Zeolite-Coated Ceramic Pervaporation Membranes - ACS Publications

Jun 9, 2005 - Department of Chemical Engineering and Chemistry, Process Development Group, Eindhoven University of Technology, P.O. Box 513, 5600 MB E...
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Ind. Eng. Chem. Res. 2005, 44, 9490-9496

Zeolite-Coated Ceramic Pervaporation Membranes; Pervaporation-Esterification Coupling and Reactor Evaluation Thijs A. Peters, Nieck E. Benes,* and Jos T. F. Keurentjes Department of Chemical Engineering and Chemistry, Process Development Group, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Pervaporation is a promising option to enhance conversion of reversible condensation reactions, generating water as a byproduct. The present study aims to develop a continuous composite catalytic pervaporation membrane reactor, as integration of reaction and separation offers advantages in terms of process efficiency and compactness. Composite catalytic membranes have been prepared by applying a zeolite coating on top of ceramic hollow fiber silica membranes. This approach allows independent optimization of the selective and catalytic properties. The performance of the composite catalytic membrane is examined in the esterification reaction between acetic acid and butanol. Additionally, a preliminary large-scale composite membrane reactor evaluation is carried out based on the obtained experimental data (e.g., membrane permeability and catalyst activity). In the pervaporation-assisted esterification reaction, the catalytic membrane is able to couple catalytic activity and water removal. A computational reactor evaluation proved that the outlet conversion for the catalytic pervaporation-assisted esterification reaction exceeds the conversion of a conventional inert pervaporation membrane reactor, with the same loading of catalyst dispersed in the liquid bulk. This shows the potential added value of such a membrane system as compared to more common reactor designs. Introduction Pervaporation is a promising option to enhance conversion of reversible condensation reactions in which water is formed as byproduct. In pervaporation-esterification coupling, one side of a membrane is exposed to the liquid phase, while at the other side of the membrane vacuum is maintained. Water permeates preferentially through the membrane and evaporates at the low-pressure permeate side. The selective removal of water from the liquid drives the esterification reaction toward the product side. Beneficial aspects of pervaporation include a low energy consumption and the possibility to carry out the esterification reaction at a selected temperature. Moreover, the separation efficiency in pervaporation is not determined by the relative volatility, as in reactive distillation.1 Several authors have used membranes in which the selective layer itself is catalytically active and have shown that equilibrium displacement can be enhanced by close integration of production and removal of water.2-4 The integration of the selective and catalytic function into one single layer, however, demands contradicting material properties. For example, to achieve a high selectivity the diffusion of products inside the material should be low, whereas efficient use of the catalytic properties requires the diffusion of products to be high. The conflicting demands on materials properties can be avoided by accommodating the selective and catalytic features in two different distinct layers, which are in close physical contact.5-7 This approach allows independent optimization of the selective and the catalytic properties. The present study aims to develop a continuous composite catalytic pervaporation membrane reactor, as * To whom correspondence should be addressed. Tel: +31 40 2475445. Fax: +31 40 2446104. E-mail: [email protected].

Figure 1. Process configurations for pervaporation-assisted esterification processes: (a) semi-batch process and (b) continuous process.

Figure 2. Definition of geometric properties for a catalytic membrane module.

integration of reaction and separation offers advantages in terms of process efficiency and compactness. Composite catalytic membranes have been prepared by applying a zeolite coating on top of ceramic hollow fiber silica membranes. Large-pore zeolites, like Y-type zeolite, have proven to be efficient catalysts for esterification reactions.8,9 Additionally, zeolite coatings have been used as catalyst in various reactions, such as condensation, acylation oxidation, and dehydrogenation reactions.10-16 The performance of the composite catalytic membrane is examined in the esterification reaction between acetic acid and butanol. A preliminary large-scale composite membrane reactor evaluation is carried out based on the obtained experimental data for membrane permeability and catalyst activity.

10.1021/ie0502279 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/09/2005

Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9491 Table 1. Geometric Parameters for the Catalytic Membrane Reactor hydraulic diameter module void fraction initial module void fraction geometric surface area volume fraction of catalyst

dh m m am φcat

[m] [-] [-] [m-1] [-]

dh ) (d2m,int - Nfd2f,o)/(dm,int + Nfdf,o) m ) 1 - Nf(df,o + 2δcat/dm,int)2 m ) 1 - Nf(df,o/dm,int)2 am ) (4df,oNf)/[L(d2m,int - Nf(df,o + 2δcat)2)] φcat ) Nf|(d2f,o + 2δcat)2 - d2f,o|/(d2m,int - Nfd2f,o)

