Comparing and Modeling the Dehydrogenation of Ethanol in a Plug

Mar 23, 2002 - Keuler, J. N.; Lorenzen, L. The dehydrogenation of ethanol and 2-butanol in a catalytic membrane reactor. 6th World Congress of Chemica...
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Ind. Eng. Chem. Res. 2002, 41, 1960-1966

Comparing and Modeling the Dehydrogenation of Ethanol in a Plug-Flow Reactor and a Pd-Ag Membrane Reactor Johan N. Keuler* and Leon Lorenzen Department of Chemical Engineering, University of Stellenbosch, P.O. Box X1, Matieland 7602, South Africa

In this study the performance of a membrane reactor was compared to that of a plug-flow reactor for the dehydrogenation of ethanol. The membrane consisted of a 2.2 µm Pd-Ag film (23 wt % Ag) deposited on the inside of an asymmetric R-alumina membrane tube (SCT). The membrane tube was packed with a 14.5 wt % Cu on SiO2 catalyst. The effects of the sweep gas flow rate, the ethanol feed flow rate, and the temperature on acetaldehyde yield and selectivity were investigated. A model was developed using measured kinetic data and membrane permeance data to predict the performance of the membrane reactor. The membrane reactor performed significantly better than the plug-flow reactor at all temperatures tested. The best results were obtained at 275 °C, where the total ethanol exit conversion increased from 45% (plug-flow reactor) to 60% at low feed flow rates and from 36% to 46% at high feed flow rates. The acetaldehyde selectivity for the membrane reactor increased from the lower 80% range to above 90% at 275 °C. The model underpredicted the total ethanol conversion, indicating that the measured reaction rate parameters for a differential reactor were lower than those for the membrane reactor. 1. Introduction A catalytic membrane reactor combines the functions of a membrane and a catalyst in a single unit. Reaction and separation can thus be performed in one step. Either the catalyst can be deposited into the membrane pores1 or the membrane can be packed2,3 with a catalyst. A third alternative is to deposit the catalyst only on the inner or outer membrane surface next to, or as part of, the separation layer. In the latter case, the catalytic surface area is very small and not effective unless the catalyst is on the inside of a membrane with a very small inner diameter (hollow fiber). Membrane tubes need to be packed to provide sufficient catalyst surface area. To create a driving force for the components being separated by the membrane, either a sweep gas or a pressure difference is used. The sweep gas enters the shell and tube reactor on the opposite side of the membrane than the reactant(s), and the sweep gas can be inert or active. In the case of an active sweep gas, the sweep gas will react with the component permeating through the membrane, for example, an oxygen sweep4 in dehydrogenation reactions. Furthermore, the sweep gas can be co- or countercurrent. The reactor can be adiabatic or isothermal. The effects of different flow patterns and reactor configurations on reaction conversions have been well documented.5-7 Keuler8 published an extensive list of alkane and alcohol dehydrogenation reactions that have been studied and modeled to some extent in a membrane reactor. Those include methane steam reforming, water gas shift reaction, ethane dehydrogenation, propane dehydrogenation, butane and butene dehydrogenation, dehydrogenation of ethylbenzene to styrene, and ethanol dehydrogenation. Deng et al.9 modified alumina membranes (500 nm pore size) with a γ-alumina layer containing Pd, Pt, Cu, or Ni. Ethanol dehydrogenation was studied in the temperature range from 250 to 310 °C, employing a CuP/SiO2 catalyst. The acetaldehyde yield for the conven* Corresponding author. E-mail: [email protected]. Current address: Sasol Technology, Andries Brink Building, B level, 1 Klasie Havenga Road, Sasolburg 1947, South Africa.

