Yield Improvements in Membrane Reactors for Partial Oxidation

Ind. Eng. Chem. Res. 2006, 45, 2697-2706

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Yield Improvements in Membrane Reactors for Partial Oxidation Reactions Christopher M. O’Neill and Eduardo E. Wolf* Department of Chemical and Biomolecular Engineering, UniVersity of Notre Dame, Notre Dame, Indiana 46556

Implementing a distributive membrane reactor for partial oxidation reactions, specifically the oxidative dehydrogenation (ODH) of propane and the partial oxidation (POx) of propylene to acrolein, is the focus of this theoretical investigation. The reactor model in this study demonstrates that the membrane reactor increases the yield of the desired product, propylene or acrolein, while suppressing the yield of COx. The membrane reactor accomplishes this by lowering the partial pressure of a reactant, oxygen, to suppress the full oxidation reaction that has a higher order dependence on oxygen. In addition to improvements for these individual reactions, the reactor model is expanded to include two different catalyst beds, the ODH reaction and the POx reaction. The dual-bed membrane reactor design improved the yield of acrolein compared to that of a dual-bed fixed bed reactor. The distributive membrane reactor is a useful tool to further enhance the performance of these POx catalysts. Introduction Over the past decade, interest in utilizing membranes as a functional component of a reactor has significantly increased.1-7 Although numerous methods exist for coupling a membrane with a reactor, two remain the most prevalent: (1) reactors in which a membrane is used to selectively remove a product (permselective) and (2) reactors in which the membrane is used to distribute one of the reactants or a catalyst along the reaction bed (porous). Both membrane configurations alter the reaction environment, which can have a drastic effect on selectivity and rates. Perm-selective membranes are especially helpful in driving equilibrium-limited reactions to completion.2,4 Porous membranes replenish a reactant all along the catalyst bed thereby promoting the rate of reactions of lower order in the distributed reactant. When changes to the reaction environment are combined with improved catalysts, higher conversions and selectivities can be achieved. In a membrane reactor, three options for positioning the catalyst arise. The catalyst can be loaded either inside the membrane tube, in the annular region between the membrane and reactor wall, or deposited directly on the membrane wall itself. The first two configurations are referred to as inert membrane reactors (IMR), the focus of this investigation, and the third configuration is a catalytic membrane reactor (CMR).5,6 Perm-selective membranes present a number of hurdles that must be overcome before they are viable in reactor designs. The major limitations inherent to the perm-selective membranes are a lack of thermal stability, inadequate perm-selectivity, and insufficient permeability as cited by Saracco et al.4 Currently, overcoming these main drawbacks requires the implementation of larger surface area membranes, to compensate for their limited permeability, that must be replaced frequently, increasing cost. Additional separation may still be required downstream of the reactor to remove unwanted compounds that were able to permeate through the membrane. Manufacturing improved permselective membranes that overcome these drawbacks is necessary before they can be implemented in applications. Continued investigations in these membranes are largely academic until improved perm-selective membranes can be manufactured. * To whom correspondence should be addressed. Tel.: (574) 6315897. Fax: (574) 631-8366. E-mail: [email protected].

CMRs exhibit similar problems, and the introduction of catalysts into the membrane often limits their permeability, thereby increasing the pressure drop and cost of reaction operation.4 For these reasons, we focused on investigating the possibilities of porous inert membrane reactors. Porous membranes affect the distribution of reactants in the reactor. Separating the reactant gases across a membrane serves to change the residence time, location, and concentration of reactants throughout the catalyst bed. With the separation of reactants, previously unattainable reactor conditions become feasible. New conditions range from using different operating temperatures and pressures to changes in gas concentrations and ratios. Feed streams existing above the explosion limit for given reactants can be realized in an IMR as the reactants do not contact one another outside of the reaction zone of the catalyst bed. Optimal reactor conditions must be reevaluated with the addition of new reactor environments. Two reaction families of particular interest for the use of membranes are the oxidative dehydrogenation (ODH) and partial oxidation (POx) of hydrocarbons. Inherently unselective, these reaction pathways are restricted by kinetics and thermodynamics which both favor complete oxidation products. Literature results for the POx of light hydrocarbons show that high selectivity of partial oxidation products can be attained when reactions are performed at low conversions. Conversely, high conversion of hydrocarbons result in low selectivity.8-17 Overcoming the kinetic and thermodynamic limitations for the POx of hydrocarbons requires the use of selective catalysts, novel reactor designs, or both to increase the production of selective oxidation products. There has been significant interest in investigating the POx of hydrocarbons in distributive membrane reactors.14,18-24 The results reported by these authors demonstrate the capability of membrane reactors to decrease total oxidation products and promote the selectivity of olefins or oxygenates. This alteration of product selectivity in these often complex series-parallel reaction networks is facilitated by the membrane which can reduce the partial pressure of a key reactant, namely, oxygen in the case of POx reactions. In the POx of hydrocarbons, total oxidation reactions are typically first-order in oxygen, while the partial oxidation reactions are close to zero-order in oxygen. Even though the activation energy for total oxidation is lower

