Structural analysis and modeling of the commercial high performance composite flat sheet membranes for membrane distillation application

Desalination 349 (2014) 115–125

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Structural analysis and modeling of the commercial high performance composite flat sheet membranes for membrane distillation application Seongpil Jeong a, Songbok Lee b, Hyo-Taek Chon c, Seockheon Lee a,⁎ a b c

Center for Water Resource Cycle, Green City Technology Institute, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea School of Civil, Environmental & Architectural Engineering, Korea University, Seoul 136-713, Republic of Korea Department of Energy Resources Engineering, Seoul National University, Seoul 151-744, Republic of Korea

H I G H L I G H T S • • • • •

Performances of commercial PTFE membranes were tested in DCMD configuration. Structural analysis and modeling of composite flat sheet membranes were conducted. Pore size, membrane thickness, and support layer structure could result in high flux. Temperature profiles across the tested membranes were computed by numerical analysis. Heat and mass transfer models were applied to know hydrodynamic conditions in module.

a r t i c l e

i n f o

Article history: Received 12 February 2014 Received in revised form 17 May 2014 Accepted 19 May 2014 Available online 16 July 2014 Keywords: Membrane distillation Flat sheet bi-composite membranes DCMD Heat and mass transfer modeling Non-fully developed flow

a b s t r a c t Direct contact membrane distillation (DCMD) module was operated with three flat sheet commercial membranes (PTFE/PP 1.0, PTFE/PP 0.45, and PTFE/PE 0.45) under various operating conditions. High flux was observed when the PTFE/PP 1.0 membrane was used and there was no membrane wetting when the feed concentration was up to 3.0 M as NaCl. In order to understand the relationship between membrane characteristics and operating conditions, the heat and mass transfer models were employed. Instrumental analyses (SEM, porosimetry and contact angle analysis) were carried out for the membrane characteristics and thermo-hydrodynamic parameters were calculated under given experimental conditions. High flux of PTFE/PP 1.0 membrane was thermodynamically possible because the flow in the feed side was not fully developed in the water channel of the module. The membrane characteristics, such as pore-size of active layer and porosity and thickness of support layer were also attributed to a high flux of PTFE/PP 1.0 membrane. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Membrane distillation (MD) is a membrane process driven by thermal potential difference between feed and permeate. MD process guarantees a extremely high rate of rejection (theoretically 100%) for non-volatile impurities under various feed solutions. Additionally, MD process does not require a high pressure pump and can be operated with relatively lowgrade thermal energy such as waste heat or solar power. Due to these advantages, the MD process has been considered as a next generation technology for water treatment, especially desalination [1]. The economic assessment by Al-Obaidani et al. depicted that cost of thermal energy and cost of MD membrane replacement occupied 61% and 30% in O & M (operation and management) cost, respectively. In capital cost, installation cost of MD membrane also occupied 61% [2]. Thus,

⁎ Corresponding author. Tel.: +82 29585829; fax: +82 29585839. E-mail address: [email protected] (S. Lee).

http://dx.doi.org/10.1016/j.desal.2014.05.027 0011-9164/© 2014 Elsevier B.V. All rights reserved.

superior membrane can decrease the total cost of MD system and promote the commercialization of MD process. Many researchers have look deeply with respect to the membrane properties including pore-size [3], porosity [4], membrane thickness [5], hydrophobicity [6] and heat transfer coefficient of the material [2]. They found that thin, porous, hydrophobic and low thermal conducting membrane is necessary for the MD process [7]. The firstly suggested MD membrane applied in direct contact membrane distillation (DCMD) configuration by Weyl [8] was thick and minimally porous PTFE membrane. The flux of the prototype MD membrane was approximately 1 L/m2h (LMH). Over the past decades, membrane technology has been developing. During the 1980s, thin, porous and hydrophobic membranes with large pores were developed by many companies and tested in various MD units [9]. Recently, various suggestions have been done to develop high flux membrane. 1) Bi-composite membranes were developed either as a flat sheet type or hollow fiber type [1] in order to achieve high flux and mechanical strength, altogether. 2) Surface modification using plasma technology was conducted in order to enhance the

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2. Experimental method Nomenclature 2.1. Materials, operation and analysis a A Bm D h J k Kb L LEP m M Nu p P Pa Pt Pr Q r R Re T Tm ΔHV τ δ ε

diffusion parameter (Eq. (22)) cross-sectional area of the channel net membrane permeability or MD coefficient (kg/m2 s Pa) water diffusion coefficient heat transfer coefficient (W/m2 K) flux (L/m2h) thermal conductivity (W/m K) constant for the boiling point elevation characteristic length (m) liquid entry pressure of water (Pa) molality molecular weight of water (g/mol) Nusselt number wetted perimeter of the channel (m) vapor pressure of water (Pa) air pressure (Pa) total pressure inside the pore (Pa) Prandtl number heat flux (W/m2) radius (m) gas constant (8.314472 J/mol K) Reynolds number temperature (K) mean temperature of the feed and permeate (K) latent heat (kJ/kg) tortuosity thickness (m) porosity