tional reactor was slightly below the equilibrium value, while the values for the alumina membrane were higher than the equilibrium value. Cu- and Ni-modified alumina yielded results similar to those of alumina membranes. The best results were obtained with the Pd- and Pt-modified alumina membranes. Raich and Foley10 studied ethanol dehydrogenation in a Pd tube with a wall thickness of 76 µm. The operating temperature varied between 175 and 225 °C, and the tube was packed with Cu or Pt on silica catalysts. The best results were obtained with a copper on silica catalyst prepared by ion exchange followed by a copper on silica catalyst prepared by impregnation. The latter catalyst gave higher selectivity but lower activity and lower overall yield. Ethyl acetate was the main byproduct at lower temperatures. They compared a palladium reactor packed with a copper on silica catalyst (prepared by ion exchange) with a conventional reactor and obtained the following results: conversion increased from 60% to 90% and selectivity from 35% to 70%. 2. Experimental Section 2.1. Experimental Procedures. Palladium membranes were prepared by depositing a thin Pd layer on the inside of asymmetric R-alumina membranes from SCT (200 nm pore size) by electroless plating. The membranes had a length of 250 mm with an inside diameter of 7 mm and an outside diameter of 10 mm. An enamel provided by SCT was used to seal the membrane edges (35 mm length on both sides). The thickness of the Pd layer was optimized to provide both high hydrogen permeance and high hydrogen to nitrogen ideal selectivity. Pd films with hydrogen permeances of between 10 and 15 µmol/m2‚Pa‚s and hydrogen to nitrogen ideal selectivities of above 200 (at 330-450 °C) were prepared.11 A silver layer was deposited on the Pd layer by electroless plating, and the separated layers were heat treated12 to obtain Pd-Ag films on the order of 2 µm with 23 wt % Ag. The membranes were tested individually in the reactor shown in work by Keuler.12 Hydrogen and nitrogen permeance and selectivity testing were performed with the setup described in work by Keuler.8 The apparatus

10.1021/ie010127c CCC: $22.00 © 2002 American Chemical Society Published on Web 03/23/2002

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1961

Figure 1. Setup used for testing ethanol dehydrogenation.

and procedures used for ethanol permeance testing was also described in work by Keuler.8 Catalytic membrane reactor experiments were performed with the setup shown in Figure 1. A 2.2 µm PdAg membrane (23 wt % Ag) was packed with 3.0 g of a 14.4 wt % Cu on silica catalyst. The catalyst particle size fraction was 500-850 µm. Reactor temperature profiles were measured and the heating system was altered until the axial temperature change along the length of the membrane varied by less than 5 °C around the required reaction temperatures. An undiluted ethanol feed passed through a 1 m coil, which was placed in a preheating oven and connected to the reactor inlet, prior to entering the membrane reactor. The preheating oven ensured that the feed entered the reactor at the reaction temperature. Exit lines were heated with a heating wire to keep the products in the gas phase. A heated syringe (kept at 120 °C) was used for taking gas samples and injecting them into a GC. The products were analyzed with a HP G1800A gas chromatograph, equipped with a mass spectrometer and a flame ionization detector. The catalyst was kept in position with quartz wool at the edges of the membrane. During the start-up procedure, the membrane reactor was heated in nitrogen at 2 °C/min from room temperature up to 275 °C. The membrane and the copper catalyst were then reduced in a hydrogen atmosphere (flow of 50 cm3/ min) for 1.5 h. After 1.5 h, hydrogen was replaced with nitrogen for a further 10 min, and then the reactor was either heated or cooled at 2 °C/min to the required reaction temperature. At the required reaction temperature, the alcohol was introduced at 10 mL/h for 1.5 h before analysis. The same membrane was used for all membrane reactor experiments. The catalyst in the tube was replaced after each set of experiments at a specific temperature. Three product samples were analyzed for every set temperature and feed flow rate. The averages of the three analyses are the values reported in this paper. Typically, the individual ethanol conversions were (3% around the average. Kinetic parameters for ethanol dehydrogenation were determined in an inert quartz U tube8 at high feed flow rates to ensure acetaldehyde selectivities of close to 100%. These pa-

Figure 2. H2 and N2 permeances for a 2.15 µm Pd-Ag film.

Figure 3. % H2 permeated with N2 sweep gas and space time ) 1.19 s (2.15 µm Pd-Ag film).

rameters were used for all modeling calculations. Plugflow reactor (PFR) experiments reported in this paper were performed in the membrane reactor but without the use of a sweep gas. When the PFR performance is referred to, it is thus indicated by values on Figures 4-9 where the sweep gas flow rate equals zero. 2.2. Theory. In the development of a membrane reactor model, the following assumptions were made: (i) isothermal operation, (ii) plug flow on both the tube and the shell side, (iii) isobaric conditions, in other words negligible pressure drop on the shell and tube

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Figure 4. Comparison between H2-to-N2 and H2-to-ethanol selectivities for a 1.5 µm Pd film.

Figure 5. Ethanol conversion at 250 °C vs sweep gas flow rate (FEt ) 2.39 × 10-5 mol/s).

