Mass-production of tubular NaY zeolite membranes for industrial purpose and their application to ethanol dehydration by vapor permeation

Journal of Membrane Science 319 (2008) 244–255

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Mass-production of tubular NaY zeolite membranes for industrial purpose and their application to ethanol dehydration by vapor permeation Kiminori Sato ∗ , Kazunori Sugimoto, Takashi Nakane Bussan Nanotech Research Institute Inc., Koyadai 2-1, 305-0074 Tsukuba, Japan

a r t i c l e

i n f o

Article history: Received 28 December 2007 Received in revised form 21 March 2008 Accepted 23 March 2008 Available online 30 March 2008 Keywords: NaY zeolite Dehydration Vapor permeation Concentration polarization Permeance

a b s t r a c t A fabrication method for tubular NaY zeolite membranes with 80 cm long for industrial purpose has been applied in mass-production to synthesize 99 membrane samples using two types of synthetic solutions with Al2 O3 :SiO2 :Na2 O:H2 O = 1:10:14:800 and = 1:20:14:1000. Sufficient reproducibility for industrial purpose with high membrane performance in mass-production were observed by pervaporation at 75 ◦ C in a feed mixture of water(10 wt.%)/ethanol(90 wt.%), resulting in the average flux (1 − ) and separation factor of 8.1 kg m−2 h−1 (1.5) and 220 for the SiO2 -rich synthetic solution and those of 9.9 kg m−2 h−1 (1.3) and 190 for the SiO2 -poor solution, respectively. Dehydration of hydrous ethanol by vapor permeation for the present membranes was studied at 90–110 ◦ C to investigate an applicability of these membranes to ethanol purification. The ethanol products with concentration up to 98.5 wt.% and with 96.1 wt.% could be obtained at 110 ◦ C by production rates at 5 kg h−1 and 20 kg h−1 , respectively. It is also confirmed that concentration polarization could limit the permeation fluxes through the present high-performance membranes for the feed flow rates at 5.5–22 kg h−1 with the Reynolds number of 5000–20,000. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Zeolite membranes are receiving an amount of attention because those have advanced properties of molecular separation and chemical, mechanical and thermal stabilities. Therefore, zeolite membranes are expected to be applied to separation processes in industry. Many types of zeolite membranes such as MFI-, LTA, MOR-, FER-, FAU-, BEA- have been studied in preparation and permselective behavior (reviewed in e.g. [1–4]). In these various types of zeolite, unique properties with a larger zeolitic pore size (0.74 nm) and high polarity are found in FAU-type zeolite that could be applied in multiple purposes for (1) dehydration [5–9], (2) organic–organic separation [5,6,10–13], (3) gas separation such as CO2 extraction [14–17] and (4) vapor–gas separation [18]. Consequently, it has been expected that establishment of a fabrication method for FAU-type zeolite membranes with an industrial scale enables us to apply them to various practical separating processes in industry. A fabrication method for tubular NaY zeolite membranes in an industrial scale using two types of synthetic chemical solutions (Table 1) has been reported [19]. The synthesized membranes exhibited high permeation fluxes in a mixture of water/ethanol. In

∗ Corresponding author. Tel.: +81 29 839 9371; fax: +81 29 839 9430. E-mail address: [email protected] (K. Sato). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.03.041

the fabrication method, a large apparatus for hydrothermal synthesis was utilized to develop the 80 cm long tubular membrane with uniformity of permselective properties in the longitudinal direction that found in pervaporation (PV) [19]. The synthesis apparatus has an advantage in that boiling by extra thermal input from the outer steam jacket could homogenize the crystallization environment in the solution. These results indicated that the developed fabrication method for tubular NaY zeolite membranes could be used in industrial mass-production. Therefore, this achievement of the fabrication method for NaY zeolite membranes in an industrial scale with high performance made us decided to conduct a technical feasibility study of this fabrication method in mass-production and to investigate the influence of chemical composition of synthetic solutions on the membrane performance. In applications of NaY zeolite membranes to various kinds of separation, solvent dehydration is an important application for the present NaY membranes. As a typical case, dehydrated ethanol is demanded for an additive to gasoline. Zeolite NaA membranes are also effective for this purpose and it was reported that an up-scaled tubular NaA membrane with 1 m long exhibited membrane performance with 5.9 kg m−2 h−1 in permeation flux and 9000 in separation factor by PV at 75 ◦ C in a feed mixture of water(10 wt.%)/ethanol(90 wt.%) [20]. On the other hand, the present NaY zeolite membranes were reported to show high permeation flux with 9.1–10.1 kg m−2 h−1 and moderate sepa-

K. Sato et al. / Journal of Membrane Science 319 (2008) 244–255

Table 2 Performance of synthesized NaY zeolite membranes in selected 68 samples by PV at 75 ◦ C in water(10 wt.%)/ethanol(90 wt.%)

Table 1 Chemical composition for membrane synthesis Solution type

Al2 O3 :SiO2 :Na2 O:H2 O

Number of tubes Synthesized

Measureda

Type-a Type-b

1:20:14:1000 1:10:14:800

24 75

18 50

Total

–

99

68

a Measured by pervaporation water(10 wt.%)/ethanol(90 wt.%).