(1) (2) (3) (4) (5)

Table 2. Model Equations for the Catalytic Membrane Reactor mass balances liquid bulk catalytic layer boundary conditions membrane catalytic layer catalytic layer-bulk interface mass transfer polarization layer

d(FCi)/dz ) -2πrcatNfDe,i(∂Ci/∂r)|rcat + Rb,imπrm2 with De,i ) cat/ζcatDi ∂2Ci/∂r2 + 1/r(∂Ci/∂r) + (catRcat,i/De,i) ) 0 with Rcat,i ) kr[(1 - cat)Fcat]/cat(aAaB - (aEaW)/K) Pi[(γixipi*) - (yipp)] ) De,i(∂Ci/∂r)|rmem De,i(∂Ci/∂r)|rcat ) kF,i(Cb,i - Cmem,i) Sh ) kF,idh/Di ) 3.66 + 1.2x(1 - m))-0.8

Theory Reactor Model. A reactor simulation study has been carried out according to the framework presented by Lipnizki and Field17 to evaluate a preliminary reactor design. Conceptual process flow diagrams for the semibatch and continuous pervaporation-assisted production of butyl acetate are presented in Figure 1. In most pervaporation-coupled esterification studies presented so far, the membranes are operated batchwise18-20 or are placed in a recycle loop1,2,21-24 (Figure 1a). However, several authors investigated the coupled pervaporation-esterification process in a continuous-flow membrane reactor.4,25-27 In the second configuration, the membranes and the reactor are combined in a single unit, which is advantageous in terms of compactness and process efficiency (Figure 1b). In the simulations a continuous process is considered, without recirculation of reactants. A cross section of the reactor is depicted in Figure 2, and the geometric properties are summarized in Table 1. In the mathematical description of the pervaporation-coupled esterification reactor, the following simplifying assumptions have been made: (i) The membrane reactor behaves as an ideal isothermal plug-flow reactor; axial diffusion in the catalytic layer and in the liquid is not taken into account. (ii) External transport limitations on the permeate side are negligible under the pressure conditions applied. Concentration polarization effects in the shell side, however, are taken into account. (iii) The membrane is completely water selective, so no other species than water will permeate through the membrane. (iv) The catalyst particles are assumed to be nonporous, and the catalytic activity is uniformly distributed in the catalyst coating layer.

Figure 3. Coordinate system for the modeling of a catalytic membrane reactor and proposed concentration profiles.

(6) (7) (8) (9) (10) (11) (12)

The adopted coordinate system is depicted in Figure 3, and the governing equations are summarized in Table 2. The set of equations comprising the model have been solved numerically in Matlab, using a Runge-Kutta procedure for the axial direction coupled with a finite difference method in the radial direction. A more elaborate description of the model, together with the influence of various model input parameters on the performance of the catalytic membrane reactor, can be found elsewhere.6 Experimental Section Pervaporation Membranes. Applications of pervaporation for enhancement of the conversion of equilibrium-limited reactions are widespread on laboratory scale.1-7,18-27 However, mainly due to insufficient area to volume ratios, the translation to industrial applications is limited. In this project, ceramic pervaporation hollow fiber membranes (TNO-TPD, The Netherlands) were used to obtain a high area to volume ratio. The membranes are 20 cm long and have an inner and outer diameter of 2.0 and 3.2 mm, respectively. The membrane supports consists of γ-alumina intermediate layers positioned on top of a porous R-alumina hollow fiber. The permselective layer is located on the outside of the support and consists of a thin (70 nm) microporous amorphous silica layer.28 In the dehydration of 1-butanol, these membranes have proved to combine a high water flux with a good selectivity.28 Coating of the Pervaporation Membranes. Deposition of a catalytic layer on the outside of a pervaporation membrane was done by means of the dipcoat process. The membrane was immersed at a certain speed in a suspension of zeolite crystals, containing a binder material, followed by drying and calcination. Prior to dipcoating, the dipcoat solution was treated in an ultrasonic bath in order to disperse the zeolite particles. A pre-hydrolyzed TEOS solution was used as binder. After calcination the adhesion of zeolite crystals was tested by an ultrasonic bath treatment for 1 h, using SEM to evaluate the amount of crystals remaining at the surface. Zeolite H-USY with a silica-to-alumina ratio of 20 was obtained from Zeolyst Int. (Valley Forge, USA) and was used after calcination in an air stream for 8 h at 500 °C. The average particle size was 1 µm, as measured by scanning electron microscopy (SEM). More details concerning the dipcoat experiments and the preparation of the catalytic pervaporation membranes can be found elsewhere.7

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Figure 4. Setup used in the catalytic tests and the pervaporation-esterification coupling.