Subscripts a active layer b bulk D diffusive flow f feed (side) or feed solution boundary layer K Knudsen flow m membrane p permeate (side) or permeate solution boundary layer s support layer w water

membrane performance [10]. 3) Block copolymer (PEBAX) was tested as a novel membrane material [11]. 4) Electro-spinning technology was also adopted to fabricate novel MD membranes [12]. Recently, Adnan et al. suggested the membranes present outstanding performance (the flux of the membrane was theoretically 120 L/m2h (LMH) when the feed temperature was 70 °C) [13]. The membrane properties are similar to Zhang et al.'s research [14], however, the flux exceeded. In this paper, promising commercial membranes will be suggested in order to share the features and structural information of high performance MD membrane. Also by comparing membrane performances of each membrane in DCMD configuration, the effect of pore size and support structure to flux will be studied based on modeling approaches. The objectives of this study are 1) investigating the flux of the suggested the membranes under given feed solutions and 2) understanding the relationship between membrane characteristic, thermo-hydrodynamic parameters and flux under given experimental conditions by instrumental analyses of and by the heat and mass transfer modeling.

2.1.1. Membrane and module The target membranes for MD were searched based on the analysis of the membrane structural parameters including pore size, thickness and material. All the accessible membranes were listed with proper pore size (0.2–1.0 μm), thickness (b200 μm) and material (polypropylene (PP), polyethylene (PE), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE)) by ignoring their original applications. Among air filtration membranes, there were some membranes with promising membrane structure for MD application. Two flat sheet type PTFE/PP bi-composite membranes with different pore size (PTF045LD0A; PTFE/PP 0.45, PTF100LD0A; PTFE/PP 1.0) and PTFE/PE bi-composite membrane (PT045LH0P; PTFE/PE 0.45) were purchased from a membrane company (Pall, U.S.A.) and prepared for the membrane distillation test (model name of membrane; abbreviation). The numbers just after the membrane model names describe the pore size (unit: μm) of the membrane active layer. The membrane pore size and liquid entry pressure (LEP) were obtained from the manufacturers' manual. The LEPs of the PTFE/PP 1.0 (pore size of 1.0 μm), PTFE/PP 0.45 (pore size of 0.45 μm) and PTFE/PE 0.45 (pore size of 0.45 μm) were N103421, N179263 and N 206842 Pa, respectively. A membrane distillation module in a plate-and-frame configuration was designed for the DCMD process (Fig. 1). The module is made with transparent acryl in order to allow observation of the membrane during operation. The effective area of the membrane for this module is 0.026 m × 0.077 m and the height of the water channel is 0.003 m. 2.1.2. Direct contact membrane distillation system and operation A schematic diagram of the experiment unit is shown in Fig. 2. The feed water is stored in a feed water tank (3 L) and moves into the heater by the gear pumps (75211-15, Cole-Parmer Instrument Company, USA). The membrane separates the water vapor from the feed and the concentrated water is kept circulating in the feed side of the MD system. The transferred water vapor is condensed at the permeate side and is kept circulating in the permeate side of the MD system via the gear pump and chiller. The feed and permeate are circulated in the MD system with counter-current flow in order to maximize the average temperature difference between them [15]. The pressure gages (0–3 bar) and flow meters (0–2.0 L/min) were installed to monitor pressure and flow rate of the feed and the permeate before the inlets of DCMD module. The increasing weight in the permeate side is monitored during the MD operation by a balance. Electronic conductivity meter probe (CDC40101, Hach, USA) connected to electronic conductivity meter (HQ40d, Hach, USA) monitors the electronic conductivity variations of the feed and the permeate. Only the weights and electronic conductivities are monitored every minute and stored automatically on a lap top computer and pressure and flow data are recorded at the beginning and end of the experiments. The fluxes are computed by using the stored weight data for at least 1 h. The DCMD system was driven in steady state under given experimental conditions. The details are summarized in Table 1. 2.1.3. Instrumental analysis The surface morphology and cross-sectional image of the membrane was examined by scanning electron microscopy, SEM (Nova 200, FEI, U.S.A.), at KIST (Korea Institute of Science and Technology). The thickness of the membrane was obtained from the cross-sectional image. The porosities of the active and support layer were analyzed by a mercury porosimeter (Autopore IV 9500 (Micromeritics, U.S.A.)) at KRICT (Korea Research Institute of Chemical Technology) after the active and support layers were separated. Contact angle analyses for active and support layer were carried out with a contact angle analyzer (DSA100, KRÜSS, Germany) at KIST.

