Pervaporation and vapor permeation dehydration of Fischer–Tropsch mixed-alcohols by LTA zeolite membranes

Pervaporation and vapor permeation dehydration of Fischer–Tropsch mixed-alcohols by LTA zeolite membranes

Separation and Purification Technology 57 (2007) 140–146 Pervaporation and vapor permeation dehydration of Fischer–Tropsch mixed-alcohols by LTA zeol...

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Separation and Purification Technology 57 (2007) 140–146

Pervaporation and vapor permeation dehydration of Fischer–Tropsch mixed-alcohols by LTA zeolite membranes Yanshuo Li, Hongliang Chen, Jie Liu, Hongbo Li, Weishen Yang ∗ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Received 11 September 2006; received in revised form 13 February 2007; accepted 26 March 2007

Abstract Aiming at the applications in Fischer–Tropsch synthesis of mixed-alcohols, microwave synthesized LTA zeolite membranes were applied to pervaporation and vapor permeation dehydration of unitary and mixed alcohols. The water to methanol separation factor was found to lie between 150 and 1000. Both water to ethanol and water to propanol separation factors were observed to be above 10,000 over a wide range of solvent concentration. It was found that the water flux was only depended on its own driving force, i.e. the pressure difference of water across the membrane. High separation factors were also obtained by vapor permeation for ethanol and isopropanol systems. At the liquid–vapor equilibrium state, the water fluxes of pervaporation and vapor permeation showed no obvious difference. Two different types of F-T mixed-alcohols were dehydrated by pervaporation and vapor permeation, respectively. One is simulated mixed-alcohols based on IFP technique, and the other is actual F-T liquid produced on Rh-based catalysts. The separation factor of the former was similar to that of methanol system, and the water flux was between that of methanol and ethanol. This is coincided with the fact that the mixed-alcohols are mainly composed of methanol and ethanol. As for the latter, good separation performance was obtained at the initial stage. However, the long chain hydrocarbon impurities in the feed fouled the membrane during the operation, which resulted in an obvious decrement of the permeate flux after 3 h of operation. © 2007 Elsevier B.V. All rights reserved. Keywords: LTA zeolite membranes; Pervaporation; Vapor permeation; Fischer–Tropsch; Mixed-alcohols

1. Introduction At the beginning of the 21st century, along with the coming of the peaking of world oil production, oil production from conventional reservoirs begin to decline, creating a gap between supply and demand. Alternative energy sources are needed to fill the gap. Among which, coal and natural gas, which have already played an important role in electricity generation, industrial manufacture and residential utilization, are expected to be the dominate choices because of its relative abundant reserves and mature infrastructure. In addition, with much attention paid to the protection of public health and the environment, environmental friendly liquid fuels based on natural gas and coal are desired when gasoline and diesel fuel are substituted by them. As the times of require, Fischer–Tropsch (F-T) synthesis, an “old” technology that began in 1920s, staged a comeback.



Corresponding author. Tel.: +86 411 84379073; fax: +86 411 84694447. E-mail address: [email protected] (W. Yang). URL: http://yanggroup.dicp.ac.cn (W. Yang).

1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.03.027

Coal-to-liquids (CTL) and gas-to-liquids (GTL) process can realize the conversion of coal and natural gas to high value liquid fuels. Besides producing mixed-hydrocarbons (ultra-clean diesel), F-T process can also selectively produce mixed-alcohols (oxygenated fuel) [1–4]. The addition of mixed-alcohols into gasoline can effectively reduce HC and CO emissions. At present, the raw products of industrial F-T process (e.g. MAS technique) contain about 5–20 wt.% of water. Cyclohexane extraction method is often used to dehydration of the product, which is low-efficiency and non-environmental friendly. Pervaporation (PV) and vapor permeation (VP) are attractive because the energy consumption is low, and the separation efficiency is not determined by the relative volatility, and the applicability for multi-component dehydration. For the dehydration of methanol, conventional PVA based hydrophilic polymeric membranes show PV separation factors of only about 2–3. This makes them inapplicable to the dehydration of F-T produced mixed-alcohols, which contain considerable amount of methanol. Since the end of last century, zeolite membrane has drawn much attention in the field of gas and liquid separation [5,6], among which, LTA zeolite membranes, due to