(PV)

at

75 ◦ C

75 ◦ C

in

feed

245

mixture

of

ration factor with ˛ = 170–190 at in a feed mixture of water(10 wt.%)/ethanol(90 wt.%). These indicate that the present NaY zeolite membranes could have an advantage of higher permeation flux in solvent dehydration. Another expected property of the NaY membrane in ethanol production from fermentation (bioethanol) might be extraction of other organic components than ethanol such as methanol that is a common by-product occurred in fermentation. Therefore, the applicability of the present NaY zeolite membranes to ethanol dehydration should be investigated to determine permselective properties by permeating experiments in practical conditions. Vapor permeation (VP) can be employed for ethanol dehydration by NaY zeolite membranes combined with distillation. In this hybrid distillation–vapor permeation system, hydrous ethanol vapor with azeotropic compositions containing 5–10 wt.% of water would be fed from the overhead of rectifier into the membrane process to break the azeotrope. The membrane performance can be influenced mainly by such operating parameters in VP process as (1) feed temperature and (2) flow rate of feed vapor. Temperature is an influencial operating parameter that an increasing of temperature enhances permeation flux. However, temperature dependency of separation behavior between water and ethanol through NaY zeolite membranes in VP has not been clarified. Therefore, it is required to determine permselective behavior as a function of temperature. It has been reported that the effect of concentration polarization which is compositional gradient in a region adjacent to the membrane surface at the feed side could be caused by depletion of permeating component concentration (e.g., [21–27]). This concentration polarization reduces the concentration of permeating component difference across the membrane, resulting in lowering its flux. A certain degree of concentration polarization on the membrane surface might be expected for the present NaY zeolite membrane because those exhibit high permeation water flux causing the deficiency of water component immediately adjacent to the membrane surface. This concentration polarization could be transport resistance and limitation for permeation flux through the present membranes. An operating parameter of the feed flow rate controls a state of concentration polarization on the membrane surface because the flow rate could influence mixing conditions at the membrane surface. Thus, it is required to investigate the influence of flow condition on permselective behavior including permeation flux and separation factor. This investigation is to determine the mass flow resistance of individual regions for the permeation through the membrane including the stagnant boundary layer with concentration polarization, the zeolite layer and the support layer. In these backgrounds, the purpose in this study is (1) to verify the applicability of the developed fabrication method for NaY zeolite membranes in an industrial scale to mass-production, (2) to apply those NaY membranes to ethanol dehydration in VP and (3) to clarify the influence of operating parameters of temperature and feed flow rate on permeating behavior.

Lot no.

Synthesized (N)

Measured (n)

Type-a 12 13 15 18 Sub-total

6 6 6 6 24

5 6 6 1 18

Type-b 1 2 3 4 5 6 7 8 9 10 11 14 16 17 Sub-total

3 6 4 4 6 4 6 6 6 6 6 6 6 6 75

3 6 4 4 5 4 4 2 2 4 4 1 6 1 50

1−

˛ (Ave.) [–]

1−

7.2 7.7 8.9 9.5 8.1

2.2 0.9 0.62 – 1.5

280 220 180 110 220

130 20 32 – 85

8.2 8.1 10.6 10.6 11.0 9.6 10.8 9.5 10.4 10.1 10.2 8.2 9.9 9.9 9.9

0.55 1.2 0.28 0.29 0.43 1.4 0.8 0.92 0.21 0.37 1.0 – 1.3 – 1.3

170 230 220 200 150 220 160 180 250 170 140 250 190 70 190

21 64 14 14 53 43 36 28 21 23 57 – 58 – 53

Q (Ave.) (kg m−2 h−1 )

2. Experimental 2.1. Fabrication method and quality evaluation by PV Ninety-nine samples of the tubular NaY zeolite membrane with 80 cm long were produced hydrothermally with two types of synthetic solution (Table 1) by a developed fabrication method using a larger scale synthesis apparatus. The details of the method and hydrothermal apparatus were reported in our previous study for a piece of tubular membrane [19]. The multiple hydrothermal furnace which is composed of six batches of this elemental apparatus was utilized for the present mass-production. Namely, a lot with six batches is a fabrication unit at once and the hydrothermal synthesis was carried out for 18 lots in this study. A secondary growth method including seeding and hydrothermal synthesis was used. The asymmetric supports (0.8 ␮m for average pore size on the surface layer and 0.4 for porosity) with 80 cm long were used. The synthetic condition was kept at 100 ◦ C and atmospheric pressure for 4–6 h (5 h for 91 samples, 4 h and 6 h for 4 samples, respectively). The membranes were recovered to be washed with water and then dried in air after the hydrothermal synthesis. A degree of reproducibility in the synthesized membranes for both types solution was investigated to determine dispersion of membrane performance represented by the standard deviation (1 − ) in the permeating flux and the separation factor measured by PV on 66 membrane samples at 75 ◦ C in a feed mixture of water(10 wt.%)/ethanol(90 wt.%) using an apparatus for elemental tubular membrane with 80 cm long. The 66 membrane samples were extracted from the all 99 membrane samples by the following criterion; (1) 1–5 samples from all lots were selected for PV measurements, (2) all 6 samples from 2 lots were evaluated for both types of solutions to investigate the dispersion within the individual lot (Table 2). The design of module for tubular membrane is shown in Fig. 1. A sealed membrane sample was set in the module and the effective membrane area was ca. 260 cm2 . The PV apparatus was equipped with a heater of 6 kW and a feed tank of 30 L to supply feed liquid with 75 ◦ C stably to the membrane module. The recirculation feed flow rate through the membrane module was held

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2.2. Vapor permeation

Fig. 1. Schematic representation of pervaporation (PV) apparatus for membranes with 80 cm long.

at 15 L min−1 . The inside of the tubular membrane was evacuated at 1–2 kPa with a vacuum pump and permeate vapor was condensed in a vessel cooled at 1 ◦ C. The feed and condensed permeate were weighted and analyzed with a gas chromatography (Shimadzu, GC14B) with equipped 3-m column packed with Polarpack Q (etylvinylbenzen–divinylbenzene filler) and with a thermal conductivity detector (TCD) to determine separation factor (˛). Temperature conditions at column and TCD were 200 ◦ C and 180 ◦ C, respectively.