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117

Fig. 1. Tested direct contact membrane distillation (DCMD) module.

2.2. Theoretical heat and mass transfer models 2.2.1. Simple mass transfer modeling for MD membrane in saline water The mass transfer of the MD system is explained by the following simplified equation:
 JW ¼ Bm ΔP ¼ Bm P f −Pp

ð1Þ

where JW is the flux of the MD membrane, and Bm is the net membrane permeability or MD coefficient. The driving force of the membrane distillation is the vapor pressure difference (ΔP) which is generated by the temperature difference of the feed (Tf) and permeate (Tp) across the membrane. The vapor pressures of the feed (Pf) and permeate (P p ) can be calculated using the Antoine equation when the solvent is water. 
 Pi ¼ P f or Pp ¼ exp 23:1964−

 3816:44 : TðKÞ−46:13

Q ¼ Q f ¼ Qa ¼ Qs ¼ Qp ð2Þ

Additionally, in order to estimate the vapor pressure of the various feed solutions, the boiling-point elevation was considered according to the NaCl concentrations of the feed solutions. The boiling-point elevation was calculated by the following equation: ΔTb ¼ Kb m

2.2.2. Heat transfer model The heat transfer model developed by Qtaishat et al. (2009) was used [17]. This model was originally developed to understand the heat transfer of a hydrophobic/hydrophilic composite membrane. The PTFE/PP and PTFE/PE bi-composite membranes also show similar characteristics with a hydrophobic/hydrophilic bi-composite membrane. In this research, Nusselt number was calculated by using the flat plate type channel model from laminar to turbulent flow. The bi-composite membrane in DCMD is described in Fig. 3 [17]. The heat transfers from the feed to the permeate across the membrane by Knudsen flow or molecular diffusion as water vapor and conduction through membrane. The total energy (heat) from the feed to the permeate was assumed to be a steady state. The heat transfer across the membrane from the feed to the permeate represented by the equation:

ð3Þ

where ΔTb is the temperate variation due to boiling-point elevation, Kb is the constant for the boiling-point elevation and m is the molality. The Kb of the water was 0.512 K kg/mol [16].

ð4Þ

where Q is the total heat flux from the feed to the permeate. Qf, Qa, Qs and Qp are the heat fluxes through the feed solution boundary layer, active layer, support layer and permeate solution boundary layer. Each heat flux is expressed as: Q f ¼ h f ðTb: f −Tm: f Þ

ð5Þ


 Q a ¼ ha Tm: f −Tm:p þ JW ΔHV

ð6Þ

Fig. 2. The schematic diagram of the MD system.

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Flat plate in laminar flow:

Table 1 Detailed experimental conditions of the DCMD system.

1=2

Nu ¼ 0:332Re

Range of the temperature variation in the feed and permeate Feed: 30, 40, 50, 60, 70, 80 °C

Types of the feed Deionized (D.I.) water, 0.6 M NaCl, 3.0 M NaCl Range of the cross-flow rate and velocity of the feed and permeate 1) For flux test of PTFE/PP 1.0, PTFE/PP 0.45, PTFE/PE 0.45 Cross-flow rate: 1.2–1.6 L/min Cross-flow velocity: 0.26–0.34 m/s 2) For the support layer comparison (PP and PE support layer) Cross-flow rate: 0.4, 0.8, 1.2, 1.6 L/min Cross-flow velocity: 0.09, 0.17, 0.26, 0.34 m/s

ð7Þ


 Q p ¼ hp Ts:p −Tb:p :

ð8Þ

The value of vaporization latent heat (ΔHV) was gained from the reference [18]. The heat convection coefficients are Heat convection coefficient of the feed boundary layer: hf ¼

kw Nu: L

ð9Þ

Flat plate in turbulent flow: 4=5

Nu ¼ 0:337Re L¼

ð10Þ

Heat convection coefficient of the support layer: hs ¼

kw ϵs þ ks ð1−ϵs Þ : δs

ð11Þ

Heat convection coefficient of the permeate boundary layer: k hp ¼ w Nu: L

ð14Þ

4A : P

ð15Þ

2.2.3. Mass transfer model The developed mass transfer model by Qtaishat et al. (2009) was applied in this study [17]. The mass transfer equation is expressed by K