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their hydrophilic properties, have been used for the PV (VP) dehydration of various organics [7,8]. They exhibit high separation factors for many alcohol–water systems, even for the dehydration of methanol [9], which shows a potentiality for the integration of PV (VP) units to the downstream process of F-T synthesis, and a promising option to integration of reaction and separation. Espinoza et al. investigated the potential of silicalite-1/ZSM5 and Mordenite membranes to separate water from F-T synthesis components [10]. A high selectivity toward water over hydrocarbons was found. Aiming at in situ removal of water from F-T reactor units to enhance the reactor efficiency and protect catalyst deactivation, Zhu et al. applied TiO2 -supported LTA zeolite membranes for the water separation from permanent gases, including CO, H2 , and CH4 [11]. Unary and binary permeation were investigated. The permeance of water in the binary mixture is about two orders of magnitude larger than that of CO, H2 , and CH4 . This is ascribed to the weak adsorption affinity of these permanent gases on LTA zeolite, but the high adsorption affinity and capacity of water inside the LTA zeolite. The latter prevents the bypassing of the former through the membrane. Microwave heating is the one of the lasted development in zeolite membrane synthesis [12–14]. Recently, high quality LTA zeolite membranes were microwave synthesized on ␣-alumina supports with high reproducibility in our group [15,16], which exhibits prospective industrial applications. Aiming at the applications in mixed-alcohols production by F-T synthesis, PV dehydration of methanol, ethanol, propanol, and simulated FT mixed-alcohols was investigated. VP dehydration of actual

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F-T products was also carried out to test the performance and stability of the as-synthesized LTA zeolite membrane. 2. Experimental 2.1. LTA zeolite membrane preparation LTA zeolite membranes were prepared according to our reported method (“In situ aging—Microwave synthesis” method, AM method) [15]. Home-made porous ␣-Al2 O3 tubes (self-made, 14 mm outside diameter, 9 mm inside diameter, ca. 0.3 ␮m pore radius, and ca. 40% porosity) were used as the supports. 2.2. Characterization The texture and thickness of the as-synthesized membranes were examined by scanning electron microscopy (SEM, JEM1200E scanning electron microscope operating at 40 kV). The membrane after vapor permeation experiments was examined using scanning electron microscopy (SEM, Philips, XL-30), and energy dispersive analysis (EDS, EDAX-Phoenix) was performed to determine the composition of the deposits on the support surface. X-ray diffraction (XRD) patterns were recorded on Rigaku Miniflex. X-ray photoelectron spectroscopic (XPS, VG Esca Labmk-II, VG Scientific Ltd., Al K␣ radiation, hν = 1486.6 eV) was performed to determine the Si/Al ratio of the as-synthesized membranes.

Fig. 1. Schematic layout of (a) the experimental apparatuses for pervaporation and (b) the vapor permeation: 1, water bath; 2, feed tank; 3, feed pump; 4, membrane module; 5, permeate collector; 6, triple value; 7, buffer tank; 8, vacuum pump; 9, microfeeder; 10, vaporizer; 11, heat tape; 12, condenser.

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2.3. Pervaporation experiments The pervaporation (PV) measurements were carried out using a standard pervaporation apparatus, as illustrated schematically in Fig. 1a. Tubular LTA zeolite membranes were sealed in place with silicone O-rings. PV experiments were carried out on alcohol (including methanol, ethanol, propanol, and simulated F-T mixed-alcohols)/water mixtures at temperature of 65 ◦ C. The effective membrane area was ca. 45 cm2 and the permeation side was kept under vacuum. The permeation flux was measured by weighing the condensed permeate collected every 30 min. Permeate and feed concentrations were measured by off-line GC. 2.4. Vapor permeation experiments The apparatus for vapor permeation (VP) experiments is illustrated schematically in Fig. 1b. Tubular LTA zeolite membranes were sealed in place with flurorubber O-rings. VP experiments were carried out on 90 wt.% ethanol, 90 wt.% isopropanol, and actual F-T products. The F-T products were obtained from Shanghai Wujing Chemical Plant. Before VP experiments, the raw F-T products were simply filtered through filter papers to remove the suspended impurities. The effective membrane area was ca. 17 cm2 . The feed liquid (100 ml) was vaporized by a superheated oven (130 ◦ C) at a flow rate of 2 ml/min. The feed vapor pressure was controlled at 0.1 MPa and the permeation side was kept under vacuum. The temperature of the membrane module was held at 130 ◦ C by heat tapes. The permeation flux was measured by weighing the condensed permeate collected every 30 min. Permeate and feed concentrations were measured by off-line GC. 3. Results and discussion 3.1. LTA zeolite membrane synthesis Fig. 2a shows the XRD pattern of the as-synthesized membrane. Only the peaks of LTA zeolite and alumina support were detected. This indicates that through AM method LTA zeolite membrane can be microwave synthesized without adding seeds to facilitate it, and no phase transforms occurred during the synthesis period although without adding seeds to inhibit them. The SEM images of the as-synthesized membranes are shown in Fig. 2b and c. It can be seen that after AM synthesis, a continuous zeolite layer was formed on the support surface. The grain boundaries are hardly to be distinguished. The thickness of the zeolite top layer of the as-synthesized membrane was about 5 ␮m, as estimated from the cross-section SEM image (Fig. 2c). From the cross section views, it can be seen that the pores of the support were occupied with reaction products, which resulted in an unclear interface between the external product layer and the support. This can result in a better mechanical strength of the obtained membrane. It is interesting to note that the as-synthesized LTA zeolite membrane had a Si/Al ratio of 1.43 as determined by XPS characterization, which is higher than the common value of LTA