Figure 7. Ethanol conversion at 275 °C vs sweep gas flow rate (FEt ) 4.77 × 10-5 mol/s).

Figure 8. Ethanol conversion at 275 °C vs sweep gas flow rate (FEt ) 9.54 × 10-5 mol/s).

Figure 6. Ethanol conversion at 250 °C vs sweep gas flow rate (FEt ) 4.77 × 10-5 mol/s).

sides, (iv) no radial concentration gradients in the catalyst bed, (v) steady-state operation, and (vi) inert membrane. Both the shell and tube sides were operated under atmospheric conditions. The pressure drop across the length of the packed membrane tube (length of catalyst bed ) 19 cm) was less than 15 mbar. Isobaric conditions along the membrane’s axis were therefore a good assumption. To account for an axial pressure drop, the Ergun equation13 can be used. Radial concentration models have been studied for a catalytic membrane reactor.14,15-17 Gobina et al.16 found that the radial concentration change was negligible for a membrane with a 7.8 mm inner diameter. Intraparticle and interphase mass-transfer resistance can be incorporated into the reaction model. The rele-

Figure 9. Acetaldehyde yield at 275 °C for a membrane reactor.

vant equations and the correlations for calculating the mass-transfer coefficient were derived in refs 8, 13, 18, and 19. A catalytic membrane reactor is a PFR with separation. In dimensionless form, with the reaction at bulk conditions the model equation for the membrane reactor on the tube side is

dFi,t d(L/L0)

[

) V νir′i,bFbη -

]

2 J ) Rm i

(

L0πRm2 νir′i,bFbη -

)

2 J (1) Rm i

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where i ) 1-4 for ethanol, acetaldehyde, hydrogen, and nitrogen, respectively, and ν1 ) -1, ν2 ) 1, ν3 ) 1, and ν4 ) 0. Similarly, on the shell side

[ ]

dFi,s

2 Ji )V R d(L/L0) m

(2)

At L ) 0, Fi,t ) Fi,t(0)

(3)

At L ) 0, Fi,s ) Fi,s(0)

(4)

The flux (J) is described by eq 5. The value of n was

Per,i n n ) ) Pm,i(Pi,t - Pi,s) (Pi,high - Pi,low l

(5)

experimentally determined to be 18 for the flow mechanism through the Pd-Ag dense films. The ethanol dehydrogenation reaction (6) is an equilibrium reaction. Experimental data8 indicated that the

CH3CH2OH(g) T CH3CHO(g) + H2 ∆H°298 ) +52 kJ/mol

(6)

k′Et(PEt,b - PAcPH2/KEt,eq) (1 + KEtPEt,b + KH2PH2 + KAcPAc)2

(7)

Parameters were determined experimentally,8 and Keq was calculated using firsts principles via the van’t Hoff equation.13 The equilibrium constant in kPa is

Ln(Keq) ) 19.014 - 8503.3/T

(8)

The initial conditions were

At L ) 0:

FEt,t ) FEt,t(0)

FEt,s ) 0

(9)

FH2,t ) 0

FH2,s ) 0

(10)

FAc,t ) 0

FAc,t ) 0

(11)

FN2,t ) 0

FN2,s ) FN2,s(0)

(12)

Finally, the partial pressures need to be expressed in terms of flow rates: 4

Pi,t ) Pt[Fi,t/

Fi,t] ∑ i)1

(13)

4

Pi,s ) Ps[Fi,s/

Fi,s] ∑ i)1

(14)

with

FN2,t ) FN2,s(0) - FN2,s

Keq (Pa)

k′ [mol/ (kg of catalyst)‚ s‚Pa] 5.19 × 10-7 1.27 × 10-6

KEt (Pa-1)

KHydrogen (Pa-1)

KAc (Pa-1)

5.14 × 10-6 3.24 × 10-6 6.94 × 10-5 4.32 × 10-6 2.91 × 10-6 6.57 × 10-5

Table 2. Permeance Data (Pm Values) for a 2.2 µm Pd-Ag Film (23 wt % Ag) at 250, 275, and 300 °C ethanol acetaldehyde T H2 permeance N2 permeance permeance permeance 2 2 2 (°C) (µmol/m ‚Pa‚s) (nmol/m ‚Pa‚s) (nmol/m ‚Pa‚s) (nmol/m2‚Pa‚s) 250 275 300