The VP experiments of aqueous ethanol solution were performed with a large scale unit with 25 kW heater and 35 L feed tank. In this VP apparatus, a set of four modules in a series connection was equipped. A schematic layout of the VP unit is shown in Fig. 2. This VP apparatus has an advantage that the process of producing the final ethanol retentate by one-through dehydration in a series of membrane modules can be clarified by measurements of compositions for permeates and retentates from every membrane modules. The feed hydrous ethanol vapor was supplied from the feed vessel into the series of membrane modules with dehydrating procedure from the first module to the fourth module, and then the dehydrated vapor was converted into liquid in the condenser for return to the feed tank. The sampling points for ethanol productions at every stage are shown in Fig. 3. The inside of the tubular membrane was evacuated at 3–4 kPa with a vacuum pump. The permeate vapor from each membrane modules was collected in a trap that cooled by liquid nitrogen to determine permeation fluxes and separation factors for individual modules. The chemical analysis on the feed and permeate was carried out by same method as in PV experiments with the gas chromatography. The degree of experimental error is estimated to be within 10% of the determined values from every single membrane in a series of four membranes by the repeated experiments at same experimental conditions. The primary operating parameters in the present dehydration are the feed temperature and the feed flow rate. These two parameters influence on the membrane performance of permeation flux and separating behavior. The relations of membrane performance and operating parameters are required for processing design of practical separations. The feed temperatures from 90 ◦ C

Fig. 2. Schematic diagrams for the double-pipe type membrane module (a) and for vapor permeation apparatus (b).

K. Sato et al. / Journal of Membrane Science 319 (2008) 244–255

247

Fig. 3. The sampling points in a series of membrane modules in the vapor permeation apparatus. The ‘SPn’ indicates sampling points for hydrous ethanol samples from the feed and the final product.

to 110 ◦ C and the 2–4 conditions at 5 kg h−1 , 10 kg h−1 , 15 kg h−1 and 20 kg h−1 in the flow rate of hydrous ethanol vapor through the membrane modules were applied. The upper values of the flow rate for 10 kg h−1 , 15 kg h−1 and 20 kg h−1 at 90 ◦ C, 100 ◦ C and 110 ◦ C, respectively, were given by its maximum capacity of the vaporizer. The flow rate of vapor through a series of membranes is represented by the product rate (kg h−1 ) because the weight of condensed ethanol product was measured. These production rates of 5 kg h−1 , 10 kg h−1 , 15 kg h−1 and 20 kg h−1 are processed from the feed rates of hydrous vapor ethanol with 10 wt.% water content of 5.5 kg h−1 , 11 kg h−1 , 16.5 kg h−1 and 22 kg h−1 , respectively. The supplementary VP experiments on the present type NaY zeolite membrane with effective membrane area of ca. 4 cm2 in the feed of pure water component to determine water permeation flux in a state of no influence of concentration polarization on the membrane surface with using a VP apparatus which was reported in our previous study [18]. 3. Theory

p

ponent of i between the feed (pfi ) and the permeate (pi ) across the membrane. The difference of vapor pressure across the membrane is used for description of the driving force because the permselective behavior in VP is investigated in this study. The permeance (˘ i ) for component of i is results of four parameters of permeability (Fi ), adsorption coefficient (Si ), diffusivity (Di ) and membrane thickness (L) described as: ˘i =

Fi SD = i i L L

The influence of membrane thickness (L) on the relation of permeation flux (Ji ) and driving force is expressed by Eq. (6) from Eq. (4) and Eq. (5); Ji =

Si Di f p (Pi − Pi ) L

 Di = D0 exp

The membrane performance is described by permeation flux J (kg m−2 h−1 ) and separation factor (˛), defined as follows:

×permeating time (h)

˛a−b =

(1)

J = J0 exp

(7)

RT −HiS

 (8)

p

Ei

 (9)

RT

p

where Ca and Cb are the weight fraction of components a and b, respectively. The effect of feed temperature (T) on the permeation flux (J) is described by activation energy for permeation flux (EJ ) through the zeolite membrane is calculated by Eq. (3): EJ RT



RT



(2)

[Ca /Cb ]feed

−EiD

Thus, the following relation results Fi = F0 exp

[Ca /Cb ]permeate



 Si = S0 exp

weight of permeate (kg) membrane area (m2 )

(6)

The adsorption coefficient (Si ) and the diffusivity coefficient (Di ) are normally dependent on temperature and the temperature dependency can be expressed as:

3.1. Temperature effect on permeating behavior

Permeation flux J(kg m−2 h−1 ) =

(5)



(3)

where J0 is the pre-exponential factor and R is the gas constant. The permeation behavior through the zeolite membrane is represented according to the solution-diffusion model (e.g., [27–30]) by a relationship between the permeation flux and partial pressure difference. The relation is described by Eq. (4): p

Ji = ˘i P = ˘i (Pif − Pi )

(4)

where Ji is the permeation flux for component i, ˘ i the permeance for component i, P is the difference of vapor pressure for com-

where Ei is the activation energy of permeability. The relation of the activation energy of diffusion (EiD ) and the enthalpy of adsorption (HiS ) of component i is described by Eq. (10); p

Ei = EiD + HiS

(10) p

It is noted that Ei is the activation energy for both permeability and permeance for a given membrane sample, presented by the Eq. (11); ˘i =

J Fi = i = L P

P  0

L

 exp

p

−Ei RT

 (11)

where P is the translayer pressure difference. The activation energy for permeability or permeances can characterize the temperature effect on the permeability dynamics of the zeolite layer represented by adsorption and diffusion [28].