The main advantage of the dipcoat method is that ready-made catalysts can be directly deposited from the dipcoat suspension. Additionally, tuning of the zeolite coating thickness, to achieve the desired loading, is possible by varying the number of dipcoat steps. Pervaporation-Assisted Esterification Reaction. The catalyst activity of the zeolite-coated pervaporation membrane was measured in the esterification reaction between acetic acid and butanol. The membrane was placed in a stainless steel module module using Kalrez O-rings. The effective membrane area used was 17 cm2. The setup used is depicted in Figure 4. The supply vessel was charged with a certain amount of acetic acid. Then, the system was heated to the reaction temperature (T ) 75 °C), after which the preheated equimolar amount of alcohol was added. The reaction temperature was maintained by means of a thermostatic water bath, in which the system was immersed. The liquid reaction mixture was recirculated through the membrane module, and the supply vessel by means of the pump (valves are in position 1) at a liquid flow rate of 40 L‚h-1. This liquid flow corresponded to a superficial velocity exceeding 2 m‚s-1, which was sufficiently high to eliminate polarization effects. On the permeate side a vacuum was maintained (10 mbar) by a cascade of a liquid nitrogen cold trap and a vacuum pump. After a short stabilization period (10 min), the flow toward the supply vessel was stopped, decreasing the reactor volume to 30 mL. Hence, the liquid was only flowing through the pump and the module containing the catalytic membrane (valves are in position 2). For kinetic measurements, samples were taken periodically and analyzed by a gas chromatograph, equipped with a flame ionization detector and a thermal conductivity detector. GC analysis confirmed that no byproducts were formed. The catalytic activity of the unsupported catalyst was evaluated in a batch reflux system (T ) 75 °C) in which the catalyst was dispersed in the bulk liquid. The reaction rate constants were evaluated from the measured time-dependent concentration curves by means of the differential method, using a nonlinear least-squares regression technique. Results Coating of the Ceramic Pervaporation Membranes. Figure 5 shows a zeolite-coated membrane obtained from a 20 wt % zeolite dipcoat solution, containing 10 wt % of TEOS binder solution. A crackfree zeolite layer with a thickness of 2 µm is obtained

on top of the water selective layer of the pervaporation membrane. In contrast to the observation in other studies,29 varying the zeolite concentration in the coating mixture between 3 and 20 wt % does not seem to affect the final layer thickness substantially. In more concentrated mixtures, however, the zeolite layer thickness increases dramatically, and cracks appear during drying and calcining. Moreover, the thick layers are easily removed during the ultrasonic treatment. To increase the layer thickness in a controlled manner, sequentional dipcoating has been performed using a 20 wt % zeolite dipcoat solution. A calcination step has been applied after every dipcoat step. The results are presented in Figure 6. The resulting coverage is uniform, as shown in Figure 6a. From Figure 6b, it can be observed that the zeolite layer thickness is approximately 10 µm (5 × 2 µm), indicating that the increase in layer thickness after each immersion is not a function of the number of previous immersions. This has also been confirmed by the weight increase after each immersion. Hence, tuning of the zeolite coating thickness to the desired loading and coating thickness is possible by simply varying the number of dipcoat steps. Zeolite-Coated Pervaporation Membrane-Assisted Esterification Reaction. The catalytic activity of a membrane, coated with a ∼70 µm thick zeolite layer, has been tested in the esterification reaction between acetic acid and butanol, without water removal. The ∼70 µm thick layer corresponds to a coverage of about 45 g catalyst per m2 membrane surface area. For a hollow fiber membrane module this translates to a loading of 220 kg of catalyst per m3 of liquid volume. The coated membrane exhibits an activity of 9.2 × 10-10 m3‚mol-1‚s-1‚gcat-1, which is about 7% lower as compared to the activity of the unsupported zeolite catalyst. The small reduction in activity is most likely due to inhibition of external acid sites by the TEOS binder.30 The reaction rate constant for the uncatalyzed reaction is equal to 1.3 × 10-9 m3‚mol-1‚s-1 as the reaction also proceeds in the absence of a catalyst. The performance of the coated membrane has also been tested in the pervaporation-assisted esterification reaction. A feed at equilibrium conversion has been used to simulate the outlet stream from a conventional reactor, with an equimolar feed mixture at equilibrium conversion. Results are shown in Figures 7 and 8. In Figure 7, the molar fractions of the reactants and products are depicted as a function of time. Initially, the molar fraction of water shows a significant decrease in time. In addition, in contrast to the observations of other studies,2,4 the collected permeate consists mainly of water. This evidently illustrates the beneficial effect of the dual-layer structure. After 3 h a major part of the water is removed from the reactor, suggesting that the performance of the membrane reactor is mainly limited by the reaction kinetics. As a consequence, the rate of water production by reaction and the rate of water removal through the membrane become equal at a relatively low water concentration, as can be seen from Figure 7. A similar conclusion can be drawn from Figure 8, where the conversion is shown as a function of time for the catalytic membrane-assisted esterification reaction. The simulated curves in Figure 8 represent two extreme cases in which the conversion is either limited by reaction kinetics (uncatalyzed reaction) or by water