3.37 3.78 4.21

22.71 21.29 20.15

5.68 5.32 5.04

5.68 5.32 5.04

Table 3. Parameters for Solving the Ethanol Dehydrogenation Model Rm [m] L0 [m] V [m] FEt [mol/s]

3.5 × 10-3 0.18 6.93 × 10-6 4.77 × 10-5

Fb [kg/m3] b m [kg]

430 0.4 0.003

and 275 °C. The permeance data are listed in Table 2, and further data necessary to solve the model are given in Table 3. 3. Results

reaction followed the dual site, surface reaction mechanism, with a rate expression of the form of eq 7.

r′Et,b )

T (°C)

250 10 918 275 24 083

The boundary conditions for cocurrent flow are

Ji )

Table 1. Ethanol Reaction Rate Parameters

(15)

To solve the model, kinetic coefficients and gas permeances must be known. Table 1 lists reaction rate coefficients that were determined experimentally at 250

3.1. Membrane Permeance. Figures 2 and 3 show hydrogen and nitrogen permeances for the Pd-Ag membrane used in the membrane reactor. Hydrogen permeances followed the Arrhenius relationship and decreased with a decrease in temperature (see Table 2). Nitrogen moved through the defects in the Pd-Ag layer mainly by Knudsen diffusion, and the permeances increased with a decrease in temperature (Figure 2). The net result was a sharp decrease in ideal hydrogento-nitrogen selectivity from about 330 at 410 °C to about 180 at 250 °C. Figure 3 indicates that when a sweep gas was used for creating a concentration gradient across the membrane, the sweep gas flow rate had a significant effect on the amount of hydrogen extracted through the membrane. The amount of hydrogen that permeated through the membrane dropped sharply when the sweep gas flow rate became too low. Figure 4 indicates the ethanol permeances for a pure Pd film. Ethanol had a lower permeance through the film than nitrogen, because of the larger molecule size. This resulted in a higher hydrogen-to-ethanol selectivity compared to the hydrogen-to-nitrogen selectivity. The nitrogen-to-ethanol selectivity varied with temperature between 3.1 and 4.4. To simplify calculations, an average of 4 was assumed for modeling. Pure-component permeances were used for modeling as a first approach. Competitive adsorption of oxygenate molecules on the Pd-Ag sites will result in lower hydrogen permeances (in a mixture) than those measured using pure hydrogen. 3.2. Membrane Reactor Modeling. The data in Tables 1 and 2 were determined experimentally, and the parameters in Table 3 represent physical properties. As a first approximation (model 1), the effectiveness factor was taken as 1, and it was assumed that there was no mass-transfer resistance. The model, furthermore, assumed 100% selectivity toward acetaldehyde and did not take side reactions into account. Equations 1-15 were solved to determine the exit ethanol conversion from the membrane reactor. The exit ethanol

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conversion is defined by eq 16. Model 1 underpredicted

Xethanol )

FEt(0),t - FEt,t - FEt,s FEt(0),t

(16)

