.8. Effect of temperature on water permeance without the electric field. Feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa.
Following Eq. (8),
E J 0 K J 0 exp J 0 RT
(11)
Rearranging Eqs. (9) and (10), one has pH 2O
J0 K E EP 0 J 0 exp J 0 P0 / l K P RT
(12)
Define
E J 0 EP 0
(13)
where ζ, which is a parameter reflecting the effect of temperature on the vapor pressure gradient, can be calculated from the difference between the activation energy based on permeation flux J0 and the activation energy based on permeance (P0/l) in the absence of the electric field. An increase in temperature raises the saturated vapor pressure of water in the feed gas, leading to an increased vapor 20
pressure gradient. The increase in permeation flux with temperature is primarily due to the increased water vapor pressure gradient. On the other hand, the water vapor pressure in the permeate was much lower than that in the feed, and the water vapor pressure gradient was approximately a constant at a given temperature, whether the electric field is present or not. That is, the value of ζ was hardly affected by the electric field. Therefore, when an electric field is applied, the activation energy of permeation based on membrane permeance EPt can be estimated from
EPt EJt
(14)
where EJt is the activation energy for water permeation based on water flux produced in the pressure-electric fields. Fig.9 shows that the activation energy for water permeation EP, which is EP0 in the absence of an electric field or EPt in the pressure-electric field, decreases with the electric field. At an applied voltage of 5 kV, the permeation activation energy was reduced by about 70%. When applying an asymmetric electric field across the membrane, the polar molecules are subjected to an electric gradient force, which reduces the thermal energy required for the water molecules to overcome the energy barrier in order to permeate through the membrane. Since the permeation flux of N2 was not affected by the electric field, the activation energy for N2 permeation remains the same with and without the electric field.
21
Fig.9. Plot of activation energy for water permeation at various electric fields. Operating conditions same as specified in Fig.7.
It should be pointed out this study is only a proof-of-concept study to demonstrate enhanced water vapor permeation from gas streams using an electric field. Much more work is needed in module design and process development to properly accommodate an asymmetric electric field in hollow fiber and spiral wound configurations, which is a subject matter for further studies.
4. Conclusions Integration of a pressure field and an asymmetric electric field was used to enhance membrane gas dehydration, and the following conclusions can be drawn: (1) Applying an asymmetric electric field across a membrane in the direction of permeation enhanced water permeation and thus water/gas permselectivity. (2) The enhancement in water permeation increased with an increase in the voltage gradient applied 22
across the membrane. (3) The asymmetric electric field provides an additional driving force for water permeation. (4) The activation energy for water permeation was decreased by the electric field.
Acknowledgements National Natural Science Foundation of China (NSFC, 21206122), China Postdoctoral Science Foundation (2014T70216; 2012M520577), Tianjin Natural Science Foundation (11JCYBJC26500; 14JCZDJC38100) and Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry (2013-693) are gratefully acknowledged.
Nomenclature A
effective area of membrane
d
electrode distance
Dp
transmembrane mass transfer coefficient driven by pressure gradient
De
transmembrane mass transfer coefficient driven by electric potential gradient
EJ
overall activation energy
EP
activation energy of permeation
F
mass flow rate
G
enhancement in permeation
I
electric current
J
permeation flux 23
k
specific heat ratio (=1.3 for water vapor)
K J, KP
pre-exponential factor
MW
molecular weight
P0/l
permeance without electric field
pf
feed pressure
pp
permeate pressure
p1
pressure at inlet of vacuum pump
p2
pressure at outlet of vacuum pump
∆pH2O
transmembrane partial pressure gradient of water vapor
Q
mole quantity of permeate
R
gas constant
t
permeation time
T
operating temperature
T1
temperature of gas entering vacuum pump
U
voltage applied on the electrodes
V
feed flow rate
W
power consumption
x
mole fraction of water vapor in feed
y
mole fraction of water vapor in permeate
Z
compressibility factor
ζ
parameter reflecting the effect of temperature on the vapor pressure gradient 24
Subscripts 0