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Figure 5. SEM pictures of a H-USY zeolite-coated membrane, prepared with a 20 wt % H-USY mixture in ethanol pretreated in a TEOS solution: (a) top view and (b) side view.

Figure 6. SEM pictures of a H-USY zeolite-coated membrane, prepared with a 20 wt % H-USY mixture in ethanol pretreated in a TEOS solution, procedure repeated four times: (a) top view and (b) side view.

Figure 7. Mole fractions in the composite catalytic membraneassisted esterification of acetic acid and butanol as a function of time, T ) 75 °C.

Figure 8. Conversion of acetic acid in the composite catalytic membrane-assisted esterification of acetic acid and butanol (2), T ) 75 °C. Dashed line is a guide to the eye.

removal (the reaction rate constant is chosen such that further increase does not alter the conversion behavior). The experimentally observed conversion lies between these two extreme cases and clearly exceeds thermodynamic equilibrium. The performance of the composite membrane can be improved by increasing the reaction kinetics, as the conversion is limited by the reaction rate. Reactor Evaluation. For the performance of a pervaporation-coupled esterification process, the ratio of water production over water removal is found to be a key factor. This ratio is for instance determined by the area-to-volume ratio, the amount of available catalyst,

and the residence time in the membrane reactor. Due to the relatively low amount of catalyst as compared to the area-to-volume ratio of the experimental setup used in the previous paragraphs (2000 m2‚m-3 in a practical membrane module), a substantial conversion could not be reached within a reasonable time scale. Therefore, in order to evaluate the performance of a larger scale practical membrane module, model simulations are performed. Additionally, the performance of a catalytic pervaporation membrane module is compared with the performance of a reactor in which the same loading of heterogeneous catalyst is simply dispersed in the bulk liquid.

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Figure 9. Effect of catalyst position and catalytic layer thickness on module performance. Catalyst activity ) 9.2 × 10-10 m3‚mol-1‚ s-1‚gcat-1; Pw ) 5.2 × 10-7 mol‚m-2‚s-1‚Pa-1.

Figure 10. Effect of catalyst position and catalytic layer activity on module performance. Catalyst activity ) 1.8 × 10-9 m3‚mol-1‚ s-1‚gcat-1; Pw ) 5.2 × 10-7 mol‚m-2‚s-1‚Pa-1.

Data used in the simulations (e.g., membrane permeability and catalyst activity) are taken from the experiments described in the preceding paragraphs. The reactor dimensions are based on an annual production of 10 000 t of butyl acetate. The feed consists of acetic acid and butanol, in a stoichiometric ratio. Along the length of the reactor water is removed continuously through the pervaporation membranes. The reactor is operated at 75 °C and 1 bar. Based on a production rate of 1 m3‚h-1 and a residence time of 6 h, a reactor volume of 6 m3 is required. This reactor volume corresponds to a catalyst loading of 1300 kg. Using hollow fiber membranes with a diameter of 3 mm and a module void fraction of 0.2, a high membrane surface-area-to-volume ratio is obtained (4700 m2‚m-3). From Figure 7, the reaction rate constant and membrane permeability are estimated to be 9.2 × 10-10 m3‚mol-1‚s-1‚gcat-1 and 5.2 × 10-7 mol‚m-2‚s-1‚Pa-1, respectively. For the unsupported catalyst the reaction rate constant is taken 6.6% higher as compared to the activity of the supported catalyst. On the basis of these conditions, the outlet conversion of the catalytic pervaporation membrane reactor is calculated to be 0.85, whereas a conventional inert pervaporation membrane reactor with the same catalyst loading shows an outlet conversion of 0.79. This indicates that, in a practical catalytic pervaporation membrane module, substantial conversions are reached within a reasonable time scale. Moreover, the conversion for the reactor in which the catalyst is coated on the membranes clearly exceeds the conversion for the reactor in which the catalyst is simply dispersed in the bulk liquid. Due to a more efficient water removal, because the water is formed at the location where it is formed, the water concentration in the bulk is lowered. Consequently, the hydrolysis of the formed ester is reduced leading to an increase in conversion. This shows the potential added value of such a membrane system as compared to more common reactor designs. It is conceivable that a further improvement in reactor performance can be realized by optimization of the catalytic layer. As can be seen by Figure 9, for the inert membrane reactor (IMR) the conversion increases monotonically with the amount of heterogeneous catalyst, whereas for the catalytic membrane reactor (CMR) an optimum in conversion is observed at a certain layer thickness. The layer thickness corresponding to optimum performance is a function of, for example, reaction kinetics and membrane permeability.6 However, under the prevailing conditions, mass transfer limitations