10.1021/ie051009i CCC: $33.50 © 2006 American Chemical Society Published on Web 03/09/2006

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Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006

than for POx,8,25 if the O2 concentration is limited, full oxidation reactions will proceed slowly, while POx reactions are unaffected. Thus, feeding oxygen into the membrane and along the length of the catalyst bed enables control of the selectivity by reducing the local partial pressure of oxygen (higher HC/O2 ratio). The total oxidation products CO2 and CO are no longer favored, and intermediate oxygenate products are formed. The implementation of a membrane reactor in partial oxidation reactions minimizes the oxygen partial pressure while allowing for near stoichiometric HC/O2 feed rates to accomplish both high conversion as well as a high oxygenate selectivity. It is possible that lowering the partial pressure of a reactant through the use of a membrane alters the kinetic constants or the reaction order for the rate law. However, due to an absence of kinetic data under these altered conditions, it was assumed for this study that the rate law derived from FBR studies holds true for the membrane reactor as well. In addition to the literature results above, research in our group has investigated new reactor designs involving membranes. Initial experiments incorporated a membrane reactor in the partial oxidation of methane (POM).26-28 The results revealed little improvement in the yield obtained in the IMR over that of the conventional fixed bed reactor (FBR) for this reaction network. The major result of this work, however, came with the addition of a second catalyst bed immediately following the membrane reactor.26 In the single-bed IMR, conversion of methane was limited to about 60%. The addition of the second bed fed independently with CO2 increased conversion of methane to about 80%. The increased conversion also corresponded to an increase in CO and H2 selectivity from 75% to 80%. Although the IMR resulted in a minimal increase of yield in OCM it was observed that a second reaction bed furthered the formation of the desired products. Improvements seen in the conversion and selectivity of membrane reactors as well as product enhancement in two-bed reactor designs led to this investigation of partial oxidation reactions in a dual-bed membrane reactor. Selective Oxidation of Propane to Acrolein. Selective partial oxidation of propane is a viable candidate for use in an IMR. Currently, acrolein is produced in a two-step process. The ODH of propane creates propylene, and then the propylene is partially oxidized to form acrolein (POA). Each of these reactions requires different catalysts, temperatures, reactant ratios, and residence times. Direct conversion of propane to acrolein in a one-step process is desirable and enables the use of the alkane instead of the higher cost alkene while simultaneously lessening the environmental impact. One of the major hurdles to a one-step process is the lack of understanding of the reaction pathway from alkane to oxygenate. In principle there are three possibilities: (1) simultaneous dehydrogenation and oxygen insertion; (2) sequential dehydrogenation of propane followed by oxygen insertion into the propylene molecule; (3) sequential but reverse of the second pathway, i.e., oxygen insertion followed by dehydrogenation. Attempts to increase the yield of acrolein in the one-step process have had little success with a maximum yield of 13%.29 The necessity for different reaction conditions for each reactor makes it more advantageous to utilize two separate catalysts. Interesting results obtained by Sinev et al.30 showed that acrolein yields could be increased to 25% by combining ODH and POA catalysts in consecutive reaction zones. It was hypothesized from these results that an intermediate is formed over the ODH catalyst that produces a yield to acrolein superior than that of propylene over the POA catalyst.

Scheme 1. Propane to Acrolein Reaction Network Used in the FBR and IMR Simulationsa

a The numbers above each arrow reference the reactions listed in Table 1.

Figure 1. Schematic of dual-bed IMR (a) and FBR (b).

In accordance with these results, specific to the selective oxidation of propane to acrolein, and our own results when adding a second reaction bed to promote a desired reaction, the use of a dual-bed membrane reactor (2B-IMR) for this reaction offers an attractive alternative to the one-step process. A 2BIMR designed for the selective oxidation of propane to acrolein will consist of the ODH of propane in the first bed followed by the POA in the second bed. The 2B-IMR should show improvement in acrolein selectivity as the ODH and POA reactions use different optimal conditions, both benefit from the use of a membrane, and consecutive reaction beds promote the formation of the oxygenate acrolein. Theory The propane to acrolein reaction network consists of series and parallel reactions (Scheme 1). The production of useful partial oxidation products such as acrolein and acrylic acid is limited by the formation of CO2.25,31-34 The 2B-IMR provides an alternate reaction environment and is intended to enhance the yield of the partial oxidation products. The ensuing theoretical analysis compares the performance of an FBR and 2B-IMR (Figure 1). Differences between the reactors arise only through the introduction of the reactants to the catalyst bed. Table 1 illustrates the reaction rate equations, activation energies, and heats of reaction for each reaction as reported in selected references. Minor differences arise between the model equations for the one-dimensional (axial) FBR and IMR. The two reactor designs

Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2699 Table 1. Reaction Rate Equations, Activation Energies, and Heats of Reaction for the Propane to Acrolein Reaction Network Depicted in Scheme 1a

1 2 3 4 5 6 7 8 a

reaction

rate equation

preexponential factor

activation energy (kJ mol-1)

heat of reaction (kJ mol-1)b

C3H8 + 0.5O2 f C3H6 + H2O C3H6 + O2 f C3H4O + H2O C3H8 + 5O2 f 3CO2 + 4H2O C3H6 + 4.5O2 f 3CO2 + 3H2O C3H4O + 3.5O2 f 3CO2 + 2H2O C3H4O + 0.5O2 f C3H4O2 C3H4O2 + 3O2 f 3CO2 + 2H2O C3H8 f CH4 + C2H4

r1 ) k1CP r2 ) k2CPP r3 ) k3CPCOx r4 ) k4CPPCOx r5 ) k5CAcCOx r6 ) k6CAc r7 ) k7CAcACOx r8 ) k8CP

5 × 106 4.75 × 105 1 × 107 3 × 106 6 × 106 1 × 105 9 × 106 1 × 103

103.4c 78.7c 65.3c 66.5c 70.0a 70.0a 75.0a 65.3d

-117.1 -368.0 -2040.0 -1930.0 -1633.0 -264.0 -1325.0 83.0

Estimated values. b Ref 39. c Ref 25. d Ref 15.

analyzed were assumed to have similar reactor dimensions and the same initial conditions to allow the membrane to be the sole factor responsible for any changes in the effluent composition. The pseudohomogeneous steady-state reactor model was developed using the following assumptions: (1) There are isobaric reactor conditions. (2) Gas velocity is only in the axial direction. (3) No radial gradients are present. (4) The ideal gas law applies (low pressure and high temperature). (5) All reactions are irreversible and occur only in the catalyst beds. (6) There are no mass transfer limitations. (7) The membrane is highly porous, inert, and evenly distributes the oxygen along the axial length of the catalyst bed. (8) Gas heat capacities are independent of temperature. A differential mass and energy balance for a homogeneous fixed bed reactor at steady-state conditions, plug flow, and reaction with power law kinetics is as follows for component i (1 ) propane, 2 ) propylene, 3 ) acrolein, ...) and catalyst bed k:

dFi

)

dz

-υi,j

∑j υ

rj,kac +

1,j

ni,k

∑i z

(1) k

with the last term representing the flux at the membrane wall. The energy balance yields

dT

)

which were solved using a fourth-order Runge-Kutta method. Under FBR conditions, the mass and energy balance are simplified by the absence of the ∑ni,k terms. Reaction Pathway Model Improvements to the simulations were made in stages, beginning with a simple network and reactor design and gradually adding complexity, to ensure the results are consistent. Preliminary results were obtained with an FBR simulation consisting of a single bed and a simplified reaction network reported in the literature.35 This simplified reaction pathway was used for both verification of the numerical technique and initial simulations of experimental data from literature in a singlebed FBR. The simulation was then run in a two-bed FBR and IMR to predict the performance of a reactor with two beds as there is currently no experimental data of a two-bed FBR or IMR for comparison. The reaction simulations require kinetic parameters (activation energy and preexponential factor), oxidation order, and heats of reaction as input parameters. All other variables are defined by the reactor design. The activation energies, heats of reaction, and oxidation orders were obtained from previously reported experiments and reference materials as listed in Table 1. Preexponential factors were unknown and were obtained from the fitting of the experimental data from literature. Verification of Numerical Solution. The numerical method used in the 2B-IMR model was tested in two ways. First, a previously simulated, simple reaction network for an FBR was reproduced.35 The network consisted of the following:

dz Uaac(Ta - T) +

∑j

(-∆Hj)(-rj,k)ac +

∑i

ni,k Cpi(Tm - T) zk

∑i FiCpi (2) In the above expression, the first term is the heat transfer to the wall, the second term the heat generation due to reaction, and the last term represents the enthalpy change due to the membrane flow. With the initial conditions

At z ) 0

Fi ) Fio

T ) Tin

The membrane flux term was included by assuming oxygen as a point source uniformly distributed along the catalyst bed. While in reality addition of oxygen occurs at the porous membrane wall, we assume plug flow (PFR), implying no radial gradients; hence, the form of the oxygen addition term is consistent with the PFR assumption. The mass and energy balances are a set of nonlinear ordinary differential equations

