ARTICLE pubs.acs.org/IECR
Multicomponent Dosing in Membrane Reactors Including Recycling— Concept and Demonstration for the Oxidative Dehydrogenation of Propane Christof Hamel,*,†,‡ Tanya Wolff,‡ Pushpavanam Subramaniam,§ and Andreas Seidel-Morgenstern†,‡ †
Institute of Process Engineering, Otto von Guericke University, Universit€atsplatz 2, D-39106 Magdeburg, Germany Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstr. 1, D-39106 Magdeburg, Germany § Department of Chemical Engineering, Indian Institute of Technology, Madras, 600036 Chennai, India ‡
ABSTRACT: The potential of a multicomponent distributed reactant dosing via membranes for enhancing selectivity and yield in a network of series and parallel reactions is investigated in this work. In a preliminary theoretical feasibility study, the optimal local concentrations of the various reactants used to improve the selectivity and conversion of the desired intermediate product were determined. These results were extended to a case study. For this examination, the oxidative dehydrogenation of propane to the kinetically limited intermediate product propylene on a VOx/γ-Al2O3 catalyst was considered. The kinetic equations and parameters were first estimated. It is shown that, compared to a single-component dosing of O2 via membranes, the yield of propylene can be almost trebled via a controlled simultaneous dosing of oxygen and propane.
1. INTRODUCTION Currently, intensive research in chemical process engineering has focused on improving the performance of simultaneous parallel-series reactions. Here, the objective is to maximize the selectivity and yield, with respect to desired but kinetically limited intermediate products. In particular, for the selective oxidations of short-chain hydrocarbons, the yield of precious activated olefins is limited. Because of the relatively high local concentrations of oxygen in the widely applied conventional fixed-bed reactors (FBRs) operating in a co-feed mode, the thermodynamically more favored CO2 is formed in large amounts.1 Improvements in selectivity of oxidation reactions can be achieved by means of an optimal discrete or continuous dosing of oxygen into catalytic fixed beds. Here, dense or porous membranes can act as distributors.2,3 So far, the possibility of increasing yields by dosing reactant mixtures in packed-bed membrane reactors (PBMRs) with an internal recycling has not been considered. Such dosing exploits the influence of local concentrations on the reaction course. This can be illustrated considering the differential selectivity of a simple scheme of two consecutive reactions: r1
A þ B f D ðdesiredÞ
It is often found that the reaction orders satisfy β1 < β2.4 Here, the selectivity of the desired intermediate product D can be enhanced by axial distribution and reducing the local concentration of B (e.g., oxygen).57 Based on distributing just B, various possibilities have been suggested that exploit different porous or nonporous membranes and this single-component dosing membrane reactor concept has been identified as promising.1,6,814 Equation 3 further shows that a high axial concentration of the precious reactant A is favorable, with respect to both selectivity and conversion. This can be supported by an additional distributed dosing of A. Of course, high conversion can be only realized by recycling unconverted A through a subsequent separation unit and recirculation. This suggests that the joint dosing of B and recycled A should result in significant improvements in selectivity and yield, with respect to the desired intermediate product D. This multicomponent dosing membrane reactor concept is investigated first theoretically for a parallel-series reaction network. The insight gain is used to analyze, both theoretically and experimentally, the industrially interesting oxidative dehydrogenation of propane to propylene.
r2
D þ B f U ðundesiredÞ Assuming power-law kinetics, the rates of these two reactions can be described by r1 ¼ k1 xA xB β1
ð1Þ
r2 ¼ k2 xD xB β2
ð2Þ
The differential selectivity (dSD) of the desired intermediate D in a simple series reaction can be expressed by eq 3: r1 r2 k2 xD ¼ 1 ð3Þ x B β2 β1 dSD ¼ r1 k1 xA r 2011 American Chemical Society
2. THEORETICAL ANALYSIS OF MULTICOMPONENT REACTANT AXIAL DOSING Principle of Multicomponent Distributed Axial Dosing, Including Recycling. The principle of multicomponent dosing Special Issue: Ananth Issue Received: January 24, 2011 Accepted: May 19, 2011 Revised: May 5, 2011 Published: May 19, 2011 12895
dx.doi.org/10.1021/ie2001692 | Ind. Eng. Chem. Res. 2011, 50, 12895–12903
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Figure 1. Principle of a multicomponent dosing including internal recycling of Ass via membranes. (Note: ts = tube side, ss = shell side.)