the total ethanol conversion. The main reason was that the reaction rate coefficients in Table 1, determined from pure kinetic experiments,8 were lower than those obtained in the membrane reactor. The hydrogen permeance for the membrane was measured at the operating temperature after all ethanol dehydrogenation experiments were completed at that temperature. The hydrogen permeance was found to be about half of the initial pure-component hydrogen permeance. This indicates that there were some molecules and/or coke adsorbed on the membrane surface (Pd-Ag sites). For model 2, the permeances of all species were halved and the overall reaction rate was doubled to test the assumption that the reaction rate was higher in the membrane reactor than previously measured in an inert quartz U tube. Model 2 does not present a fit based on measured parameter values but rather indicates how changes in the reaction rate and permeances shift the exit ethanol conversion. 3.3. Effect of the Feed Flow Rate on Ethanol Conversion. Most work performed to date on dehydrogenation in catalytic membrane reactors has been done with very low feed flow rates. This approach exaggerates the theoretical advantages of the membrane reactor and provides very good conversion results.1,9,10,20,21 Such low feed flow rates result in extremely low product production rates even though the conversion is increased. In the present study, higher feed rates were employed to determine the practical importance of a catalytic membrane reactor. If a catalytic membrane reactor can significantly improve the reaction conversion when the feed rate is fast enough not to limit the reaction by the equilibrium conversion, then the reactor becomes of practical importance. Figures 5 and 6 illustrate the effects of the sweep gas (nitrogen) flow rate and the ethanol feed flow rate on the total ethanol conversion at 250 °C. The equilibrium conversion at 250 °C is just over 30%. The PFR is indicated by the ethanol conversion in the absence of a sweep gas. At the lower flow rate (FEt ) 2.39 × 10-5 mol/s; LHSV ) 0.68 h-1), the ethanol conversion for the PFR was similar to the equilibrium conversion (Figure 5). For the lower feed flow rate, the total ethanol conversion increased from 32% to 52% for the membrane reactor at a high sweep gas flow rate. When the ethanol feed flow rate was increased (Figure 6; LHSV ) 1.37 h-1), the total ethanol conversion for the PFR declined to 28.5%. The membrane reactor was still able to increase the total ethanol conversion to 34.5% at the higher sweep gas flow rate. There is significant scatter in the data represented in Figure 6. There is, however, a very clear trend of increasing conversion with increasing sweep gas flow rate for all other experiments (Figures 5 and 7-9). Experiments represented in Figure 6 were not repeated because those conditions are not optimum for good membrane performance. 3.4. Effect of Temperature on Ethanol Conversion. Three temperatures (250, 275, and 300 °C) were tested. The reaction rate and total ethanol conversion improves with temperature. In the low ethanol feed flow rate regime, i.e., in the equilibrium restricted domain, the membrane reactor performed significantly better

than the PFR at all temperatures. At the higher ethanol feed flow rates, the following improvements in the total ethanol conversion were obtained with the membrane reactor. (i) 250 °C, FEt ) 4.77 × 10-5 mol/s, LHSV ) 1.37 h-1: 5.7% absolute or 20% relative improvement. (ii) 275 °C, FEt ) 9.54 × 10-5 mol/s; LHSV ) 2.74 h-1: 10.4% absolute or 29% relative improvement. These LHSVs are similar to those (0.4-2.0 h-1) used for industrial dehydrogenation reactions8 and significantly higher than those studied by other researchers22 in a membrane reactor, which vary between 0.5 and 0.0038 h-1. At 300 °C, coking began to take place, and it lead to inaccurate data. From an operating point of view, there seemed to be an optimum working temperature. If the temperature was too low, the reaction kinetics was slow and the conversion poor, even with a membrane reactor. Organic molecules adsorb more strongly onto the PdAg membrane film at low temperatures than at higher temperatures and reduce hydrogen permeance more substantially at lower temperatures. Higher temperatures improve hydrogen permeance in two ways: by increasing the hydrogen permeance kinetics and by reducing the organic molecule adsorption onto the Pdalloy surface. At high temperatures, catalyst deactivation occurs quickly, which reduces the activity of the copper-based catalyst and leads to poor conversion. Coking will also occur on the membrane surface and reduce hydrogen permeance severely. 3.5. Performance of the Membrane Reactor at 275 °C. Figures 7 and 8 show measured and model data for ethanol dehydrogenation at 275 °C. The feed flow rate in Figure 7 resulted in the equilibrium acetaldehyde yield for the PFR. The equilibrium ethanol conversion and the measured acetaldehyde yield differed by only about 2%. As the feed flow rate increased, the acetaldehyde yield decreased and the difference between the equilibrium ethanol conversion and the measured acetaldehyde yield increased (see Figure 8). The same principles regarding feed flow rate that were discussed for Figures 5 and 6 apply to the experiments performed at 275 °C. Figure 9 shows the combined effect of feed flow rate and sweep gas to feed molar ratio on the total ethanol conversion. The feed multiple is the multiple of the standard feed rate (4.77 × 10-5 mol/s). The total ethanol conversion increased more sharply at the lower feed rates with an increase in the sweep gas flow rate. Toward the higher feed rates, the total ethanol conversion for the PFR (sweep gas to feed molar ratio ) 0) dropped slightly. Figure 10 indicates an improvement in acetaldehyde selectivity at the lower ethanol feed rates with an increase in sweep rates. Selectivity ranged between 80% and 95%, with the highest ethanol feed rates giving the best selectivities at the lower sweep gas ratios. At the higher sweep gas ratios, selectivity improved to above 90% for all feed rates. Table 4 indicates the measured improvements obtained by using the membrane reactor at 275 °C. The higher the equilibrium conversion, the more difficult it becomes to push the reaction’s equilibrium further toward the product side. The improvements in the total ethanol conversion with a membrane reactor will become smaller at higher temperatures. Absolute and