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K. Sato et al. / Journal of Membrane Science 319 (2008) 244–255

support is equal to the total flux through the whole membrane as described by Eq. (13). 3.3. Flow condition in the tubular membrane module The Reynolds number (NRe ) which is a representative for a state of turbulence in membrane modules is described by the following equation: NRe =

De V 

(18)

where De is the equivalent diameter,  the density of feed vapor, V the velocity of feed vapor,  the viscosity. The equivalent diameter for the present annulus type tubular module is D2 − D1 , which is the difference between inner diameter of module tube (D2 ) and the outer diameter of membrane tube (D1 ). 4. Results and discussion Fig. 4. Schematic representation of layers of the boundary layer, the zeolite layer and the support layer in the whole membrane and the definition of pressure at individual interfaces.

3.2. Mass flow resistances for the layers of boundary, zeolite and support The mass flow resistance can be expressed by the resistancesin-series model, in which the overall resistance of mass transfer (1/Kt ) is described as the sum of those across the individual layers of the boundary layer (1/kb ), zeolite layer (1/km ) and the support layer (1/ks ) [27] described as: 1 1 1 1 = + + Kt kbl km ks

(12)

In this study, a modified resistances-in-series model is utilized to describe the vapor pressure-driven permeation. Fig. 4 shows that the overall permeation flux through the present NaY zeolite membrane in the module of specific i is expressed for three layers of the chemical boundary layer, NaY zeolite layer and support by Eq. (13): i i i i Ji = ˘ti Pti = ˘bl Pbl = ˘M PM = ˘si Psi i (pi − pi ) = ˘ i (pi i ) = ˘ti Pti = ˘bl − PMO M MS MS f

= ˘si (piMO − pip )

(13) (14)

4.1. Mass-production of 99 tubular NaY zeolite membranes Ninety-nine samples of the tubular NaY zeolite membranes were synthesized with two types of chemical solutions and the selected 66 samples were evaluated by PV for clarification of applicability of the developed fabrication to mass-production for industrial purpose. The reproducibility of fabrication for both types by the mass-production evaluated by PV is shown in Fig. 5. The average permeation fluxes (J) and separation factors (˛) for two synthetic solutions are listed in Table 2. The synthesized NaY zeolite membranes exhibit higher membrane performance in permeation fluxes and separation factors than those reported in the previous studies of laboratory scale membranes [5–7], proving the success of NaY zeolite membrane synthesis by the present fabrication method. The average values for both types of NaY membranes are similar in separation factors (˛) with ca. 200, on the other hand, the higher permeation fluxes were found systematically in the typeb NaY zeolite membrane. Both types of samples exhibit negative correlations between the permeating flux (J) and separation factor (˛) (Fig. 5). These results indicate that the NaY membranes synthesized by the type-b solution could be appropriate for dehydration applications, because those membranes exhibit higher permeation

i ) is Therefore, the analogous overall mass flow coefficient (1/˘total described by Eqs. (4) and (14):

1 ˘ti

=

1 i ˘bl

+

1 i ˘M

+

1 ˘si

(15)

The mass flow resistance in the support layer is represented as the pressure drop over the present porous support. Our previous studies on the same porous asymmetric support showed that Knudsen flow could be flow mechanism. The pressure drop is calculated by the following equation for Knudsen diffusion: Ji =

εDKn (pMO − pf ) lRT

(16)

Here ε is the support porosity, l the support thickness and DKn the Knudsen diffusivity described by the following equation:



DKn

d = 3

2kB T M

(17)

where M is the molecular mass. The pressure drop of (pMO − pf ) across the support can be calculated by the Eq. (16), because the permeation flux through the

Fig. 5. The distribution of data from PV experiments at 75 ◦ C for synthesized NaY zeolite membranes with 80 cm long by two types of synthetic solution (; type-a, 䊉; type-b) in a feed mixture of water(10 wt.%)/ethanol(90 wt.%) with comparison of previous studies ( [6],  [7]).

K. Sato et al. / Journal of Membrane Science 319 (2008) 244–255

fluxes than those by the type-a solution for a similar separation factors (˛) around 200. Although precise experimental data were not obtained to explain the higher permeation fluxes in the type-b membrane samples than those in the type-a samples, we estimate based on a series of experimental observation in a type of NaA zeolite membrane [29] that the flux differences could be attributed to thinner zeolite layer in the type-b samples. It is noted that chemical compositions in two types membrane is similar [19], suggesting that adsorption feature should not play a significant role in the flux difference. Four tubular membrane samples synthesized by the typeb solution with performance of 9.6–11.3 kg m−2 h−1 in flux and ˛ = 110–170 in separation factor were chosen from the same lot (lot no. 7 in Table 2) for the present ethanol dehydration experiments in VP. Those performance is close to the average performance for btype NaY zeolite membranes that is relevant to samples to a feasible study of ethanol dehydration. 4.2. Ethanol dehydration in VP experiments through NaY membranes 4.2.1. Product quality of dehydrated ethanol Applicability of the present NaY zeolite membranes in the ethanol dehydration was investigated by VP at 90–110 ◦ C and the product flow rate of 5–20 kg h−1 from a mixture of water(10 wt.%)/ethanol(90 wt.%). The compositional evolutions in retentate with proceeding of dehydration through the series of membrane modules for the vapor flow rates with 5–20 kg h−1