Fig. 2. (a) XRD pattern, (b) SEM top view, and (c) cross-sectional view of the as-synthesized LTA.

zeolite. Other authors have also reported that microwave synthesized zeolite possessed a higher Si/Al ratio compared with that obtained under conventional heating [17]. The reason why microwave heating synthesis results in products with higher

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Fig. 3. PV performance of the as-synthesized LTA zeolite membrane for dehydration of methanol at 65 ◦ C.

Si/Al ratio is not clear now. Nevertheless, this higher Si/Al ratio may be related to a better acid resistant ability [18]. 3.2. Alcohol/water binary mixture pervaporation Fig. 3 shows the PV performance of the as-synthesized LTA zeolite membrane for dehydration of methanol at 65 ◦ C. The concentration of water in the permeate (WP ) and the flux of water/methanol through the membrane are plotted as functions of concentration of water in the feed (WF ). As WF decreased from 30 to 5.0 wt.%, WP maintained above 95 wt.%. When WF was below 3.0 wt.%, WP was still above 90 wt.%, which was corresponding to a separation factor above 250. As WF decreased from 30 to 2.5 wt.%, the water flux for the system was observed to decrease nearly linearly from 0.72 to 0.07 kg/m2 h, while the finite methanol flux fluctuated around 0.01 kg/m2 h. Fig. 4 shows the PV performance of the as-synthesized LTA zeolite membrane for dehydration of ethanol at 65 ◦ C. In the whole range of WF , WP maintained very high, i.e. 100 wt.% (below the detection limit of GC), except the point of 1.5 wt.%,

Fig. 4. PV performance of the as-synthesized LTA zeolite membrane for dehydration of ethanol at 65 ◦ C.

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Fig. 5. PV performance of the as-synthesized LTA zeolite membrane for dehydration of propanol at 65 ◦ C.

where WP decreased to 99.1 wt.% (the corresponding separation factor is still above 7000). The water flux decreased slowly initially and then processed a remarkable decrease when WF was below 20 wt.%. The ethanol flux is negligible in the whole range of WF . Fig. 5 shows the PV performance of the as-synthesized LTA zeolite membrane for dehydration of propanol at 65 ◦ C. It can be seen that both the flux trend and WP trend are similar to that of ethanol–water system. This indicates that the pervaporation processes are basically similar for these two alcohol–water systems. Wijmans and Baker developed a simple predictive treatment of the permeation process in pervaporation [19]. According to their model, the partial pressure difference cross the membrane is the driving force for permeation. As for zeolite membrane, pervaporation flux was also reported by several authors to be correlated with feed fugacity [20,21]. In Fig. 6, the water flux is plotted against the feed water driving force, i.e. the water fugac-

Fig. 6. Variation of water flux with feed water driving force for various alcohol–water mixtures at 65 ◦ C.

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Table 1 Typical compositions of the F-T mixed-alcohols (IFP technique) and the composition of the simulated mixed-alcohols No.