AfB

-r1A ) k1ACA

(3)

2A f C

-r2A ) k2ACA2

(4)

The comparison between our solution and the reported results for the same reaction pathway agree within 1% confirming that the Runge-Kutta solution technique used is accurate and can be used with confidence for solving the membrane reactor model.35 Modeling: POA of Propylene Kinetics. After confirming the accuracy of the numerical solution, we extended its application to the reaction pathways involved in the partial oxidation of propane to acrolein. Two different reaction networks, propane ODH (Scheme 2) and propylene POx (Scheme 3), were simulated individually with the purpose of utilizing them to predict the performance of a two-bed reactor. The combination of the two networks creates the series-parallel network, shown in Scheme 1, to partially oxidize propane to acrolein. Reactor simulations were first conducted on the POA reaction pathway. Prior investigations performed in FBRs were often

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Scheme 2. ODH Reaction Network Used in the FBR and IMR Simulationsa


Scheme 3. POA Reaction Network Used in the FBR and IMR Simulationsa


conducted using either Bi-Mo derived oxides or Fe-Sb oxides as catalysts.8,11,32,36-38 For our purpose we focused on the Bi-Mo oxide catalysts as these have shown to have the most promise and also correspond to the type of catalyst that will be utilized in our experimental investigations. The reactor was modeled as an FBR, and the published experimental data were fitted by varying the preexponential factor in the rate constant. Once a satisfactory fit was obtained, the IMR was simulated using the same rate constants to project the results as if a membrane reactor had been utilized. This simulation served only to predict the overall trends of the reaction network due to the simplifying assumptions and limited kinetic data available. Krenzke and Keulks36 studied the mechanism of POA over different Bi-Mo-(Fe) oxide catalysts by using oxygen-18 as a tracer. Reactions were carried out in an FBR with a 1:1:8 ratio of C3H6/O2/He. Conversion of C3H6 in all the reactions was held between 10% and 20%. This was achieved by varying the mass of catalyst in the reactor bed and the reactor temperature (higher temperature, less catalyst). Figure 2a presents the experimental data (points), reported by Keulks and Lo,37 along with the predicted results from the single-bed FBR model (dashed line). Since the exact experi-

mental conversion is not reported by Keulks and Lo,37 the conversion is estimated to be a constant 15% as seen in Figure 2. The model prediction fits the experimental data well at higher temperatures, but it underpredicts the selectivity at lower temperatures. The preexponential factors for the POA reactions are listed in Table 1 along with the activation energies and heats of reaction. The preexponential factors obtained from these simulations were then utilized for predicting the performance of the 1B-IMR. Figure 2b compares the 1B-FBR to the 1BIMR for the POA reaction. It can be seen that the conversion is higher in the FBR (dashed line) than in the IMR (solid line); however, the selectivity toward acrolein is significantly higher in the IMR. The data for the POA pathway is limited as the reaction is only run at low conversion, but the trends of the reaction are of more importance. Modeling: ODH of Propane Kinetics. The other reaction of interest in the reaction pathway shown in Scheme 1 is the ODH of propane (Scheme 2). Vanadium oxide based catalysts are typically employed to perform the ODH of propane due to its good conversion and selectivity. Other metal oxides are often added to the vanadium oxide catalyst to improve the conversion and selectivity toward propylene.32 These metals include antimony, tungsten, magnesium, molybdenum, and bismuth. Zanthoff et al.15 used a V-Sb-W oxide catalyst as a way to enhance the selectivity of propylene in the ODH reaction network. Their single-bed FBR is run under atmospheric pressure with a gas mixture of C3H8/O2/N2 that is equal to 2:1:6. Figure 3 shows our first attempt to fit the experimental data of Zanthoff et al.15 with the single-bed FBR model. As evidenced by these graphs, one can see that the conversion trend is not properly predicted and the selectivity is underpredicted at high temperatures when using the reaction pathway including reactions 1, 3, and 4 as shown in Scheme 2. The conversiontemperature relationship switches from proportional to inverse above 775 K. This simulation predicts incorrect trends because at high temperature the reaction rate is sufficiently high to completely deplete the oxygen in the reactor and thus terminate the reaction. The conflicting results between the FBR model and literature can be resolved by adding in the propane cracking reaction to form methane and ethylene (reaction 8 in Scheme 2). With the addition of this reaction, the FBR model prediction fits the experimental data from literature and its trends well as shown in Figure 4. Increasing temperature leads to an increase

Figure 2. (a) Experimental FBR data from Keulks and Lo (ref 37) for the POA reaction fit with the single-bed FBR model. (b) Fit of the FBR model alongside the predicted performance of the single-bed IMR model.

Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2701

Figure 3. Zanthoff et al. (ref 15) experimental FBR data for the POx of propane fit with the single-bed FBR model without the inclusion of the propane cracking reactions. The points represent the literature data, and the lines are the fits from the FBR model. (a) Conversion. (b) Selectivity to propylene.

Figure 4. Zanthoff et al. (ref 15) experimental FBR data for the POx of propane fit with the single-bed FBR and IMR model with the inclusion of the propane cracking reactions. The points represent the literature data, while the lines are the fits from the model (- - - FBR, s IMR). (a) Conversion. (b) Selectivity to propylene.

in conversion and a decrease in selectivity. Therefore, at high temperatures with low availability of oxygen, the cracking reaction composes a significant contribution to the conversion of propane in addition to the ODH and combustion reactions. Table 1 lists the reaction parameters and the corresponding heats of reaction for the ODH of propane. These same parameters were also implemented to predict the products of the ODH reaction in a single-bed IMR. Figure 4 shows the improvement seen in both the conversion of propane and selectivity to propylene for the single-bed IMR over those of a traditional one-bed FBR in the ODH reaction pathway. The results show that there should be a benefit in employing membrane reactors for the conversion of propane to propylene and ultimately to acrolein. Preexponential factors used to fit the Zanthoff et al.15 ODH data will be utilized for the prediction in the two-bed model later.

Parametric Sensitivity Single-Bed FBR versus IMR: Effect of Operating Conditions. A parametric sensitivity study was conducted on the FBR and IMR model in a one-bed configuration to identify the conditions best suited for each reactor. The single-bed operation simplifies the model allowing each reaction (ODH, Scheme 2 or POA, Scheme 3) to be optimized independently under the assumption that there is no interaction between the catalyst beds when the FBR and IMR are run in a two-bed mode. Looking at each reaction pathway separately focuses the results on the effect of the reactor design (fixed bed or membrane) and not on the effect of the catalyst bed configuration (single or dual bed). Parameters for the sensitivity test were obtained from the preceding fits of the ODH and POA reactions from experimental data in the literature. Residence time and hydrocarbon-to-oxygen (HC/O2) ratio effects were investigated in the sensitivity study.

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Figure 5. Conversion (a) and selectivity (b) effects for the single-bed FBR and IMR with respect to temperature for the propylene to acrolein reaction network with a residence time of about 0.2 s. The final two FBR data points represent values that were obtained from the model where the FBR was under runaway conditions.

The POA reaction (Scheme 3) simulation produces the expected proportional relationship between conversion and temperature in both the single-bed FBR and IMR (Figure 5a). Of immediate note is the ability of the IMR to run safely at higher temperatures. At high temperatures, the FBR has the potential for instability and runaway, leading to uncontrolled reactions as shown by the points in Figure 5. These runaway points were determined from the simulation of both the temperature and concentration profiles within the catalyst bed. When a sharp spike in temperature corresponded with the simultaneous consumption of all available oxygen, the simulation became numerically unstable (negative concentrations and conversion greater than one) and thus we chose to classify this as a runaway condition. Implementation of the IMR reduces the temperature gradient through the reactor thus reducing hot spots and avoiding runaway conditions in the reactor. The selectivity to acrolein reaches a maximum near 400 °C as seen in Figure 5b suggesting an optimal reactor temperature. The selectivity to acrolein is higher in the IMR than in the FBR, but conversion is higher in the latter. However, because of the thermal stability gained in the IMR, the temperature can be increased to a higher value than in the FBR, and so comparable conversions can be achieved with higher selectivity leading to a higher yield in the IMR. Hereafter, the yield is displayed rather than selectivity because the latter ignores the difference in conversion between the two reactor types and gives a more favorable performance for the IMR that what it really is. The yield is normalized to a per carbon atom basis and defined as the moles produced of a given product times the number of carbon atoms in the product divided by three (number of carbon atoms in propane or propylene) times the conversion of the hydrocarbon. Residence times will have an effect on the performance of reactors. Adjusting the time reactants contact one another over a catalyst bed changes both conversion and yield in a given reactor. Figure 6 shows the effect of residence time on conversion and the yield of both acrolein and COx in the POA reaction. Two residence times, 0.1 and 0.2 s, are selected with increases in the temperature accounting for the increasing conversion. As expected in both the FBR and IMR, higher conversions are obtained at longer residence times. Looking at