249

Table 3 Ethanol loss ratio (%) Temperature (◦ C)

Flow rate (kg h−1 )

90 100 110

5

10

15

20

6.2 10 14

3.6 5.5 7.5

– 3.5 5.1

– – 3.5

at 100 ◦ C and 110 ◦ C (Fig. 6). At 90 ◦ C, the ethanol compositions in retentate reached to 97.6 wt.% and 96.9 wt.% at 5 kg h−1 and 10 kg h−1 in the flow rate, respectively. The results indicate that the ethanol retentate with ethanol contents of 96.1–98.5 wt.% could be obtained through the series of four membranes with effective membrane area of ca. 1050 cm2 at feed temperatures of 90–110 ◦ C in dehydration using the present NaY zeolite membranes. The retentate quality and productivity (kg h−1 ) exhibit a tradeoff relation. The ethanol concentrations in retentate are increased with decreasing of the flow rate of feed vapor (Fig. 6). These results suggest for a practical utilization of these membranes in ethanol dehydration that the smaller amount of dehydrated products should be expected when the more higher ethanol concentration products are needed. 4.2.2. Recovery ratio of ethanol in the dehydration The recovery ratio is one of the criteria to estimate ability of dehydration through membranes. The ethanol loss ratio (L# ) is defined by Eq. (19): L# (%) =

Ep × 100 ER

(19)

where ER is ethanol weight in the retentate and Ep is ethanol weight in the permeate for a given amount of feed hydrous ethanol. Namely, the lower values of ethanol ratio indicate the higher productivity by effective separation. The ethanol loss ratios (L# ) in the individual feed conditions are shown in Table 3. The values are systematically changed in that two factors of lower feed temperature and higher flow rate of hydrous vapor encourage the lower ethanol loss ratios (Table 4). 4.3. Influence of operating parameters on permeation behavior 4.3.1. Influence of the feed flow rate on membrane performance The membrane performance for the present NaY zeolite at 100 ◦ C and 110 ◦ C is represented by partial permeation fluxes for water and ethanol through the present NaY zeolite membranes (Fig. 7). The partial fluxes of water are decreased with the proceeding of dehydration at a given flow rate of feed vapor. The gradual reduction of partial water permeation fluxes through dehydration at all production rates should be caused by the decreasing of water contents in the feed vapor, as described in Eq. (4). The influence of the feed flow rate on the permeation fluxes is significantly exhibited in water permeation fluxes that are increased with increasing of the flow rate of vapor from 5 kg h−1 to 15 kg h−1 or 20 kg h−1 at both temperatures of 100 ◦ C and 110 ◦ C (Fig. 7). It is typically shown at the first stage in dehydration that partial water fluxes are Table 4 VP performance in pure water feed

Fig. 6. The compositional evolutions in ethanol retentates with the product rate of 5–20 kg h−1 ( 20 kg h−1 ,  15 kg h−1 , 䊉 10 kg h−1 ,  5 kg h−1 ) at 100 ◦ C and 110 ◦ C.

Temperature (◦ C)

PD for support Flux Jw (kg m−2 h−1 ) PSw (kPa)

w water permeance ˘M (mol m−2 s−1 Pa−1 )

100 110

47.2 80.0

7.8 × 10−6 1.1 × 10−5

12 20

w Psw : PD for support: pressure drop across the support layer; ˘M : water permeance across the zeolite layer.

250

K. Sato et al. / Journal of Membrane Science 319 (2008) 244–255

Fig. 7. The partial fluxes of water (solid symbols) and ethanol (open symbols) at individual modules through ethanol dehydration at 100 ◦ C and 110 ◦ C with variation of the feed flow rate (♦ 20 kg h−1 ,  15 kg h−1 , 䊉10 kg h−1 ,  5 kg h−1 ).

increased from 10.5 kg m−2 h−1 to 23.2 kg m−2 h−1 with increasing the flow rate of vapor from 5 kg h−1 to 20 kg h−1 at 110 ◦ C (Fig. 7b). Fig. 8 shows the relations between the water permeation fluxes and the Reynolds number in the feed vapor at 90–110 ◦ C for the

Fig. 8. The relation between partial water flux and the Reynolds number at a feed mixture of water(10 wt.%)/ethanol(90 wt.%) in vapor permeation experiment at 90 ◦ C (), 100 ◦ C (䊉) and 110 ◦ C ().