Water

Methanol

Ethanol

Propanol

Butanol

Amyl alcohol

DME

1# F-T product 2# F-T product Simulated mixed-alcohols

5.55a

51.60 62.83 60.00

26.30 20.45 22.00

11.92 9.41 10.00

3.91 3.07 –

0.37 0.18 –

5.90 4.06 –

a

8.39 8.00

Mole percentage.

ity in the feed (here, we neglect the permeate fugacity). It can be seen that the water flux through the membrane varies linearly with the driving force. In other word, the water permeability of the LTA zeolite membrane is a constant, which indicates that there are no swelling effects for zeolite membrane [9].

course of experiments, thus the parts of the membrane near the O-ring sealed ends were rubbed out in certain degree. Nevertheless, the separation factor was larger than 150 in the whole range of WF for the dehydration of mixed-alcohols. 3.4. Vapor permeation dehydration of actual F-T products

3.3. Pervaporation dehydration of simulated F-T mixed-alcohols Two typical compositions of the F-T mixed-alcohols on Cu–Co catalyst (IFP technique) are listed in Table 1. These data were provided from State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS. Base on our preliminary studies, for the PV dehydration of butanol and amyl alcohol, the permeate was only water. Besides, the butanol, amyl alcohol, and DME concentrations in the feed were relatively small. Therefore, a mixed-alcohols consisting of water, methanol, ethanol and propanol was used to simulate the F-T mixed-alcohols. The mole fractions of each component are listed in Table 1. Fig. 7 shows the PV performance of the as-synthesized LTA zeolite membrane for dehydration of mixed-alcohols at 65 ◦ C. The water flux of the mixed-alcohol was between that of methanol/water mixture and ethanol/water mixture. This is coincided with the fact that the mixed-alcohols are mainly composed of methanol and ethanol. As WF decreased from 9.0 to 0.8 wt.%, WP varied from 94 to 72 wt.%, and the water flux decreased from 0.44 to 0.06 kg/m2 h. The methanol flux fluctuated around 0.03 kg/m2 h, which is larger than that of the methanol–water system. The reason is probably that the membrane module was assembled and unassembled for several times during the whole

In the above section, PV dehydration of simulated F-T mixed-alcohols by LTA zeolite membranes was discussed. The composition of the simulated F-T mixed-alcohols was simple, which contains only methanol, ethanol, propanol, and water. As for actual industrial F-T products, however, the compositions are complex. In present study, F-T products which were produced on Rh-based catalysts [4] were obtained from Shanghai Wujing Chemical Plant. The composition of the F-T products is list in Table 2. It can be seen that this F-T product is rich in ethanol and contains 2.0 wt.% of acetic acid. The pH value of this liquid is about 5–6. Commonly, at pH values below 6, the zeolite layer of LTA zeolite membranes will be irreversibly leached from the supports, which limited their applications even in weak acidic conditions. As described in membrane synthesis section, the LTA zeolite membrane synthesized by AM method processed a Si/Al ratio of 1.43. This relatively higher Si/Al ratio may expand the application of this membrane to the weak acidic condition, e.g. the F-T products used in current study. In consideration of that VP operation is applicable to the in situ removal of water from reactor units and thus integrates reaction and separation together. Therefore, VP dehydration of the F-T products was investigated in current study. At first, VP experiments were carried out to evaluated the performance of the AM synthesized LTA zeolite membrane for the dehydration of ethanol and isopropanol. The results are shown in Table 3. The PV results are also given for comparison. It can be seen that no matter in the PV or VP operation, the separation factors toward water are always very high (>10,000). At the liquid–vapor equilibrium state (e.g. 90 wt.% isopropanol, 80 ◦ C, 1 atm), there is no significant difference between the Table 2 Composition of the actual F-T product (Rh-based catalysts)

Fig. 7. PV performance of the as-synthesized LTA zeolite membrane for dehydration of the simulated mixed-alcohols at 65 ◦ C.

Component

Concentration (wt.%)

Water Methanol Ethanol n-Propanol i-Propanol Butanol Acetic acid

47.2 3.8 38.6 2.1 2.4 3.8 2.0

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Table 3 PV and VP performances of the as-synthesized LTA zeolite membrane for the dehydration of ethanol and isopropanol Feed

90 wt.% ethanol Temperature

PV VP a

65 110

(◦ C)

90 wt.% i-propanol Separation factor

Total flux

>10,000 >10,000

0.52 1.71

(kg/m2

h)

Temperature (◦ C)

Separation factor

Total flux (kg/m2 h)

80 80a

>10,000 >10,000

1.16 1.00

VP experiment at 80 ◦ C was carried out by putting the membrane in the vapor phase which was equilibrated with 90 wt.% i-propanol solution at 80 ◦ C.