D

J þJ ¼

" #−1  
 3 τt δt πRT 1=2 τt δt Pa RT þ Tm: f −Tm:p 2 εt rp;t 8M εt Pt D M    ΔHV 3841 exp 23:238−  2 Tm −45 RTm

ð16Þ

where JK and JD are the fluxes by the Knudsen flow and diffusive flow, R is the gas constant (8.314472 J/mol K), T is the temperature at given condition (K), M is the molecular weight of water, Pa is the air pressure (Pa), Pt is the total pressure inside the pore, D is the water diffusion coefficient, and Tm is the mean temperature of the feed and permeate (K). The T, Pa and PD are given by T¼

Tm: f þ Tm:p 2

ð12Þ

The water thermal conductivity (kw) and gas thermal conductivity (kg) were obtained from references [18,19]. Thermal conductivities of active layer (ka) and of support layer (ks) were determined by their materials. The thermal conductivities of PTFE, PP and PE were assumed as 0.25, 0.12, and 0.5 (W/m K), respectively [20]. The Nusselt numbers were calculated by the following equations [19].

1=3

Pr

Where A is the cross-sectional area of the channel, L is the characteristic length, and p is the wetted perimeter of the channel. The temperature variations across the membrane (Tm.f, Tm.a, Tm.s) and heat transfer flux (Qf, Qa, Qs, Qp) were calculated by numerical analysis.

Heat convection coefficient of the active layer: kg ϵa þ ka ð1−ϵa Þ : ha ¼ δa

ð13Þ


 5 Reb5  10 ; 0:6bPrb50 :

Permeate: 20 °C


 Q s ¼ hs Tm:p −Ts:p

1=3

Pr

Pa ¼

ð17Þ

P f þ Pp 2

ð18Þ −5 2:072

Pt D ¼ 1:895  10

T

:

ð19Þ

However, this model has limitations due to the assumption that this equation is acceptable only for small trans-membrane bulk temperature differences (Tb.f − Tb.p ≤ 10 K). 3. Results and discussion 3.1. Membrane characteristics of commercial membranes

Fig. 3. Heat transfer across the composite membrane in DCMD [17].

3.1.1. SEM analysis The SEM images of the active and support layers for three tested membranes (PTFE/PP 0.45, PTFE/PP 1.0 and PTFE/PE 0.45) were shown (Figs. 4–5). The surface morphologies of the PTFE/PP 0.45 and PTFE/PP 1.0 active layers were presented in Fig. 4(a) and (b), respectively. And the support layers of PTFE/PP 0.45 and PTFE/PE 0.45 were also examined as Fig. 5(a) and (b). The PE support layer was a non-woven fabric-type fiber structure and has irregularly shaped pores. On the other hand, the PP support layer was a scrim-type mesh structure and has elliptical, regular pores. The cross sectional images of PTFE/PP 0.45 membrane (Fig. 6(a)) and PTFE/ PE 0.45 membrane (Fig. 6(b)) were used to measure the thickness of each membrane. In Fig. 6(b), the total thickness of PTFE/PE 0.45 membrane was measured and the other image was used to measure the thickness of the PE support membrane only. The PTFE/PP 0.45 and PTFE/PP 1.0 membranes have the identical PP support layer (Fig. 5(a)) and similar cross section

S. Jeong et al. / Desalination 349 (2014) 115–125

a) Active layer of PTFE/PP 0.45

119

b) Active layer of PTFE/PP 1.0

Fig. 4. Active layers of the tested bi-composite membranes.

(Fig. 6(a)) and the PTFE/PP 0.45 and PTFE/PE 0.45 membranes have the same PTFE active layer (Fig. 4(a)). The thicknesses of tested membranes were summarized in Table 2. 3.1.2. Contact angle and porosity Instrumental analysis for contact angles and porosities were conducted to compare the membrane characteristics among tested membranes (Table 2). The surfaces of PFTE active layers had high contact angles (140–148°) and could be considered nearly superhydrophobic (N155°). In case of the support layer, the PE support was hydrophilic (contact angle: 69°) while the PP support was hydrophobic (contact angle: 118°). However, the hydrophobic PP support can be easily wetted because the pore size is large (500 μm × 200 μm) and the pore is cylinder like in structure (Fig. 5(a)). The porosity (66.8%) of the PTFE/PP 1.0 active layer was relatively lower than the porosity (72.6–72.8%) of the PTFE/PP 0.45 active layer. And the porosity (55.6%) of PE support layer was smaller than that of PP support layer (67.2%). The significant differences between PTFE/PP and PTFE/PE membranes were values of the contact angle and porosity of the support layer. 3.2. Comparisons of the fluxes according to temperatures under various feed solutions 3.2.1. Experimental fluxes of the bi-composite membranes in D.I. water The flux tests were conducted based on the given conditions as shown in Table 1. The feed water was deionized water (D.I. water) and the cross-flow velocity of the feed and the permeate side was