both the IMR and FBR data in Figure 6, longer residence times correspond to a decrease in the yield percentage of acrolein with a simultaneous increase in the yield percentage to COx. The acrolein yield remains between 50% and 60% at both residence times in the IMR until higher temperatures and conversions limit the POx product. The FBR, meanwhile, is in the 40-50% yield range for both residence times at the same temperature. The higher temperatures from the IMR are unattainable in the FBR because of the onset of the runaway reaction. This further supports our prediction of the advantage of the IMR over the FBR. The IMR demonstrates a higher (∼10%) yield of acrolein and lower (∼10%) yield of COx than the FBR. Although in this study, the residence time is not found to drastically alter the yields attained in the reactor, shorter residence times produce higher yields for the POx product, acrolein, in both the IMR and FBR. These results suggest that the best conditions for both the IMR and FBR are short residence times at low conversion to maximize the yield of acrolein. The second parameter of interest is the hydrocarbon-tooxygen ratio. The simulation was run with two different HC/ O2 ratios, 1:1 and 2:1, at a residence time of about 0.1 s. The conversion increase is attained in the simulation by increasing the reactor temperature. Higher HC/O2 ratios at a given temperature decrease the conversion in both the IMR and FBR while increasing the yield of acrolein (Figure 7a). Simultaneously, the COx yield percentage decreases with an increase in the HC/O2 ratio (Figure 7b). When the two reactor types are compared in Figure 7 it is shown that, at a given HC/O2 ratio, the acrolein and COx yield percentages are higher and lower by about 10%, respectively, in the IMR than in the FBR. The HC/O2 ratio also has a significant impact on the products as well. Increasing the HC/O2 ratio from 1:1 to 2:1 raises the yield percentage of acrolein by about 15% in both reactor types. The yield of COx is also lowered by about 15% in both the IMR and FBR as well. In summary, the partial oxidation product (acrolein) is enhanced in either reactor configuration when the partial pressure of oxygen is lowered (HC/O2 ratio increased). Not only does this suggest the use of higher HC/O2 ratios for the reaction in either configuration, but it also supports the argument that the IMR will outperform the FBR due to the inherent nature of the IMR to distribute the oxygen throughout


Figure 6. Obtaining the yield profiles of the single-bed FBR and IMR simulations for the POA reaction was done by increasing the temperature from 350 to 500 °C to raise the conversion. All other parameters were held constant. The simulation was run at two different residence times, 0.1 s (s) and 0.2 s (- - -), for both the FBR (lines with open symbols) and IMR (lines).

Figure 7. Obtaining the yield profiles of the single-bed FBR and IMR simulations for the POA reaction was done by increasing the temperature from 350 to 500 °C to raise the conversion. All other parameters were held constant. The simulation is run at two different HC/O2 ratios, 1:1 (- - -) and 2:1 (s), for both the FBR (lines with open symbols) and IMR (lines). The / represents data points where the FBR exhibited runaway.

the catalyst bed and thereby reduce the local oxygen partial pressure. The reduced local partial pressure of oxygen in combination with an unaltered hydrocarbon partial pressure further raises the local HC/O2 ratio and subsequently increases the yield of the desired POx product, acrolein. Therefore, maintaining higher HC/O2 ratios will increase the yield percentage of acrolein, decrease the yield of COx, and lower the overall reactant conversion. Simulation of the ODH of propane (Scheme 2) in the singlebed FBR and IMR results in similar behavior as the POA due to the identical reaction rate laws with first- and zero-order dependence in oxygen. This parametric sensitivity study on the one-bed IMR and FBR demonstrate that the IMR produces a higher percentage of desired partial oxidation products. Additionally, the IMR also produces fewer waste products in the form of CO and CO2. Dual-Bed: Effect of Catalyst Activity and Selectivity. The dual-bed reactor consists of two sequential catalyst beds in one

reactor. In the one-bed simulation, the benefits of the IMR over the FBR were evident. The dual-bed investigation utilizes the reactor operating parameters obtained for the single-bed reactor and focus on the effect that a hypothetical catalyst would have on the production of acrolein from propane in a two-bed reactor. The hypothetical ODH and POA catalysts were assumed to have a lower activation energy for either the ODH reaction (reaction 1) or the partial oxidation reaction (reaction 2) than those reported in the literature while holding all other activation energies for the reactions in Scheme 1 constant. The catalysts were altered consecutively to allow the effect of an improved catalyst to be more easily identifiable. With the addition of a second reactor bed, the dual-bed FBR (2B-FBR) and 2B-IMR can no longer be run under identical conditions due to the significant difference of reactant introduction between the two reactors. The 2B-IMR has separate oxygen feeds to each bed, while oxygen in the 2B-FBR is cofed with the hydrocarbon at