water flux of PV and that of VP (e.g. 1.16 kg/m2 h for PV and 1.00 kg/m2 h for VP, respectively). As the operation temperature increased (e.g. VP dehydration of ethanol at 110 ◦ C), water flux increased remarkably. Water flux of 1.71 kg/m2 h for VP dehydration of 90 wt.% ethanol was obtained. Water molecular transport through the zeolite channel is an activated diffusion process, and the diffusion coefficient increases with temperature. Therefore, in a certain range of temperature, the water flux increases with temperature. The VP experiment for dehydration of actual F-T products was carried out at 130 ◦ C (at this temperature, all the main components of the F-T product can be vaporized) with an original feed volume of 100 ml. The result is shown in Fig. 8. Due to the large water content in the F-T products (47.2 wt.%), the initial flux was quite large, i.e. 3.36 kg/m2 h. As the dehydration progressed, the water concentration in the feed decreased, and the ethanol concentration in the feed increased accordingly. After 11 h of VP dehydration, the water concentration in the feed decreased to 21.8 wt.%, and the ethanol concentration increased to 59.2 wt.%. At the same time, the permeate flux declined, especially after 3 h of operation. In the whole course of VP experiment, the permeate contained only water. This indicates that the zeolite membrane was not destroyed by acetic acid, even at the last stage of operation, when the acetic acid concentration in the feed was increased to 3.7 wt.%. The increased acid resistance ability of the LTA zeolite membrane

Fig. 8. VP performance of LTA zeolite membrane for the dehydration of 100 ml actual F-T product at 130 ◦ C. The inset shows the water flux and the water concentration in the permeate as functions of operation time.

Fig. 9. (a) Optics photogram, (b) SEM top view, and (c) cross-sectional view of the LTA zeolite membrane after 11 h of VP dehydration of the actual F-T product.

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may be attributed to the increased Si/Al ratio as a result of AM synthesis. Further investigation on this aspect is in progress. It should be noted that the water flux (total flux) decreased sharply from 3.20 kg/m2 h (after 3 h of operation) to 0.44 kg/m2 h (after 11 h of operation). As compared with the value of VP dehydration of 90 wt.% ethanol (1.71 kg/m2 h at 110 ◦ C, as shown in Table 3), the water flux of 0.44 kg/m2 h at the WF of 21.8 wt.% was too small, which indicates that the membrane might have been fouled during the VP experiment. After 11 h of VP operation, the membrane module was unassembled, and it was seen that some yellow contamination deposited on the membrane, especially near the inlet (Fig. 9a). Fig. 9b shows the SEM top view image of the membrane after VP experiment. It can be seen that some spongy deposits covered the support surface, which contained carbon atoms as confirmed by EDS. The cracks appearing in the zeolite layer should be formed during the sample preparation. From the SEM crosssectional view (Fig. 9c), it can be seen that the membrane was still very compact after VP experiment. It should be note that the different membrane thickness between Figs. 2 and 9 should be originated from the difference between two samples but not due to the degeneration of the membrane during experiment. The yellow contamination, which was most likely to be the longchain hydrocarbon that deposited from the feed (oleic impurities can be seen in the F-T product), can be eliminated by calcination at 500 ◦ C. However, the membrane after calcination showed an obviously decreased separation performance. This implies that the membrane was destructed in certain degree during the calcination regeneration. The over-speedy heating rate (2.5 ◦ C/min) might be the reason. Therefore, fouling prevention techniques and effective regeneration methods are strongly demanded before these zeolite membranes can get actual application in F-T process. 4. Conclusions Through in situ aging-microwave synthesis (AM) method, high quality LTA zeolite membranes were microwave synthesized without seeding. The obtained membrane process a higher Si/Al ratio (1.43) than the conventional synthesized ones. PV dehydration of methanol, ethanol, propanol, and simulated F-T mixed-alcohols were investigated. Very high selectivities were obtained over wide range of concentration for all the above systems. The water flux was only depended on its own driving force, i.e. the pressure difference of water across the membrane. As for simulated mixed-alcohols (IFP technique), over the entire range of feed concentration, the permeate contained only water and little methanol. The water flux was between that of methanol and ethanol. The as-synthesized LTA zeolite membrane showed definite acid resistance during the VP dehydration of actual F-T products (produced by Rh-based catalysts), which is probably due to its higher Si/Al ratio. The long chain hydrocarbon impurities in the feed fouled the membrane during the operation, which resulted in an obvious decrement of flux. Simple calcination was proved not to be a suitable regeneration method, which would destruct the membrane irreversibly.

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