a) PP support layer

0.34 m/s. The permeate temperature was controlled at 20 °C while the feed temperatures were increased at 30, 40, 50, 60, 70 and 80 °C. The experimental fluxes of tested bi-composite membranes were shown in Fig. 7. All the fluxes of the tested membranes were increased according to the feed temperature. The PTFE/PP 1.0 membrane showed the best flux among the tested membranes (190 L/m2h, at feed temperature: 80 °C) and the slope was exponentially increased. However, in the case of PTFE/PP 1.0 membrane, it was sometimes torn or the active layer and support layer were detached due to the pressure difference between the feed and permeate and high pressure near the inlet of the module. The main reason for this lack of mechanical strength is because this membrane was designed for air filtration membrane not for water filtration application. Therefore, PTFE/PP 1.0 membrane required much careful operation controlling pressure difference of the feed and permeate side than PTFE/PP 0.45 and PTFE/PE 0.45 membranes. 3.2.2. Experimental fluxes of the bi-composite membranes in saline solutions The flux tests were conducted on the same given experimental conditions in the previous chapter except the feed solution (Table 1). The two additional saline feed waters (0.6 M and 3.0 M NaCl) were prepared in order to confirm flux variations in highly concentrated saline water and pore wetting was checked. The experimental fluxes of the tested membranes according to the feed temperature in saline feed solutions were shown in Fig. 7. The fluxes of tested membranes in saline solution (0.6 M NaCl) were decreased 2–16% compared to that of the same membranes in D.I. water.

b) PE support layer

Fig. 5. Support layers of the tested PTFE/PP and PTFE/PE membranes.

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a) PTFE/PP 0.45 membrane

b) PTFE/PE 0.45 membrane

Fig. 6. Cross sections of the tested PTFE/PP and PTFE/PE membranes.

Table 2 Analyzed properties of the tested membranes. Membrane

PTFE/PP PTFE/PE

Pore size (μm)

0.45 1.0 0.45

Thickness of the membrane (μm)

Contact angle (°)

Porosity (%)

Total

Active layer

Support layer

Active layer

Support layer

Active layer

Support layer

89 89 120

36 34 20

53 55 100

140 148 150

118 118 69

72.6 66.8 72.8

67.2 67.2 55.6

This phenomenon might be a result of boiling water elevation or the NaCl crystallization may decrease the flux of the MD membrane. Therefore, the simplified mass transfer model Eq. (1) was adopted to find out the theoretical flux under the same experimental given conditions. The electronic conductivities of the permeates were between 0.8 and 1.9 μS/cm at the end of the tests, which meant pore wetting did not occur obviously during all the tests.

3.2.2.1. Results of simple model and MD coefficient. The MD coefficients were calculated from the obtained data of the DCMD experiments on the bi-composite membranes in D.I. water (Table 3). The vapor pressures of the feed and the permeate were computed by using the Antoine equation. The flux was obtained by the DCMD operation according to the tested membranes according to the various feed temperatures from 30 °C to

80 °C. The MD coefficients were decreased with feed temperature increasing in all cases except one data (PTFE/PE 0.45 at feed temperature of 40 °C). The calculated MD coefficients of the PTFE/PP 1.0 showed the highest values among the tested membrane. The MD coefficient could be thought as a mixture of all the possible variables or parameters in MD membrane and process except the vapor pressure difference between the feed and the permeate. The MD coefficients decreased while the theoretical vapor pressure and latent heat of vaporization increased as the feed temperature increased. Furthermore, the fluxes of tested membranes were increased steadily according to the feed temperature, which meant that the vapor pressure difference affected flux increase more than the membrane properties or operating conditions at high feed temperatures. However, their flux differences among the tested membranes were continuously maintained or increased steadily even at high feed temperatures. 3.2.2.2. Theoretical flux by the simplified model and flux ratio. By using the simplified model, the theoretical fluxes were calculated according to the feed temperature (30–80 °C) in saline feed solutions (0.6 M NaCl and 3.0 M NaCl) (Figs. 8–9) considering boiling-point elevation. The calculated fluxes by the simplified model were similar to the actual fluxes of the experiments except the PTFE/PP 0.45 membrane when the feed was 0.6 M NaCl (Fig. 8(a)). The overall agreement between experimental flux and theoretical flux means that the main flux decreases with 0.6

Table 3 Calculated MD coefficients according to membrane under a given operating condition (feed: D.I. water). Temperature of the feed and permeate (°C, °C)

Fig. 7. Experimental fluxes of the tested membranes according to feed temperature (feed solution: D.I. water).