same feed composition of water (10 wt.%)/ethanol(90 wt.%). The water fluxes are increased with increasing of the Reynolds number at all three temperatures, indicating that the flow and turbulent conditions with the Reynolds number up to 20,000 at 110 ◦ C were not sufficient to eliminate completely concentration polarization on the membrane surface. This could be explained by that the present NaY membranes exhibit so high permeation property that concentration polarization could not be excluded even by the higher feed flow rates of the Reynolds number with 20,000. These results suggest that the higher flow rates over 20 kg h−1 per one elemental tubular membrane are required for the present membranes to exploit their maximum membrane performance. The significant influence of the flow rate of feed vapor on the partial ethanol fluxes was not found in results that the similar values for the partial ethanol fluxes at a given stage for the range of feed rate of vapor from 5 kg h−1 to 20 kg h−1 at individual feed temperatures of 100 ◦ C and 110 ◦ C. The influence of the feed flow rate on separation factors are shown in Fig. 10 that separation factors are decreased with increasing of the feed flow rate. These results can be interpreted that the lower separation factors at the lower flow rates could be attributed to lower concentrations of water in the permeates that caused by lower water fluxes due to reduced concentrations of water at the membrane surface in comparison to the feed bulk, that is the effect of concentration polarization. On the other hand, the ethanol permeating fluxes are not significantly influenced by the feed flow rates, resulting in rather constant ethanol fluxes at a given membrane module (Fig. 7). Therefore, the concentration of water in the permeates were decreased with decreasing of the feed flow rate, resulting in lower separation factors at the lower feed flow rates.

K. Sato et al. / Journal of Membrane Science 319 (2008) 244–255

251

Fig. 10. The influence of the feed flow rate ( 20 kg h−1 ,  15 kg h−1 , 䊉 10 kg h−1 ,  5 kg h−1 ) on the separation factors (˛) in a feed mixture of water(10 wt.%)/ethanol(90 wt.%) in vapor permeation at 110 ◦ C.

tion factors are decreased with the increasing of feed temperature. This could be caused by the temperature influence on the increasing of partial ethanol fluxes.

Fig. 9. Effect of temperature on total () and partial fluxes of water () and ethanol (䊉) through the NaY zeolite membrane of the results in VP data at 90–110 ◦ C with addition of those data (open symbols) in PV at 75 ◦ C.

4.3.2. Influence of temperature on membrane performance Fig. 9 shows the influence of temperature on permeation fluxes by an Arrhenius type plot (ln flux vs. 1/T) for partial fluxes of water and ethanol through the present NaY zeolite membrane in a feed mixture of water(10 wt.%)/ethanol(90 wt.%), including the results by PV at 75 ◦ C. The linear trends in the plot suggest the permeation fluxes through the NaY membrane are an activated phenomenon. The calculated values of activation energy for partial fluxes indicate that more thermally activated behavior for ethanol permeation by J the high value of activation energy for ethanol partial fluxes (Ee ). ◦ The value of activation energy for partial ethanol flux at 90–110 C in VP (49 kJ mol−1 ) is similar or slightly high to those for pure ethanol flux (34–46 kJ mol−1 ) that was determined in our previous study [16]. On the other hand, the value (22 kJ mol−1 ) of activation energy for partial fluxes of water at 90–110 ◦ C in VP is smaller than that for ethanol flux, however, it is similar to values of activation energy for partial water fluxes through an NaA zeolite membrane in VP at the same feed conditions [30]. These might imply certain of common features in permeation mechanism of water through these hydrophilic zeolite membranes. Fig. 10 shows the separation factors (˛) for every membrane module at the four production rates of 5–20 kg h−1 at 110 ◦ C. Rather constant of separation factors (˛) at a given production rate is found with an exceptional case of the production rate of 20 kg h−1 in which the factors are decreased from ˛ = 28 to ˛ = 21 with proceeding of dehydration. The separation factors (˛) are increased with increasing of the production rate. This could be caused by the elevation of water partial fluxes with increasing of the feed rate, because the ethanol partial fluxes are rather similar at every stage of membrane module for different feed rates at the constant temperature (Fig. 7). The ranges of separation factors (˛) at 90 ◦ C and 100 ◦ C through dehydration at the same production rate of 10 kg h−1 are ˛ = 24–31 and ˛ = 19–21, respectively. These results indicate that the separa-

4.3.3. Comparison of membrane performance with previously reported membranes Several groups reported performance of NaA zeolite membranes for industrial purpose in hydrous ethanol dehydration [20,31,32] at higher temperatures of 120–130 ◦ C by PV and VP (Table 5). The reported NaA zeolite membranes exhibited higher separation factors, on the other hand, the NaY zeolite membrane in this study has an advantageous property of higher permeating flux. 4.4. Permeating mechanism implied by results of VP experiments Fig. 11 shows the water vapor pressure drops across the individual layers of the boundary layer, the zeolite layer and the support layer for permeation fluxes at 110 ◦ C in an inlet feed mixture of water(10 wt.%)/ethanol(90 wt.%) with the feed rate of 5–20 kg h−1 . The external mass-flow resistances are caused by the pressure drop inside the support and that on the membrane surface. The water pressure drop across the support is calculated by Eq. (16). The pressure drop across the zeolite layer is by Eq. (14). An approach by VP in pure water permeations at 100–110 ◦ C to eliminate the influence of concentration polarization on the membrane surface was employed for determination of the water permeances across the w ). The pressure drop through the support was zeolite layer (˘M corrected by taking consideration in this pure flux permeating experiment (Table 4). In this approach to calculate the pressure drop across the zeolite layer, it is premises that the influence of ethanol w ) is negligible. The water vapor pressure on water permeance (˘M drops across the boundary layer are decreased with increasing of the feed rates. For the permeation flux at the feed rate of 5 kg h−1 , the similar values of water vapor pressure are found in those for the boundary layer and the zeolite layer, indicating that the same degree of mass-transport resistance in the two layers. These results show a significant role of boundary layer for mass-flow resistance at the lower flow rate with the lower Reynolds number. The decrease in the pressure drop across the boundary layer with increasing of the Reynolds number (NRe ) caused by elevation of the feed flow rate results in reduction of mass-flow resistance by elimination of concentration polarization in the boundary layer.