conventional process without electric field
e
electric field process
t
pressure-electric coupled field process
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[17] W.-g. Kim, S. Nair, Membranes from nanoporous 1D and 2D materials: a review of opportunities, developments, and challenges, Chem. Eng. Sci. 104 (2013) 908-924. [18] X. Yu, Z. Wang, Z. Wei, S. Yuan, J. Zhao, J. Wang, S. Wang, Novel tertiary amino containing thin film composite membranes prepared by interfacial polymerization for CO2 capture, J. Membr. Sci. 362 (2010) 265-278. [19] M. Wang, V. Janout, S.L. Regen, Gas transport across hyperthin membranes, Accounts Chem. Res. 46 (2013) 2743-2754. [20] N. Joseph, P. Ahmadiannamini, R. Hoogenboom, I.F.J. Vankelecom, Layer-by-layer preparation of polyelectrolyte multilayer membranes for separation, Polym. Chem. 5 (2014) 1817-1831. [21] R. Pethig, Dielectrophoresis: status of the theory, technology, and applications, Biomicrofluidics 4 (2010) 022811. [22] A. Kuzyk, Dielectrophoresis at the nanoscale, Electrophoresis 32 (2011) 2307-2313. [23] J.R. Du, W. Du, X. Feng, Y. Zhang, Y. Wu, Membrane distillation enhanced by an asymmetric electric field, AIChE J. 60 (2014) 2307-2313. [24] S. Basu, A.L. Khan, A. Cano-Odena, C. Liu, I.F.J. Vankelecom, Membrane-based technologies for biogas separations, Chem. Soc. Rev. 39 (2010) 750-768. [25] C.A. Scholes, G.W. Stevens, S.E. Kentish, Membrane gas separation applications in natural gas processing, Fuel 96 (2012) 15-28. [26] B. Bolto, M. Hoang, Z. Xie, A review of water recovery by vapour permeation through membranes, Water Res. 46 (2012) 259-266.
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[27] B. Kraftschik, W.J. Koros, J.R. Johnson, O. Karvan, Dense film polyimide membranes for aggressive sour gas feed separations, J. Membr. Sci. 428 (2013) 608-619. [28] J.R. Du, L.Liu, A. Chakma, X. Feng, Using poly(N,N-dimethylaminoethyl methacrylate)/ polyacrylonitrile composite membranes for gas dehydration and humidification, Chem. Eng. Sci. 65 (2010) 4672-4681. [29] R. Du, X. Feng, A. Chakma, Poly(N,N-dimethylaminoethyl methacrylate)/polysulfone composite membranes for gas separations, J. Membr. Sci. 279 (2006) 76-85. [30] R.
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poly(N,N-dimethylaminoethyl
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[36] J. Potreck, K. Nijmeijer, T. Kosinski, M. Wessling, Mixed water vapor/gas transport through the rubbery polymer PEBAX 1074, J. Membr. Sci. 338 (2009) 11-16. [37] D.W. Green, R.H. Perry. Perry’s Chemical Engineer’s Handbook (8th ed.), McGraw-Hill (2008).
29
Figure Captions: Fig. 1. Schematic diagram of experimental setup for membrane gas dehydration in a pressure-electric coupled field. Fig. 2. Effect of electric field on permeation of water vapor and N2 gas. (+): anode; (0): grounded; voltage: 0~5 kV; electrode distance: 23.5~63.5 mm. Water vapor permeation test: operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa; J0(H2O): 0.2 mol/(m2.h). N2 gas permeation test: operating temperature: 20oC; feed pressure: 200 kPa; permeate pressure: 100 kPa; J0(N2): 0.008 mol/(m2.h). Fig. 3. Schematic diagram of membrane gas dehydration. (a) conventional system and (b) pressureelectric coupled field system. Fig. 4. Water flux contributed by the pressure field (a) and mass transfer coefficient Dp (b) at different partial pressure gradient of water vapor. Operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 10~30 kPa. Fig. 5. Water flux contributed by the electric field at different permeate pressures. Needle⊥plate electric field; needle electrode: anode; plate electrode: grounded; electrode distance: 23.5 mm. Operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 10~30 kPa. Fig. 6. Effect of partial pressure of water vapor on mass transfer coefficient De. Operating conditions same as specified in Fig.5.
30
Fig. 7. Permeation flux of water vapor and N2 gas at different temperatures. Needle⊥plate electric field; needle electrode: anode; plate electrode: grounded; electrode distance: 23.5 mm. Feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa. Fig. 8. Effect of temperature on water permeance without the electric field. Feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa. Fig. 9. Plot of activation energy for water permeation at various electric fields. Operating conditions same as specified in Fig.7.
Highlights:
Asymmetric electric field generated a driving force for water permeation.
Water permeation and water/gas separation were improved by the electric gradient.
Activation energy for water permeation decreased with an increase in the voltage.