30, 20 40, 20 50, 20 60, 20 70, 20 80, 20

Theoretical vapor pressure (Pa)

MD coefficient, Bm (kg/m2 Pa s)

Feed

Permeate

PTFE/PP 1.0

4184 7300 12,236 19,785 30,971 47,086

2292 2292 2292 2292 2292 2292

1.69 1.69 1.41 1.28 1.27 1.18

× × × × × ×

10−6 10−6 10−6 10−6 10−6 10−6

PTFE/PP 0.45 1.66 1.38 1.19 1.14 1.12 1.03

× × × × × ×

10−6 10−6 10−6 10−6 10−6 10−6

PTFE/PE 0.45 9.65 1.06 8.99 8.27 6.98 6.39

× × × × × ×

10−7 10−6 10−7 10−7 10−7 10−7

S. Jeong et al. / Desalination 349 (2014) 115–125

121

Fig. 9. Flux ratio of the tested membranes according to NaCl concentration of the feed (feed solution: (a) 0.6 M NaCl, (b) 3.0 M NaCl). Fig. 8. Experimental and theoretical fluxes of the tested membranes according to feed temperature (feed solution: (a) 0.6 M NaCl, (b) 3.0 M NaCl).

M NaCl solution feed resulting from boiling-point elevation and the model could estimate the experimental fluxes of the tested membranes in the PTFE/PP 1.0 and the PTFE/PE 0.45 membranes. In most cases, the experimental flux was lower than the theoretical one. This tendency seemed obvious at high feed temperature. Indeed, the experiments were conducted continuously by increasing the feed temperature and by concentrating the feed solution simultaneously. It could be stated that the concentration of the feed solution at high temperature was also higher than that of the feed solution at the initial temperature. Therefore, an additional graph explaining the relationship between flux ratio and NaCl concentration of the feed was shown as Fig. 9. The flux ratio was calculated as follows. Experimental flux Flux ratio ð%Þ ¼  100: Theoritical flux

PTFE/PP 0.45 membrane at some given conditions could be explained by additional flux decreasing factors such as fouling or scaling. Because the feed solutions only contained NaCl except D.I. water, scaling could be the missing factor. 3.2.3. Heat transfer across the test membranes In order to obtain temperature profile across each membrane by numerical analysis, fluxes under given operation conditions were employed for the heat transfer model (Eqs ((4)–(15)). The feed and permeate side velocities were assumed the same value as 0.34 m/s. The feed temperatures were 30, 40, 50, 60, 70 and 80 °C and the permeate temperature was 20 °C. The hydrodynamic indicators (Re, Pr and Nu) were also Table 4 Hydrodynamic characteristics under given condition.

ð20Þ

The flux ratios of PTFE/PP 1.0 and PTFE/PE 0.45 membranes were 92–106% except the initial flux ratio when the feed was 0.6 M NaCl solution (Fig. 9(a)). However, the PTFE/PE 0.45 membrane showed 87–88% of flux ratio under the same given condition when the feed was concentrated over 0.7 M NaCl. In the highly concentrated feed (3.0–3.7 M NaCl), the flux ratios of all the tested membranes were decreased to lower than 90% (Fig. 9(b)). The relatively low flux ratio of

Temperature

Feed side

Permeate side

°C

K

30 40 50 60 70 80 20

303 313 323 333 343 353 293

Re

Pr

2283 2780 3307 3850 4428 5011 1822

5.4 4.3 3.6 3.0 2.6 2.2 7.0

Nu Laminar

Turbulent

27.9 28.5 29.1 29.7 – – 27.1

– – 36.9 39.4 41.8 44.0 –

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S. Jeong et al. / Desalination 349 (2014) 115–125

computed under given operating conditions (Table 4). In Table 4, the flow changed from laminar to turbulent with the feed temperature. Fig. 10 shows the computed temperature profiles across the tested membranes. Severe temperature polarizations were observed at high feed temperature in all tests. Δ(Tmp − Tsp) shows linear increase in all results although the support layer structure and material are different between PTFE/PP membrane and PTFE/PE membrane. This is because the cooling conditions at permeate side boundary layer are identical in hp (3016 w/m2), temperature (20 °C) and cross-flow velocity (0.34 m/s) in all tests. Most of the temperature profiles showed gradual decrease across the membrane from the feed to permeate, however, the temperature profiles under high feed temperature (N70 °C) and turbulent region showed temperature drop at Tmf. However, temperature drop at Tmf is thermodynamically impossible because the vapor pressure difference resulted from temperature difference between Tmf and Tmp is the driving force of MD system. There is a possibility that Eq. (14) estimated a low Nu value at high feed temperature. In order to check the thermo-hydrodynamic state in the module, thermal entrance length for turbulent flow was calculated by the equation; Thermal entrance length ¼ 10 L