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Table 5 Comparison of membrane performance with previously reported membranes for industrial purpose in water/ethanol separation Zeolite type

Mode

Temperature (◦ C)

Flux (kg m−2 h−1 )

Water permeance (mol m−2 Pa−1 s−1 )

LTA LTA LTA FAU

PV VP VP VP

120 125 130 110

8.4 11 20 23

1.4 × 10−6 1.6 × 10−6 3.0 × 10−6 3.0 × 10−6

Fig. 11. The pressure drops across the boundary layer, the zeolite layer and the support layer with variation of the feed rate from 5 kg h−1 to 20 kg h−1 ( 20 kg h−1 ,  15 kg h−1 , 䊉 10 kg h−1 ,  5 kg h−1 ) at 110 ◦ C.

˛ 46,000 >1,000 10,000 28

Reference [31] [32] [20] This study

Fig. 12 shows the effective permeances of water (˘w ) and ethanol (˘ e ) through the total membranes at 100 ◦ C and 110 ◦ C as a function of the flow rate of feed vapor. The influence of water content in the feed and of the feed flow rate on the water permeances at 100 ◦ C is clarified by that (1) the water permeanes are decreased with proceeding of dehydration that causes decreasing of water content in the feed, (2) those are increased with the increasing of feed rate. The occurrence of water-content dependency of water permeance might be rather unexpected because the water permeance is partial water flux divided driving force which is caused by primary water content at a given temperature (Eq. (8)). Therefore, these results imply that the effective driving force for permeation flux through the zeolite layer could be decreased with decreasing of water content. A competitive or interfering behavior between water and ethanol molecules in adsorption and migration to NaY zeolite might be a reason for this water permeance behavior. It is natural to take consideration of the effect of interaction of water and ethanol into the separation behavior through NaY zeolite, because the difference of adsorption to NaY zeolite is the main reason for this separation. The molecular sieving should not be worked in this water–ethanol separation through NaY zeolite membrane because the kinetic diameters of molecules (water with 0.265 nm and ethanol with 0.430 nm) are smaller than zeolitic pore with 0.74 nm [3]. The second point can be attributed to concentration polarization on the membrane surface. In the water permeances at 110 ◦ C, the similar permeating properties are found for water and ethanol components with those

Fig. 12. The permeances of water and ethanol through the NaY zeolite membrane with proceeding of dehydration at 100 ◦ C and 110 ◦ C with variation of the feed flow rate ( 20 kg h−1 ,  15 kg h−1 , 䊉 10 kg h−1 , 5 kg h−1 ).

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253

Fig. A1. The differential element that used in calculation of permeances of water and 2-propanol.

Fig. 13. The influence of temperature on water and ethanol permeances in a feed mixture of water(10 wt.%)/ethanol(90 wt.%) through the present type-b NaY zeolite membrane with variation of the Reynolds number ( 19,000,  15,000,  9900 for water permeance, and 䊉 for ethanol permeance with the Reynolds number 9900).

at 100 ◦ C. However, the water permeances are lower than those at 100 ◦ C at the same feed vapor rates, implying that there could be temperature dependency of water permeance. A more detailed mechanism for temperature dependency is referred in a later section. From a point of view of concentration polarization on the membrane surface, the higher feed rate of feed vapor could not be sufficient for elimination of concentration polarization on the membrane surface, because there was still gap in water permeance at production rate condition between 15 kg h−1 and 20 kg h−1 . Therefore, it is concluded that the mass-flow resistance across the boundary layer could have a significant role for limitation of permeation flux at the even high Reynolds number up to 20,000. A particular influence of the feed vapor flow rate on ethanol permeanes at both temperatures of 100 ◦ C and 110 ◦ C (Fig. 7) is not observed to suggest that concentration polarization for ethanol component on the membrane surface could not be significant. The influence of feed ethanol content on ethanol permeances was also observed at both 100 ◦ C and 110 ◦ C. The ethanol permeances are decreased with proceeding of dehydration, resulting in decreasing of ethanol content in the feed. Fig. 13 shows temperature dependency of permeances of water and ethanol in VP. It is clear that the water permeances are decreased with increasing temperature, indicating there should be temperature dependency of water permeance. The activation P ) at the given flow rate of 10 kg h−1 energy for water permeance (Ew with the Reynolds number of 20,000 is determined by Eq. (11) to be −39 kJ mol−1 . The negative activation energy for water permeance is confirmed even if the water permeance at a flow rate of 20 kg h−1 and 110 ◦ C is employed to take consideration of lower concentration polarization on the membrane surface.

The negative activation for water permeance with −21 kJ mol−1 was also observed in an NaA zeolite membrane [19]. This negative value for the NaA zeolite membrane was interpreted to be results from contributions of the activation of diffusivity (ED ) and adsorption enthalpy (HS ), because the activation energy for permeance is the sum of those two terms described by EP = ED + HS (Eq. (10)). The both zeolite membranes of NaY and NaA have hydrophilic characteristic but it has been reported that the amount of water adsorption is decreased with increasing temperature [33]. Namely, the hydrophilic property could be gradually lost at higher feed temperatures. These results show that the driving force of pressure difference of water across the membrane is not the only constant parameter controlling the water permeance. The results that the water permeances are not always constant in this study show that water permeances depend on the feed and temperature conditions. On the other hand, the trend for ethanol permeances is rather constant. The ethanol permeances with 1.8–2.2 × 10−7 mol m−2 s−1 Pa−1 are similar values of ethanol permeances in the pure ethanol VP experiments at the same feed

Fig. A2. A case for the calculated water (the above) and ethanol (the below) vapor pressures and partial water and ethanol permeation fluxes at any position in the 70 cm (effective length) long membrane at 110 ◦ C and in a inlet feed mixture of water(10 wt.%)/ethanol(90 wt.%).