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Figure 8. Obtaining the yield profiles of the dual-bed FBR (open) and IMR (closed) simulations for the ODH reaction in the first bed and the POA reaction in the second was done by increasing the temperature from 475 to 550 °C in the first bed to raise the conversion. All other parameters were held constant with a HC/O2 ratio of 4:3 and the second bed temperature held at 450 °C. The simulation is run with the activation energy of the POx catalyst equal to 100%, 95%, 90%, and 85% of its initial value. The additional data points are partially optimized conditions for the 2B-IMR run in both the FBR and IMR. The labels in (b) are the same as in (a).

the reactor entrance. Therefore, an overall HC/O2 ratio was maintained and the oxygen was split between the beds in the 2B-IMR. As all reactions are sensitive to the activation energy, it was expected that decreasing the activation energy of reaction 1 or 2 in Scheme 1 would result in an increase in the yield of the partial oxidation product (acrolein). The activation energy for the ODH reaction (reaction 1 in Scheme 1) was reduced from its initial value in Table 1 by 5%, 10%, and 15%. These values were chosen to demonstrate a realistic improvement of the catalyst as opposed to higher, but less probable, improvements. For the hypothetical catalyst performance, the HC/O2 ratio was set at 4:3 and the second bed temperature was held constant at 450 °C. Figure 8 shows the results for the 2B-FBR and 2BIMR under these conditions. The closed points in Figure 8 correspond to the 2B-IMR, while the open points are for the 2B-FBR. The points at low acrolein yield (]) (Figure 8a) represent the full activation energy (100%) of the catalyst used in the ODH reaction bed. If the activation energy for this catalyst is improved by 5% (0) then both the conversion and yield percentage of acrolein are improved, by 2-3% and 3-4%, respectively. Further reduction of the activation energy of the catalyst continues to improve both conversion and yield percentage of acrolein in the two-bed reactor. Improvements to the ODH catalyst are improving the yield percentage of acrolein by increasing the selectivity of propane to propylene in the first bed. With a higher yield of propylene entering the second bed (POx reaction), more acrolein is formed over the POA catalyst and the overall yield percentage of acrolein is consequently increased. Another area of interest is the difference in conversion and yield of acrolein for the 2B-FBR and 2B-IMR. The 2BFBR maintains a higher conversion than the 2B-IMR through all the improvements to the ODH catalyst. As the catalyst is improved, the gap in conversion between the 2B-FBR and 2BIMR increases from about 3% for the original catalyst all the way to 10% for the ODH catalyst with an activation energy 15% lower (O). Although the conversion is higher in the 2BFBR, the 2B-IMR attains a yield of acrolein equal to or greater than the 2B-FBR. Higher yield percentage of acrolein at a lower conversion means that the initial reactant is used more efficiently

to create the selective product acrolein and there is less waste in the form of undesired byproducts (COx). Figure 8b shows that the 2B-IMR does indeed produce a lower yield percentage of COx than the 2B-FBR. Where the yield percentage of acrolein in the 2B-IMR was equal to or greater than that of the 2BFBR, the yield percentage of COx in the 2B-IMR is always less than that of the 2B-FBR. This means that the 2B-IMR produces less waste (CO, CO2) than the 2B-FBR. These results are expected, as a catalyst that favors a desired reaction should increase the yield of desired product and decrease the yield of waste products. An additional case was simulated for a slightly optimized system. It was assumed that the HC/O2 ratio is almost 3:1 and the activation energy is 90% of the initial value. Under these conditions, the yield to acrolein should increase due to the increase in the HC/O2 ratio from the prior simulation (4:3). The lower activation energy should also increase the yield of acrolein. The results for this slightly optimized case (- and +) are consistent with the prior results as seen in Figure 8. This further strengthens the assertion that the 2B-IMR is a more efficient reactor than the 2B-FBR for the propane to acrolein reaction pathway (Scheme 1). In addition to improving the ODH catalyst, improvements to the POA catalyst in the second bed were simulated. The activation energy of the POA catalyst for reaction 2 in Scheme 1 was lowered from its initial value in Table 1 by 5% and 10%. As seen in Figure 9, improvements to the POA catalyst do little to change the overall conversion as conversion is based upon the initial reactant, propane, entering the reactor, and the POA catalyst is not very effective in promoting this reaction (reaction 1). However, lowering the activation energy of the POA catalyst does increase the yield of the desired product, acrolein, through an increase in the conversion of propylene in the second reaction bed. Figure 9 shows the results for the simulation of an improved catalyst in the second bed of the 2B-IMR and 2B-FBR. The first bed is held at 525 °C, and a 2:1 HC/O2 ratio is utilized. The solid points correspond to the 2B-IMR, whereas the open points are for the 2B-FBR. Consistent with the prior results, the 2B-FBR maintains a higher conversion than the 2B-IMR. The yield percentage of acrolein is higher in the 2B-IMR than