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1965 Table 5. Model Differences for Ethanol Conversion at 275 °C

Figure 10. Measured acetaldehyde selectivity (275 °C). Table 4. Improvements in the Total Ethanol Conversion for the Membrane Reactor at 275 °C flow rate (mol/s) 4.77 × 10-5 9.54 × 10-5 1.43 × 10-4

absolute relative sweep to feed improvement improvement molar ratio in conversion (%) in conversion (%) 4 4 3

13.8 7.0 10.4

29.9 16.0 28.9

relative improvements are defined by

absolute improvement ) MRV - PFRV

(17)

relative improvement ) (MRV - PFRV)/PFRV (18) with MRV the membrane reactor value and PFRV the PFR value. Deng et al.,9 Raich and Foley,10 and Liu et al.23 studied ethanol dehydrogenation in a membrane reactor, under conditions that differed significantly from those used in the present study. The feed rates that they employed were more than an order of magnitude lower than the feed rates used in this study. Liu et al.23 tested ethanol dehydrogenation as a function of the feed rate. At a W/F ratio of 1000 g‚min/mol similar to that of Liu et al.,23 this study achieved a maximum of 65% conversion at 300 °C compared to their 30%. 3.6. Model Performance. Model 1 underpredicts the ethanol conversion for the membrane reactor because of a higher reaction rate in the membrane reactor than expected. This suggests that the assumption of an inert membrane may not be valid and that the Pd-Ag membrane surface catalyzes the reaction. The surface area of the dense film is expected to be small, and further work will have to be done to quantify the contribution of the Pd-Ag sites toward ethanol dehydrogenation. A second shortcoming of the current model is to disregard side reactions. Ethanol conversion and acetaldehyde yield should be similar to the equilibrium conversion for a PFR if the acetaldehyde selectivity is 100%. At low feed flow rates for a PFR (Figures 5, 7, and 8), the ethanol conversion is clearly higher and the acetaldehyde yield lower than the equilibrium value. This indicates that both ethanol and acetaldehyde are consumed in side reactions. The main side reactions are ethyl acetate production from ethanol and acetaldehyde, as well as diethyl ether, ethene, and 1-butanone formation. The model does not account for this additional consumption of ethanol at this stage and will underpredict the ethanol conversion. The fact that both the ethanol conversion and the acetaldehyde yield are higher than the model-predicted ethanol conversion for

feed (mol/s)

average absolute error in total conversion for model 1 (%)

average absolute error in total conversion for model 2 (%)

4.77 × 10-5 9.54 × 10-5 1.43 × 10-4

8.0 11.9 9.9

3.3 4.3 2.3

some experiments indicates that side reactions are not the only factor responsible for the model deviation. There must also be a difference between the kinetic parameters measured in the U tube and those observed in the membrane reactor. Model 2, where higher reaction rate constants were assumed for testing this assumption, gave much more accurate predictions of the exit ethanol conversion. Table 5 compares the model values with measured values at 275 °C. 4. Conclusions The theoretical benefits of a catalytic membrane reactor as compared to a PFR were realized for the dehydrogenation of ethanol. For ethanol dehydrogenation, catalyst deactivation above 280 °C limited the upper operating temperature of copper-based catalysts. The optimum working temperature for ethanol dehydrogenation with copper on silica catalysts was about 275 °C. The equilibrium conversion in this temperature range is just over 40% for an undiluted ethanol feed at atmospheric pressure. The membrane reactor improved the total ethanol exit conversion from 45% (PFR) to 60% at low feed flow rates and from 36% to 46% at high feed flow rates. The acetaldehyde selectivity of the reaction was above 80% for all experiments performed. The selectivity was most sensitive to the feed flow rate, and the selectivity improved with an increase in the feed flow rate. The selectivity also improved with an increase in the sweep gas flow rate but changed little with temperature from 250 to 300 °C. At a high sweep gas flow rate, the acetaldehyde selectivity was above 90% for the majority of the experiments. Acknowledgment We want to acknowledge Sasol and the FRD (both in South Africa) for their financial contributions toward the project. List of Symbols Am ) surface area of the metal film [m2] F ) flow rate [mol/s] J ) flux [mol/m2‚s] k ) reaction rate constant [mol/m3‚Pa‚s] k′ ) reaction rate constant [mol/(kg of catalyst)‚Pa‚s] K ) adsorption coefficient [Pa-1] Keq ) equilibrium constant [Pa] l ) film thickness [m] L ) distance from the reactor inlet [m] L0 ) reactor length [m] LHSV ) liquid hourly space velocity [h-1] m ) catalyst mass [kg] n ) pressure exponent P ) pressure [Pa] Per ) permeability coefficient [mol‚m/m2‚Pa‚s] Pm ) permeance [mol/m2‚Pa‚s] rA ) rate of generation for component A in the reaction [mol/m3‚s]