become pronounced at relative thick catalytic layers (>300 µm) because diffusion inside the catalytic layer is sufficiently fast as compared to the reaction rate as the applied zeolite catalyst has a relatively low activity. In addition to increasing the catalyst layer thickness to its optimal value, the performance of the CMR can also be improved by using a more active catalyst. Figure 10 shows the effect on the conversion of an increase in catalyst activity with a factor of 2. The attained conversion increases with the catalyst activity, whereas the optimum in catalytic layer thickness is observed at a lower value. Due to the increased reaction rate, diffusion limitations inside the catalytic layer become pronounced at thinner catalytic coatings (∼200 µm). For the membrane reactor in which the catalyst is dispersed in the bulk liquid, also an increase in conversion is observed due to the higher activity of the catalyst. The difference in reached conversion between the two reactor concepts, however, is still visible (CMR: 0.96; IMR: 0.93). Conclusions Composite catalytically active membranes can be used to enhance conversion of esterification reactions as the reaction and separation function are coupled very efficiently. An additional advantage is that both the selective layer and the catalytic layer can be optimized independently. In this work, composite catalytic membranes are prepared by a dipcoating technique. Catalytic zeolite H-USY layers have been deposited on silica membranes using TEOS as binder material. Tuning of catalytic layer thickness is possible by varying the number of dipcoat steps. This procedure helps to avoid failure of the coating due to high stresses that can occur in thicker coatings during drying and calcining. In the pervaporation-assisted esterification reaction the catalytic membrane was able to couple catalytic activity and water removal. The catalytic activity of the H-USYcoated catalytic pervaporation membrane is comparable to the activity of the bulk zeolite catalyst. The collected permeate consists mainly of water; thus, acid, alcohol, and ester losses through the membrane are negligible. A reactor evaluation proved that the outlet conversion for the catalytic pervaporation-assisted esterification reaction exceeded the conversion of a conventional inert pervaporation membrane reactor, with the same loading of catalyst dispersed in the liquid bulk. The performance of the zeolite-coated pervaporation membranes can be increased by optimization of the catalytic layer thickness or by an increase in catalytic activity.

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Acknowledgment This work was performed in a cooperative project of the Centre for Separation Technology and was financially supported by TNO and NOVEM. We would like to thank Henk Woestenberg for taking the SEM pictures. Nomenclature Ci ) concentration [mol‚m liq-3] d ) diameter [m] De,i ) effective diffusion coefficient [mliq2‚s-1] Di ) diffusion coefficient [mliq2‚s-1] F ) flow rate [mliq3‚s-1] k ) reaction rate constant [mliq3‚mol-1‚s-1] kr ) reaction rate constant per catalyst concentration [mliq6‚mol-1‚gcat-1‚s-1] kF,i ) mass transfer coefficient [mliq‚s-1] K ) equilibrium constant [-] N ) number [-] p ) pressure [Pa] pi* ) vapor pressure [Pa] Pi ) permeance [mol‚m-2‚s-1‚Pa-1] r ) radius [m] Ri ) reaction rate [mol‚mliq-3‚s-1] Sh ) Sherwood [-] xi ) molar fraction retentate [-] yi ) molar fraction permeate [-] z ) axial coordinate [m] Greek Letters δ ) thickness [m]  ) void fraction [-] γi ) activity coefficient [-] F ) density [g‚m-3] ζ ) tortuosity [-] Subscripts b ) bulk liquid cat ) catalyst f ) fiber h ) hydrodynamic i ) component i in ) in int ) internal liq ) liquid m ) module mem ) membrane surface o ) outside obs ) observed out ) out p ) permeate

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Received for review February 22, 2005 Revised manuscript received May 4, 2005 Accepted May 4, 2005 IE0502279