31
PII: DOI: Reference:
S0376-7388(15)30047-8 http://dx.doi.org/10.1016/j.memsci.2015.07.019 MEMSCI13838
To appear in: Journal of Membrane Science Received date: 4 October 2014 Revised date: 8 July 2015 Accepted date: 11 July 2015 Cite this article as: Jennifer Runhong Du, Xinyang Shi, Xianshe Feng, Yanfen Li, Yufeng Zhang and Xi Chen, Membrane gas dehydration in a pressure-electric coupled field, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.07.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Membrane Gas Dehydration in a Pressure-Electric Coupled Field
Jennifer Runhong Dua*, Xinyang Shia, Xianshe Fenga,b, Yanfen Lic, Yufeng Zhanga, Xi Chena
a
State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic
University, Tianjin 300387, China b
c
Department of Chemical Engineering, University of Waterloo, Ontario N2L 3G1, Canada
Tianjin University of Technology and Education, Tianjin 300222, China
*Corresponding author: 86-15022718286; [email protected]
Abstract A novel membrane gas dehydration process in a pressure-electric coupled field was proposed to enhance water vapor permeation and water/gas separation. An electric potential gradient generated by an asymmetric electric field across the membrane was utilized as an additional driving force for polar molecules (e.g., water) to transport through the membrane. The enhancement in water permeation was evaluated and the improvement in water/gas separation was confirmed. The effects of operating parameters (including voltage gradient, pressure gradient, and operating temperature) on permeation were investigated. The results also showed that the activation energy for water permeation was reduced by the electric field.
1
Keywords: electric field; electric potential gradient; membrane gas separation; dehydration; water vapor permeation
1. Introduction Membrane gas separation, based on the difference in the permeation rates of different components in the feed mixture through a membrane, has become a commercially viable process for various applications in the gas industry.[1,2] Commercial polymeric membranes for gas separation are based on the solution-diffusion transport in dense membranes. However, the well-known trade-off between permeability and selectivity with polymeric membranes (that is, a high permeability is often associated with a low selectivity, and vice versa) usually leads to a separation upper-bound for the current generation of membranes. In general, it is desirable to maximize the productivity so as to minimize the membrane area required; therefore, to develop membrane systems with improved separation performance has been a subject of study with extensive efforts in the field of membrane gas separation.[3-6] A great deal of interest has been focused on optimizing physicochemical properties and structures of membranes to improve membrane performance. Facilitated transport using certain active carriers for specific and reversible reactions with the targeted molecules can enhance the permeation of the targeted species to achieve a high permeability and selectivity. However, facilitated transport membranes have a major potential drawback of saturation of carriers under an elevated pressure.[7-10] Immobilization of suitable room-temperature ionic liquids in a porous support to form supported liquid membranes may increase the solubility of the fast permeating gas and thus 2
increase the permselectivity. However, the long term stability of supported liquid membranes remains a challenge for practical applications.[11,12] An alternative technique is to introduce molecular sieving mechanism. Polymers of intrinsic microporosity possess highly rigid and contorted molecular structures which dictate a low packing density and thus a high free volume. These polymers behave as molecular sieves and generally have a high permeability, but their selectivity is often limited by the broad distribution of pore sizes.[13,14] Mixed matrix membranes, prepared by dispersing inorganic particles (e.g., zeolites, metal-organic frameworks, and nanotubes) into a continuous polymeric matrix, have also been attempted to improve the permselectivity of the membrane by taking advantage of the favorable sorption characteristics and molecular sieving effect of the inorganic fillers. However, to improve the dispersion and orientation of ultrafine particles within the polymeric matrix as well as the interfacial contact between the polymeric and inorganic phases are still an important issue for practical use of mixed matrix membranes.[15-17] Some other research groups have been engaged on improving membrane formation techniques. For instance, interfacial polymerization and layer-by-layer assembly can produce an ultrathin selective layer with a high permeation rate. However, it is difficult to fabricate a defect-free ultrathin film on a large scale for gas separations.[18-20] There have been advances on development of novel membrane materials, membrane structures and membrane formation processes to increase the separation performance of membranes, but much more work is still needed to apply them in practice. Currently, commercial gas separation membranes are still limited to only a few traditional membranes and materials. 3
In addition to tailoring membranes for improved separation performance, process design is also important. Conventional membrane gas separation is a pressure driven process where a partial pressure gradient across the membrane acts as the driving force for mass transport. Although an increase in the pressure gradient can increase the permeation flux, it requires the membrane and the equipment to be robust enough to withstand the high pressure or vacuum. Alternatively, when an additional driving force is applied to enhance the permeation of the fast permeating gas, the separation efficiency can be increased. This motivates us to look into coupled pressure-electric fields to enhance gas separation. In our previous study, an electric potential gradient generated by an asymmetric electric field worked as an additional driving force for water vapor transport in membrane distillation process, based on dielectrophoresis of polar molecules.[21-23] Mixtures of polar/non-polar molecules are commonly encountered in gas separations (e.g., dehydration of air and natural gas, removal of hydrogen sulfide from natural gas), where the polar molecules usually have a higher permeability in the membranes.[24-27] This study proposes to utilize an asymmetric electric field to provide an additional driving force to enhance the permeation of polar gases, thereby improving the separation of the polar/non-polar gas mixtures. For purpose of demonstration, the dehydration of nitrogen was selected in this study as a simple system to prove the concept of permeation enhancement of polar gases by applying an electric field. The effects of operating parameters (including voltage gradient, pressure gradient, and operating temperature) on the overall separation performance were investigated.