ðindependent of PrÞ:

ð21Þ

Because there is no pre-inlet channel in tested module providing fully developed flow, the hydrodynamic and thermal entrance length (0.0537 m) is included in the flow length of the module (0.077 m). Therefore, 69.7% of the flow channel in the DCMD module was operated in developing flow. In the non-fully developed flow, Nu could be higher than that in the fully developed flow due to thinner boundary layer of developing flow.

When the Nusselt number of the feed flow (Nuf) is increased theoretically, Qf starts to increase and Tmf, Tmp and Tsp also increase (Fig. 11). When the Nuf is larger than 2 times of predicted Nuf by Eq. (14), all the data show a thermodynamic agreement. Therefore, significant care will be required to employ Nusselt number into heat transfer model with respect to shape or size (i.e. thermal entrance length) of the module. 3.3. Comparisons of the fluxes according to membrane characteristics 3.3.1. Pore size and tortuosity of active layer (PTFE/PP 1.0 vs. PTFE/PP 0.45) One obvious reason for flux difference between PTFE/PP 1.0 and PTFE/PP 0.45 is pore-size. In membrane properties, pore size, porosity, tortuosity and thickness could be considered as important structural parameters of the MD membrane. The simple relationship was suggested in the previous research like in the following equation [9]. N∝

ra ϵ τδ

ð22Þ

where N is the permeability, r is the membrane pore size, a is a diffusion parameter (the average pore size for Knudsen diffusion (a = 1) or the average squared pore size for viscous flux (a = 2)), ε is the membrane porosity, τ: is the pore tortuosity and δ is the membrane thickness. Therefore, it could be mentioned that the flux of the membrane is proportional to the pore size. However, the flux of the PTFE/PP 1.0 was only 2–22% higher than that of the PTFE/PP 0.45 (Fig. 4). Therefore, other parameters might be working as flux-limiting properties in the PTFE/PP 1.0 membrane. Although the membrane tortuosity is a significant parameter limiting the mass vapor flux, it is not measurable due to the complicated nature of the membrane's inner structure. While three-dimensional (3-D)

a) PTFE/PP 1.0

b) PTFE/PP 0.45

c) PTFE/PE 0.45 Fig. 10. Temperature profiles across the tested membranes (a: PTFE/PP 1.0, b: PTFE/PP 0.45, c: PTFE/PE 0.45).

S. Jeong et al. / Desalination 349 (2014) 115–125

123

a) Employed Nu : equation (14) driven (Nuf)

b) Employed Nu : 2 times of Nuf

c) Employed Nu : 3 times of Nuf

d) Employed Nu : 4 times of Nuf

Fig. 11. Temperature profiles across the PTFE/PP 1.0 membrane with various Nusselt numbers.

structure analysis can calculate tortuosity, this method still needs an exact inner structure to accurately calculate it [21]. It is unconvincing that the PTFE/PP 1.0 membrane had a higher tortuosity than the PTFE/ PP 0.45 membrane by the SEM image analysis. Therefore, the tortuosities of the tested membranes were calculated using the heat and mass transfer modeling approach by using the experimental data and membrane structure analysis results (Table 5). The tortuosities of the PTFE/ PP 1.0 and PTFE/PP 0.45 were 6.6 and 4.2, respectively. The calculated tortuosities of both tested membranes by MD modeling can be acceptable because they are quite similar to the generally assumed tortuosity (τ = 2) in the MD modeling [1]. Consequently, it could be mentioned that the flux increase by larger pore size of the PTFE/PP 1.0 was partially compensated by the flux decrease by higher tortuosity of the same membrane. 3.3.2. Porous structure of support layer (PTFE/PP 0.45 vs. PTFE/PE 0.45) The flux of the PTFE/PP 0.45 membrane was higher than that of the PTFE/PE 0.45 membrane (Fig. 4) although the pore sizes of the two active layers were identical and the porosities of both PTFE composite membranes were similar (Table 2). Therefore, it could be assumed