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temperatures through the present type of NaY zeolite membrane [19]. These results suggest that ethanol permeations could not be influenced significantly by the permeating water component. The driving force of pressure difference of ethanol across the membrane is the primary factor determining the permeation behavior of ethanol component through the present NaY membranes at 90–110 ◦ C. 5. Conclusion Ninety-nine samples of tubular NaY zeolite membranes in an industrial scale of 80 cm long were fabricated by the application of a developed fabrication method using with two types of synthetic solutions. The degree of reproducibility by this fabrication method was evaluated by PV experiments at 75 ◦ C in a feed mixture of water(10 wt.%)/ethanol(90 wt.%). The different trends of membrane performance for two types of synthetic solutions were observed to suggest that the NaY zeolite membranes synthesized in the solution with Al2 O3 :SiO2 :Na2 O:H2 O = 1:10:14:800 could be effective to dehydration applications because of its higher water permeation fluxes. The four samples of synthesized NaY zeolite membranes in a series were applied to dehydration of aqueous ethanol with 10 wt.% water content in vapor permeation at 90–110 ◦ C at different production rates of 5–20 kg h−1 . The ethanol products up to 98.5 wt.% could be purified by the present NaY zeolite membranes with effective membrane area of ca. 1040 cm2 . These results demonstrate that the present NaY zeolite membranes could be applied to dehydration of aqueous ethanol. It is found that concentration polarization might occur on the membrane surface by the high permeation flux through the present NaY membrane. The permeation fluxes are increased with increasing of feed flow rate or the Reynolds number. It is found that concentration polarization could not be excluded even by high Reynolds number with 20,000 at 110 ◦ C. Appendix A The permeances of water and ethanol are determined in this study based on the adsorption–diffusion model to clarify the influence of operating parameters on the membrane performance. The calculation of permeances for membrane with 80 cm long is carried out with the following permeating equations for mass flow through the membrane. It is noted that in this model for calculation we have the premises of (1) the permeance is considered to be constant in the range of 80 cm long membrane and (2) the partial vapor pressure in horizontal direction in the permeate side is constant. The total pressure through the membrane module (PF ) is confirmed to be constant by measurement with the gauges. The following Eq. (A-1) describes the relation of mass balance for water component within the permeation flux, the mass flow difference between inlet and outlet, and the integrated permeation flux through the whole the membrane (Fig. A1);

 Jw A = Win − Wout = 0

A

˘w pw dA k

(A-1)

On the other hand, the differential equations for the mass flow balance in an infinite region of k;



−dW = ˘w PF

  ¯

Wk − PP Wk + Ek  ¯ + ε¯

dA

(A-2)

where dW is the water mass flow through the area of dA in the membrane, w the water permeance, PF the total feed pressure, Wk and Ek the mass flow rate (mol s−1 ) in a finite region of k for

water and ethanol component, respectively, PP the total pressure in the permeate side, ω, ¯ ε¯ the mol fractions of water and ethanol in the sampled permeate, respectively. The permeating equation for mass flow of ethanol component is described as:



−dS = ˘S PF

Ek ε¯ − PP Wk + Ek  ¯ + ε¯

 dA

(A-3)

The permeances are determined by calculation solving the simultaneous differential equations of ((A-2) and (A-3)) using the numerical simulation. Representative results for calculated distribution of partial vapor pressures and partial permeation fluxes from a case of feed mixture of water(10 wt.%)/ethanol(90 wt.%) at 110 ◦ C are shown in Fig. A2.

Nomenclature Ci d De Di DKn J Ei p

Ei Fi HS Ji k, K kB l, L L# NRe Pif p Pi P R S T V

concentration of component of i pore diameter (m) equivalent diameter (m) diffusivity for component of i Knudsen diffusivity (m2 s−1 ) activation energy for permeation flux of component i activation energy for permeance of component i permeability for component of i (mol m−1 s−1 Pa−1 ) enthalpy of adsorption (kJ mol−1 ) permeation flux of component i (kg m−2 h−1 ) mass transfer coefficient (mol m−2 s−1 Pa−1 ) Boltzmann’s constant (1.38 × 10−16 g cm2 s−2 K−1 ) layer thickness (m) ethanol loss ratio Reynolds number vapor pressure of component i in feed (kPa) vapor pressure of component i in permeate (kPa) pressure drop (kPa) gas constant (kJ mol−1 K−1 ) adsorption coefficient (mol m−3 Pa−1 ) temperature in Kelvin (K) velocity of feed vapor

Greek letters ˛ separation factor ε porosity  viscosity (Pa s) ˘i permeance for component of i (mol m−2 s−1 Pa−1 ) Subscripts b boundary layer e ethanol M zeolite layer MO interface between zeolite and support layers MS interface between zeolite and boundary layers s support layer t total w water Superscripts D diffusion p permeance S sorption w water

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