2. Experimental 4
2.1 Materials Water vapor can interact strongly with hydrophilic polymers via hydrogen bonding, and it tends to have a much higher solubility and diffusivity in hydrophilic polymers than permanent gases do. Poly(N,N-dimethylaminoethyl methacrylate) (PDM) contains hydrophilic amino groups and appears to be effective in selective removal of water vapor from N2.[28] Therefore, PDM was used in this study as the membrane material for N2 dehydration. PDM was synthesized by free radical bulk polymerization, which has been described previously.[29] Polysulfone (PSF) ultrafiltration membrane (molecular weight cut-off of ca. 100 kDa, provided by Vontron China) was thoroughly rinsed with pure water before use as a substrate. Research grade N2 (99.99% pure) was used for the gas permeation experiments. Pure water used in this study had a conductivity of 14.6 μs/cm. 2.2 Membrane preparation PDM/PSF composite membrane was prepared by interfacial crosslinking of a PDM coating layer on the PSF substrate [30]. A solution of PDM in ethanol (10 g/L) was coated onto the surface of the PSF substrate for 15 min. The excess coating solution was removed and the membrane was air dried. Then the membrane surface was allowed to contact with p-xylylene dichloride (XD) dissolved in heptane (10 g/L) for 4 h, during which period PDM was crosslinked with XD at the solid-liquid interface. The crosslinking led to a quaternary ammonium network through alkylation reaction between the tertiary amino groups in the side chains of PDM and the chloromethyl groups in XD. Then the excess crosslinking solution was removed and the membrane surface was washed with heptane to remove the unreacted residual XD on the membrane surface. After the membrane was air dried, the above coating-crosslinking procedure was repeated one more time to form a double-coated 5
composite membrane, which was shown to be defect-free on the membrane surface. 2.3 Design of membrane permeation cell suitable for use in an asymmetric electric field Two stainless steel electrodes connected to a DC power supply (maximum 20 kV and 1 mA, Dongwen, China) were located on the two sides of the permeation cell. An asymmetric electric field was formed by placing a disk electrode on the feed side (parallel to the membrane surface) and a needle electrode at the center of the permeate side (perpendicular to the disk electrode). The local electric field strength in such a needle⊥plate electric field increases from the plate electrode to the needle electrode. As a comparison, a uniform electric field was generated as well by placing a second disk electrode on the permeate side, parallel to the disk electrode on the feed side. The detailed design has been described in our previous study.[23] 2.4 Permeation tests A schematic diagram of the bench-scale permeation system is shown in Fig.1. The membrane was mounted on a sintered porous polytetrafluoroethylene plate embedded in the permeation cell. The effective membrane area for permeation was the same as the disk electrode size (15.9 cm2). The permeation fluxes of water vapor and N2 were measured at different voltages, pressures and temperatures. All the experiments were carried out with the same membrane with a PDM skin layer thickness of a few micrometers. The membrane was tested with pure gas of CO2 and N2, and the ideal separation factor of CO2/N2 reached 50, indicating that the membrane was defect-free [30]. 1) Measurement of water vapor permeation A stream of N2 gas was bubbled through a humidifier, and the humidified gas was essentially saturated with water. The flow rate of the gas stream was controlled at 50 ml(STP)/min by a mass 6
flow controller (S49-32B/MT, Huibolong, China). The operating temperature was controlled using a thermal bath for the humidifier. A paraffin oil seal was used to prevent water vapor in air from entering the system through the retentate line. The feed side was kept constant at atmospheric pressure, and the permeate side was controlled at a low pressure in the range of 10~30 kPa. When varying the permeate pressure, the purging air entering the vacuum pump was dried by two-stage desiccators: anhydrous calcium chloride granules were used as desiccant in the first stage, and silica gel particles were used in the second stage. The water contents were monitored by a dew point meter (DM70, Vaisala, Finland). The permeated water vapor was collected in a cold trap immersed in liquid nitrogen, and the permeation flux of water vapor was measured by weighing the permeate water collected in the condenser.