Figure 9. Obtaining the yield profiles of the dual-bed FBR (open) and IMR (closed) simulations for the ODH reaction in the first bed and the POA reaction in the second was done by increasing the temperature from 375 to 450 °C in the second bed to raise the conversion. All other parameters were held constant with a HC/O2 ratio of 2:1 and the first bed temperature held at 525 °C. The simulation is run with the activation energy of the POA catalyst equal to 100%, 95%, and 90% of its initial value.

the 2B-FBR for each of the hypothetical POA catalysts simulated. Although the yield improvements for the 2B-IMR are not significantly higher than those of the 2B-FBR, the 2BIMR has a few adjustable parameters, whereas the second reactor bed temperature is the only adjustment possible in the 2B-FBR. In addition to the temperature, the HC/O2 ratio can be altered through the feed to the membrane of the second bed. From the earlier simulations (Figure 7), the HC/O2 ratio was shown to have a significant impact on the yield of the desired product and the 2B-IMR allows for optimizing this ratio in each bed. In summary, the 2B-IMR increases the yield percentage of the desired product over that produced by the 2B-FBR. The 2BIMR creates this improvement at a lower reactant conversion thereby reducing the production of undesired side products (COx). The ODH of propane is the critical step in the propane to acrolein reaction pathway. Without a high yield of propylene in the first bed, the yield of acrolein in the second bed is limited. The catalyst in the second bed only aids in increasing the yield toward acrolein. The catalysts in each bed must be optimized to obtain the highest possible yield of the selective oxidation product. Finally, with future improvements to the two reactor models the impact of the 2B-IMR should be further increased. Gasphase reactions between the catalyst beds introduce the possibility for further product enhancement. Sinev et al.30 proposed in their investigation of the propane to acrolein reaction pathway that there is an intermediate formed in the propane ODH reaction that is more selective on the acrolein catalyst than propylene. This improvement of selectivity toward acrolein is made by merely enabling the intermediate to be present at the second catalyst bed. Other improvements to the two reactor models, such as lifting some of the simplifying assumptions, will serve to aid in the design of an optimal 2B-FBR and 2B-IMR.

a distributed reactant, in this case oxygen. The simulations presented in this work all demonstrate the advantage of the 2BIMR. In all cases, the membrane reactor results in higher yield percentages toward the desired product than the 2B-FBR. Simulating improvements to both the ODH and POA catalysts led to improved yields of the desired product acrolein and lowered the overall yield of COx. The 2B-IMR is also less wasteful than the 2B-FBR by reducing the COx yield. For this reason, it is of interest to continue the investigation of the potential of membranes in the production of acrolein. Although this is a simplified one-dimensional simulation of the ODH and POA reaction networks for the partial oxidation of propane to acrolein, it demonstrates the strong prospect for membranes in this functionality. The next steps include improving the reaction network to include all of the reactions that may occur in the catalyst bed, including those in the gas phase between the beds or those that occur with intermediates formed in reaction. The reactor models can also be increased from one-dimensional to two-dimensional, allowing for radial temperature and concentration profiles to be studied. Experiments are necessary to obtain more exact kinetic parameters for the catalysts used in this reaction system. The availability of accurate experiments leading to well-defined kinetics will allow for the development of a more complex and accurate model. This initial theoretical investigation has demonstrated that the 2B-IMR will provide a higher yield of desired POx products than the 2B-FBR. The improved efficiency of the 2B-IMR makes it an attractive reactor design to not only increase yield but decrease waste as well, leading to both economical and environmental benefits. To take full advantage of these benefits from the 2B-IMR, it is necessary to design highly active and selective catalysts for both the ODH of propane and POA reaction pathways. Acknowledgment We gratefully acknowledge the support from the NSF Grant CTS0224435 for funding this work. Nomenclature ac ) cross-sectional area of the reactor (m2) Cp ) heat capacity (J mol-1 K-1) F ) molar flow rate in FBR or shell side of IMR (mol s-1) n ) molar flow rate through the membrane tube (mol s-1) r ) rate of reaction (mol s-1 g-1) T ) temperature (K) Ua ) overall heat transfer coefficient (J m-3 s-1 K-1) z ) length of catalyst bed (m) Greek Letters ∆H ) heat of reaction (J mol-1) υ ) reaction equation coefficients Subscripts a ) ambient i ) component in the gas stream j ) reaction number k ) catalyst bed m ) membrane

Conclusions

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ReceiVed for reView September 8, 2005 ReVised manuscript receiVed January 11, 2006 Accepted February 2, 2006 IE051009I

Yield Improvements in Membrane Reactors for Partial Oxidation

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