that support structure was concerned with the flux difference between PTFE/PP 0.45 and PTFE/PE 0.45. The contact angles of the PP and PE support layer were 118° and 69°, respectively (Table 2). Although the contact angle of the PP support layer was 118°, the water could directly contact the inner active layer because the PP support had mesh type identical large pores (500 μm). In case of the PE support layer, the water should be moved into the highly tortuous (Fig. 5(b)) and relatively long (Table 2) water channel though it is relatively hydrophilic. In order to compare the water transport velocities via support layer for both membranes, additional tests were conducted. The feed water was deionized water (D.I. water) and the cross-flow velocities of the feed and the permeate side were increased step by step like 0.09, 0.17, 0.26 and 0.34 m/s. The permeate temperature was controlled as 20 °C while the feed temperature was 60 °C. The fluxes variations were presented in Fig. 12 according to the cross-flow velocities. The flux of the PTFE/PP and PTFE/PE membranes increased when the cross-flow velocity increased, however, the PTFE/ PP curve was always over the PTFE/PE curve under given conditions. The flux differences could be explained such that the relationship

Table 5 Ced temperatures and vapor pressures across the membranes and tortuosity according to membrane. Membrane

PTFE/PP PTFE/PE

Pore size (μm)

Flux (L/m2h)

Temperature (K) across the membrane Tmf

Tmp

Tsp

Pmf

Pmp

Psp

1.0 0.45 0.45

11.5 11.3 6.6

299.6 299.7 300.1

297.9 297.8 298.6

296.5 296.5 296.0

3422 3438 3521

3100 3080 3231

2848 2836 2764

Vapor pressure (Pa) across the membrane

Tortuosity

6.6 4.2 10.1

(Tbf: 303 K (30 °C), Tbp: 293 K (20 °C), Pbf: 4183.6 Pa, and Pbp: 2291.8 Pa). (T: temperature, P: vapor pressure, Xbf: X in bulk feed, Xmf: X at the membrane surface (active layer) on the feed side, Xmp: X between the active and support layer, Xsp: X at the membrane surface (support layer) on the permeate side, and Xbp: X in bulk permeate).

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operating condition, such as high value of Nu due to the non-fully developed flow through the feed channel. However, the PTFE/PP 1.0 membrane sometimes was torn because of pressure differences near the inlet and outlet in the module. Therefore, additional enhancement of mechanical strength is required for the long term MD application. Boiling-point elevation provided significant agreement between experimental flux and theoretical flux when the feed was saline solution (0.6 M NaCl and 3.0 M NaCl) and tortuosities of the tested MD membranes could be calculated by employing experimental data to the heat and mass transfer models. Acknowledgments This research was supported by the Korean Ministry of Environment as an “Eco-Innovation Program (Environmental Research Laboratory)” (414-111-011). References Fig. 12. Fluxes of the tested membranes according to cross-flow velocity.

between the flux and Re according to the cross-flow increases by the heat transfer theory. Table 6 shows the calculated values of heat transfer coefficients and other parameters of the water flows. If values of Re and Nu are increased by the acceleration of the feed and permeate flow, more heat is transported from the feed and permeate to the membrane. However, due to the difference of the porosity and thermal conductivity of each support layer, the hs of PP and PE support layers denoted as 8362 W/m K and 5561 W/m K, respectively. Because of high hs of PP support layer, the Tmp and Tsp of PTFE/PP membrane will become lower than that of PTFE/PE membrane and the driving force (Δ(Tmf − Tmp)) of PTFE/PP membrane will be higher than that of PTFE/PE membrane.

4. Conclusion Three candidate MD membranes (PTFE/PP 1.0, PTFE/PP 0.45 and PTFE/PE 0.45) were tested under given experimental conditions. Membrane characteristic were analyzed by instrumental approaches and thermo-hydrodynamic status was computed by the heat and mass transfer modeling. The PTFE/PP 1.0 bi-composite membrane showed the high flux (190 LMH) in the DCMD configuration. The electronic conductivities in the permeates were maintained between 0.8 and 1.9 μS/cm at the end of the tests, which means that no membrane wetting occurred during the MD operation. The major reasons for the high flux of PTFE/PE 1.0 membrane were pertinent membrane characteristics, such as large pore size of active layer and highly porous and thermal conductive support layer and

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Table 6 Calculated values of heat transfer coefficients and other parameters according to the cross-flow velocities. Cross-flow velocity (m/s)

Feed (Temp.: 60 °C)

Permeate (Temp.: 20 °C)

Re Pr Nu hf (W/m K) Re Pr Nu hp (W/m K) hs (W/m K) (PP support layer) hs (W/m K) (PE support layer)

0.09

0.17

1019

1925

0.26

0.34

2944

3850

25.9 2898 1393

29.7 3314 1822

23.7 2645

27.1 3024

3.0 15.3 1705 482

21.0 2343 911

13.9 1556

19.1 2139

7.0

8362 5561

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