paraffin oil seal ball valve silica gel desiccator pressure regulator
mass flow controller
2
permeation cell needle valve
FC
electrode
N2
thermal bath humidifier & demister
DC
dew point meter
power supply
switch valve
vacuum gauge
1 CaCl2 desiccator
vacuum pump
collection tube
bubble flow meter
7
air
Fig.1. Schematic diagram of experimental setup for membrane gas dehydration in a pressure-electric coupled field.
2) Measurement of nitrogen permeation The permeability of N2 was measured with the constant pressure-variable volume method. In the feed side, a water-saturated N2 gas stream was admitted to the permeation cell. The feed gas pressure was controlled at 200 kPa, while the permeate was kept at atmospheric pressure. The permeation flux of nitrogen was measured by a bubble flow meter. The permeation flux J of water vapor or N2 was determined by
J
Q At
(1)
After the permeation experiments in the absence of the electric filed, a voltage up to 5 kV was applied to investigate the effect of the electric field on the permeation. The permeation flux was measured again after removing the electric field to examine if the membrane experienced any permanent change in the electric field.
3. Results and Discussion 3.1 Verification of enhanced water permeation in an asymmetric electric field When a needle⊥plate electric field was applied across the membrane with an increasing field intensity along the direction of permeation, water molecules were subjected to an electric gradient force at the same direction of permeation, causing an enhancement in the mass transport. The enhancement in permeation G was defined as 8
G
Jt J0 100% J0
(2)
It is expected that increasing the electric potential gradient will help align polar molecules more orderly and make them migrate faster. Fig.2 shows that the permeation flux of water vapor was increased with an increase in the voltage gradient applied in the asymmetric electric field, irrespective of the field directions. An enhancement in water permeation of as high as 46% was achieved at a voltage gradient of 213 kV/m. When the electric field was removed, the water permeation flux was found to gradually decline to the initial value before the voltage was applied, demonstrating that the membrane was intact in the electric field under the experimental conditions.
Fig.2. Effect of electric field on permeation of water vapor and N2 gas. (+): anode; (0): grounded; voltage: 0~5 kV; electrode distance: 23.5~63.5 mm. Water vapor permeation test: operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa; J0(H2O): 0.2 mol/(m2.h). N2 gas permeation test: operating temperature: 20oC; feed pressure: 200 kPa; permeate pressure: 100 kPa; J0(N2): 0.008 mol/(m2.h). 9
The experimental data of water permeation presented in Fig.2 follow the equation derived previously to express the permeation of polar molecules in the presence of an asymmetric electric field:
J t J 0 De
U d
(3)
where De is the transmembrane mass-transfer coefficient driven by the electric potential gradient.[23] Eq.3 suggests that polar molecules exhibit a dual mode transport through the membrane under a pressure-electric coupled field: (1) diffusion of polar molecules from the feed side to the permeate side driven by a partial pressure gradient across the membrane, as in a conventional membrane gas separation process, and (2) orientation of polar molecules and migration to the direction of increasing field intensity driven by an electric potential gradient (Fig.3). The overall driving force for the polar molecules to permeate through the membrane derives from the partial pressure gradient and the electric potential gradient, and the permeation flux is thus enhanced by the electric field as compared to conventional permeation process where the transmembrane pressure is the only driving force for permeation. The observed water permeation flux is the sum of both modes of permeation. When two plate electrodes are arranged in parallel, the resulting electric field will be uniform. In such an electric field, the force on the positive end of a polar molecule will balance the force on the negative end, and thus there is no net force on the polar molecule. Therefore, the polar molecules may line up along the field direction in a uniform electric field but they will not migrate under the voltage gradient. When a plate‖plate electric field was applied across the membrane, water permeation was not accelerated by the uniform electric field, and there is no enhancement in water 10
permeation (Fig.2). This result is consistent with our previous findings for membrane distillation[23], confirming that only an asymmetric electric field can provide an additional driving force for mass transport of polar molecules.
11
Fig.3. Schematic diagram of membrane gas dehydration. (a) conventional system and (b) pressureelectric coupled field system.