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r′A ) rate of generation for component A in the reaction [mol/(kg of catalyst)‚s] Rm ) inner radius of the membrane tube [m] T ) temperature [K] V ) reactor volume [m3] XA ) conversion of A Greek Symbols b ) void fraction of the packed bed η ) overall effectiveness factor Fb ) bulk density of the catalyst bed [kg/m3] νi ) stoichiometric coefficient for component i in the reaction Subscripts 0 ) inlet conditions Ac ) acetaldehyde b ) at bulk conditions Et ) ethanol high ) high-pressure side H2 ) hydrogen i ) component i low ) low-pressure side N2 ) nitrogen s ) on the shell side of the reactor t ) on the tube side of the reactor

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genation of ethanol and 2-butanol in a catalytic membrane reactor. Ph.D. Thesis, University of Stellenbosch, Stellenbosch, South Africa, 2000. (9) Deng, J.; Cao, Z.; Zhou, B. Catalytic dehydrogenation of ethanol in a metal-modified alumina membrane reactor. Appl. Catal. A 1995, 132, 9. (10) Raich, B. A.; Foley, H. C. Ethanol dehydrogenation with a palladium membrane reactor: An alternative to Wacker chemistry. Ind. Eng. Chem. Res. 1998, 37, 3888. (11) Keuler, J. N.; Lorenzen, L.; Miachon, S. Preparing and testing Pd films between 1 and 2 micron thickness with high selectivity and high hydrogen permeance. Sep. Sci. Technol. 2002, to be published. (12) Keuler, J. N.; Lorenzen, L. Developing a heating procedure to optimise hydrogen permeance through Pd-Ag membranes of thickness less than 2.2 µm. J. Membr. Sci. 2002, 195, 203. (13) Fogler, H. S. Elements of Chemical Reaction Engineering, 2nd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1992. (14) Becker, Y. L.; Dixon, A. G.; Moser, W. R.; Ma, Y. H. Modelling of ethylbenzene dehydrogenation in a catalytic membrane reactor. J. Membr. Sci. 1993, 77, 233. (15) Gobina, E.; Hou, K.; Hughes, R. Equilibrium-shift in alkane dehydrogenation using a high-temperature catalytic membrane reactor. Catal. Today 1995, 25, 365. (16) Gobina, E.; Hou, K.; Hughes, R. Ethane dehydrogenation in a catalytic membrane reactor coupled with a reactive sweep gas. Chem. Eng. Sci. 1995, 50, 2311. (17) Gobina, E.; Hou, K.; Hughes, R. Mathematical analysis of ethylbenzene dehydrogenation: Comparison of microporous and dense membrane systems. J. Membr. Sci. 1995, 105, 163. (18) Collins, J. P. Catalytic decomposition of ammonia in a membrane reactor. Ph.D. Thesis, Oregon State University, Corvallis, OR, 1993. (19) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGraw-Hill Book Co.: New York, 1980. (20) Itoh, N. A membrane reactor using palladium. AIChE J. 1987, 33, 1576. (21) Ali, J. K.; Newson, E. J.; Rippin, D. W. T. Deactivation and regeneration of Pd-Ag membranes for dehydrogenation reactions. J. Membr. Sci. 1994, 89, 171. (22) Keuler, J. N.; Lorenzen, L. The dehydrogenation of ethanol and 2-butanol in a catalytic membrane reactor. 6th World Congress of Chemical Engineering, Melbourne, Australia, Sept 2327, 2001. (23) Liu, B.; Cao, Y.; Deng, J. Catalytic dehydrogenation of ethanol in Ru-modified alumina membrane reactor. Sep. Sci. Technol. 1997, 32, 1683.

Received for review February 7, 2001 Revised manuscript received January 11, 2002 Accepted January 15, 2002 IE010127C