Non-polar molecules (e.g., N2) do not possess permanent dipole moments. Although non-polar molecules may obtain a slight temporary dipole moment induced by an external electric field, they usually respond to the electric field quite weakly. Therefore, non-polar molecules are not subjected to a significant net force and do not exhibit preferred orientation and directional migration in an asymmetric electric field. While the mass transport of polar molecules was intensified by the asymmetric electric field, there is little effect on the permeation of non-polar molecules, resulting in an increased separation of gas mixtures comprising of polar and non-polar molecules in the pressure-electric coupled field. As shown in Fig. 2, the permeation of water vapor was enhanced, but there is no enhancement in nitrogen permeation in the experimental range of field intensities tested. 3.2 Effect of gas pressure gradient on permeation The appropriate operating conditions often depend on the specific applications. For example, in natural gas dehydration, the feed gas is already at a high pressure and the permeate can thus be withdrawn at a low pressure (close to 1 atm) or preferably at a subatmospheric pressure in order to reduce methane loss
[31,32]
. An industrial-scale membrane process for natural gas dehydration was
reported to be operated at a feed pressure of 5.0 MPa and a permeate pressure of 10~90 kPa
[33]
. In
air or flue gas dehydration, a low partial pressure of water vapor on the permeate side is often maintained by using a vacuum pump or a sweeping gas. In a field test for flue gas dehydration, a feed pressure of 0.1 MPa and a permeate pressure of 2~2.5 kPa were used 12
[34]
. In the present work,
the feed side was kept at atmospheric pressure (i.e., 0.1 MPa) while the permeate pressure was varied in the range of 10~30 kPa in order to evaluate the significance of permeation enhancement by the electric field at different permeate pressures. The driving force for mass transport of a conventional membrane gas separation process is the partial pressure gradient of the gas across the membrane, and the permeation flux J0 may be expressed by [35] J 0 Dp pH 2O
(4)
pH 2O p f x p p y
(5)
where Dp is the transmembrane mass-transfer coefficient, and ∆pH2O is the partial pressure gradient of water vapor across the membrane. The transmembrane pressure gradient of water vapor was increased by decreasing the permeate pressure in this study. The relationship between water flux and water vapor pressure gradient is shown in Fig.4a. The water flux was found to increase with an increase in the water vapor pressure gradient, and so does the corresponding mass-transfer coefficient Dp (Fig.4b). Water vapor is a condensable gas, and it can cause plasticization of hydrophilic membranes. At a higher vapor pressure gradient, water concentration in the membrane matrix increases, and thus the polymer will possess a greater segmental mobility leading to an increase in the value of Dp [36]. When an asymmetric electric field is applied, the water flux contributed by the electric field (Jt-J0) increased linearly with the voltage gradient (Fig.5). These straight lines at different permeate pressures do not overlap, showing that the mass-transfer coefficient De also depends on the water vapor pressure gradient (Fig.6). It is believed that the pressure or concentration dependence of De, 13
similar to that of Dp, is due to the plasticization and/or swelling of the hydrophilic membrane. The increased polymer segmental mobility at an elevated vapor pressure gradient will decrease the membrane resistance and favor the mass transport. It is understandable that De depends upon the properties of the permeant (e.g., size and dielectric constant) and the membrane properties (e.g., resistance).
Fig.4. Water flux contributed by the pressure field (a) and mass transfer coefficient Dp (b) at different 14
partial pressure gradient of water vapor. Operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 10~30 kPa.
Fig.5. Water flux contributed by the electric field at different permeate pressures. Needle⊥plate electric field; needle electrode: anode; plate electrode: grounded; electrode distance: 23.5 mm. Operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 10~30 kPa.
15
Fig.6. Effect of partial pressure gradient of water vapor on mass transfer coefficient De. Operating conditions same as specified in Fig.5.
In conventional membrane gas dehydration processes, vacuum is often applied to the permeate side to evacuate the water vapor and to provide the driving force for permeation. The permeation flux of water vapor can be increased by lowering the permeate pressure in a conventional process. Fig. 5 shows that the transport of water vapor through the membrane in the asymmetric electric field is favored by decreasing the permeate pressure. As such, a decrease in the permeate pressure has a positive effect on the permeation of water vapor in both of the pressure field and the asymmetric electric field. Therefore, similar to the conventional process, a further increase in the water flux of the membrane gas dehydration process in the pressure-electric coupled field can be achieved by decreasing the permeate pressure. Introducing an asymmetric electric field in a membrane gas dehydration process is shown to increase the water flux. The power consumption We due to the use of an electric field can be 16
estimated by
We UI
(6)
The water permeation was increased by 0.1 mol/m2·h in a needle⊥plate electric field at 5 kV at a permeate pressure of 20 kPa and operating temperature of 20oC (Fig. 2). The current was found to be below 0.1 μA (which was the minimum value that could be displayed on our instrument). Thus the maximum electric power consumption to achieve the increased flux would be 0.5 mW, which is very insignificant. On the other hand, in a conventional membrane gas dehydration process without the electric field, the permeation flux can be enhanced by decreasing the permeate pressure. The permeate pressure would have to be decreased by 6 kPa to achieve the same increase in the flux (Fig. 5). In view of the vacuum conditions on the permeate side, a further decrease in the permeate pressure by 6 kPa represents a significant increase in the energy consumption of the vacuum pump as the energy consumption of a vacuum pump is determined by the outlet (i.e., 101 kPa) to inlet pressure (i.e., the permeate pressure) ratio to the power of 0.23 based on the following estimation [37]: k W0 F Z k 1
k 1 R p2 k T1 1 MW p1
(7)
When the permeate pressure decreased from 20 kPa to 14 kPa, the power consumption of the vacuum pump would increase by about 27%. 3.3 Effect of operating temperature on permeation The operating temperatures of conventional membrane gas dehydration processes are mainly determined by the feed gas conditions, and they are often in the range of 5~50oC [32-34]. In this study, the operating temperature was varied in the range of 15~30oC. The permeation fluxes of water vapor 17
and N2 gas at various operating temperatures are shown in Fig.7. The water flux through the membrane under the pressure-electric coupled field was higher than that of the conventional process in the range of the operating temperatures tested. That is, an increased water flux can be achieved at a relatively low operating temperature compared to the conventional process. The permeation fluxes increased with temperature, and the temperature dependence of permeation fluxes was found to follow an Arrhenius type of relationship
E J K J exp J RT
(8)
where EJ is the apparent activation energy characterizing the overall temperature dependence of permeation flux.
Fig.7. Permeation flux of water vapor and N2 gas at different temperatures. Needle⊥plate electric field; needle electrode: anode; plate electrode: grounded; electrode distance: 23.5 mm. Feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa.
18
During the permeation experiments, the feed gas was fully saturated with water. Thus, the permeation is affected by operating temperature on two aspects: the membrane permeability and the transmembrane pressure gradient. In order to separate these two contributions of temperature, the membrane permeance without the electric field (P0/l) was estimated from the flux J0 normalized by the partial pressure gradient. P0 J0 l p H 2O
(9)
It is well known that the temperature dependence of membrane permeance (P0/l) generally follows an Arrhenius relationship, P0 E K P exp P 0 l RT
(10)
where EP0 is the activation energy of permeation characterizing the effect of temperature on the membrane permeability in the absence of an electric field. This is shown in Fig.8, where the membrane permeance to water vapor decreased with an increase in temperature. It appears to suggest that the reduction in solubility of water in the hydrophilic membrane prevailed over the increase in diffusivity as the temperature increased. Similar results on the relationship between water permeance in PDM membrane and the operating temperature have been observed in our previous work. [28]
19
Fig.8. Effect of temperature on water permeance without the electric field. Feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa.
Following Eq. (8),
E J 0 K J 0 exp J 0 RT
(11)
Rearranging Eqs. (9) and (10), one has pH 2O
J0 K E EP 0 J 0 exp J 0 P0 / l K P RT
(12)
Define
E J 0 EP 0
(13)
where ζ, which is a parameter reflecting the effect of temperature on the vapor pressure gradient, can be calculated from the difference between the activation energy based on permeation flux J0 and the activation energy based on permeance (P0/l) in the absence of the electric field. An increase in temperature raises the saturated vapor pressure of water in the feed gas, leading to an increased vapor 20
pressure gradient. The increase in permeation flux with temperature is primarily due to the increased water vapor pressure gradient. On the other hand, the water vapor pressure in the permeate was much lower than that in the feed, and the water vapor pressure gradient was approximately a constant at a given temperature, whether the electric field is present or not. That is, the value of ζ was hardly affected by the electric field. Therefore, when an electric field is applied, the activation energy of permeation based on membrane permeance EPt can be estimated from
EPt EJt
(14)
where EJt is the activation energy for water permeation based on water flux produced in the pressure-electric fields. Fig.9 shows that the activation energy for water permeation EP, which is EP0 in the absence of an electric field or EPt in the pressure-electric field, decreases with the electric field. At an applied voltage of 5 kV, the permeation activation energy was reduced by about 70%. When applying an asymmetric electric field across the membrane, the polar molecules are subjected to an electric gradient force, which reduces the thermal energy required for the water molecules to overcome the energy barrier in order to permeate through the membrane. Since the permeation flux of N2 was not affected by the electric field, the activation energy for N2 permeation remains the same with and without the electric field.
21
Fig.9. Plot of activation energy for water permeation at various electric fields. Operating conditions same as specified in Fig.7.
It should be pointed out this study is only a proof-of-concept study to demonstrate enhanced water vapor permeation from gas streams using an electric field. Much more work is needed in module design and process development to properly accommodate an asymmetric electric field in hollow fiber and spiral wound configurations, which is a subject matter for further studies.
4. Conclusions Integration of a pressure field and an asymmetric electric field was used to enhance membrane gas dehydration, and the following conclusions can be drawn: (1) Applying an asymmetric electric field across a membrane in the direction of permeation enhanced water permeation and thus water/gas permselectivity. (2) The enhancement in water permeation increased with an increase in the voltage gradient applied 22
across the membrane. (3) The asymmetric electric field provides an additional driving force for water permeation. (4) The activation energy for water permeation was decreased by the electric field.
Acknowledgements National Natural Science Foundation of China (NSFC, 21206122), China Postdoctoral Science Foundation (2014T70216; 2012M520577), Tianjin Natural Science Foundation (11JCYBJC26500; 14JCZDJC38100) and Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry (2013-693) are gratefully acknowledged.
Nomenclature A
effective area of membrane
d
electrode distance
Dp
transmembrane mass transfer coefficient driven by pressure gradient
De
transmembrane mass transfer coefficient driven by electric potential gradient
EJ
overall activation energy
EP
activation energy of permeation
F
mass flow rate
G
enhancement in permeation
I
electric current
J
permeation flux 23
k
specific heat ratio (=1.3 for water vapor)
K J, KP
pre-exponential factor
MW
molecular weight
P0/l
permeance without electric field
pf
feed pressure
pp
permeate pressure
p1
pressure at inlet of vacuum pump
p2
pressure at outlet of vacuum pump
∆pH2O
transmembrane partial pressure gradient of water vapor
Q
mole quantity of permeate
R
gas constant
t
permeation time
T
operating temperature
T1
temperature of gas entering vacuum pump
U
voltage applied on the electrodes
V
feed flow rate
W
power consumption
x
mole fraction of water vapor in feed
y
mole fraction of water vapor in permeate
Z
compressibility factor
ζ
parameter reflecting the effect of temperature on the vapor pressure gradient 24
Subscripts 0
conventional process without electric field
e
electric field process
t
pressure-electric coupled field process
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Figure Captions: Fig. 1. Schematic diagram of experimental setup for membrane gas dehydration in a pressure-electric coupled field. Fig. 2. Effect of electric field on permeation of water vapor and N2 gas. (+): anode; (0): grounded; voltage: 0~5 kV; electrode distance: 23.5~63.5 mm. Water vapor permeation test: operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa; J0(H2O): 0.2 mol/(m2.h). N2 gas permeation test: operating temperature: 20oC; feed pressure: 200 kPa; permeate pressure: 100 kPa; J0(N2): 0.008 mol/(m2.h). Fig. 3. Schematic diagram of membrane gas dehydration. (a) conventional system and (b) pressureelectric coupled field system. Fig. 4. Water flux contributed by the pressure field (a) and mass transfer coefficient Dp (b) at different partial pressure gradient of water vapor. Operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 10~30 kPa. Fig. 5. Water flux contributed by the electric field at different permeate pressures. Needle⊥plate electric field; needle electrode: anode; plate electrode: grounded; electrode distance: 23.5 mm. Operating temperature: 20oC; feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 10~30 kPa. Fig. 6. Effect of partial pressure of water vapor on mass transfer coefficient De. Operating conditions same as specified in Fig.5.
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Fig. 7. Permeation flux of water vapor and N2 gas at different temperatures. Needle⊥plate electric field; needle electrode: anode; plate electrode: grounded; electrode distance: 23.5 mm. Feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa. Fig. 8. Effect of temperature on water permeance without the electric field. Feed flow rate: 50 ml(STP)/min; feed pressure: 100 kPa; permeate pressure: 20 kPa. Fig. 9. Plot of activation energy for water permeation at various electric fields. Operating conditions same as specified in Fig.7.
Highlights:
Asymmetric electric field generated a driving force for water permeation.
Water permeation and water/gas separation were improved by the electric gradient.
Activation energy for water permeation decreased with an increase in the voltage.
31