including an internal recycling of component A for the two single-series reactions introduced above is illustrated in Figure 1. In the membrane reactor tube (tube side = ts) filled with catalyst particles, the precious reactant A is fed directly (Ats,0). The second reactant B is distributed uniformly via the shell side of the membrane (Bss,0), which is closed at the end (dead-end configuration). In order to realize a multicomponent dosing, an additional amount of A is also distributed via the shell side of the membrane (Ass). The desired product (D) and unconverted reactants A and B as well as U leave the membrane reactor. In a subsequent standard separation unit, which can be, for example, also a membrane-based separation unit, a fixed amount of Ass is separated completely and recycled into the membrane reactor in a closed loop and distributed again. Thus, only an amount Aout leaves the overall unit, which corresponds to Ats,out MR Ass (i.e., the amount of Ass dosed in the membrane reactor is equivalent to the amount of Ass separated afterward). For the following computational study illustrating the principle of a multicomponent dosing including an internal recycling, a perfect separation between D and A is assumed. Reaction Network and Reactor Model. To study the various dosing strategies introduced, an isothermal, isobaric, plug-flow membrane reactor model was applied and implemented in MATLAB.5 The model describing the reaction/catalyst zone placed in the tube of the membrane allows the continuous dosing of reactants in a discrete or distributed manner. A broad parameter range can be covered for the realized dead-end configuration. Based on this configuration and the asymmetric ceramic membranes used in the experimental part in section 2, a detailed modeling of the membrane itself is not necessary.15 Assuming the occurrence of M reactions with rates rj and under steady-state conditions, the mass balance for component i is given as M d_ni ¼ AZtotal νi, j rj þ PZtotal ðji ðξÞ þ jrecycle, i ðξÞÞ dξ j¼1
∑
ðfor i ¼ 1,..., N; j ¼ 1,..., M; ξ ¼ z=Ztotal Þ
ð4Þ
subject to the following boundary conditions: ðRecycling and continuous feeding in a PBMRÞ: n_ i ðξ ¼ 0Þ ¼ n_ 0i ,
jrecycle, i 6¼ 0
ð5Þ
ðRecycling and discrete feeding in a FBRÞ: n_ i ðξ ¼ 0Þ ¼ n_ 0i þ n_ recycle, i ,
jrecycle, i ¼ 0
ð6Þ
To examine the potential of the different dosing strategies, a simple reaction network was considered, consisting of a target reaction delivering the desired product D (with rate r1) and two undesired consecutive and parallel reactions (with rates r2 and r3) (see Figure 2).
Figure 2. Studied parallel-series reaction network.
The kinetics of these reactions is assumed to follow a powerlaw dependency: r1 ¼ k1 xA R1 xB β1
ð7Þ
r2 ¼ k2 xD γ1 xB β2
ð8Þ
r3 ¼ k3 xA R2 xB β3
ð9Þ
This network can be representative of the oxidation of hydrocarbons where component B is oxygen. The reaction scheme for the oxidative dehydrogenation (ODH) of propane to propylene discussed later is more complex (section 2) but can be simplified to the scheme illustrated in Figure 2. Using the reactor model presented above and the rate laws specified in eqs 79, a feasibility study was performed. Selected results will be reported below for the case where Ats is fed exclusively at the reactor inlet and Ass, as well as B, are distributed over the reactor wall. Preliminary Feasibility Study. Figure 3 illustrates the molar flux profiles along the axis of the membrane reactor for three different dosing and recycling strategies. The first strategy has Ats,0 = Bss,0, Ass = 0 (i.e., single-component dosing of B alone). In the second strategy, both A and B are dosed and we take Ats,0 = Bss,0, Ass = 0.5Ats,0. The third strategy is the conventional fixedbed reactor operating in a co-feed mode (Ats,0 = B ts,0 fed discretely at the reactor inlet, Ass = 0). Note that, without the occurrence of chemical reactions, the molar fluxes at the reactor outlet of FBR and PBMR for Ass = 0 are identical for all investigated dosing strategies in this work. In order to fulfill the criterion on the reaction orders β1 < β2 given in ref 6, we choose β1 = 1 and β2 = β3 = 2. The concentration profiles of all components in a packed-bed membrane reactor (PBMR) with single or multicomponent dosing differ from those in a conventional fixed-bed reactor (FBR), with respect to local molar fluxes and total molar flux (ntotal). Based on the separated reactant feeding in a membrane reactor, a higher local concentration of reactant A and a lower local concentration of B can be observed. This effect is more pronounced in the case of multicomponent dosing when Bss,0 and an additional Ass are fed via the membrane. Hence, both concepts are promising to realize a high differential selectivity. Hereby, the highest molar outlet flux of the desired component D and the lowest molar flux of the undesired series reaction product U can be indeed obtained using multicomponent dosing. We see that, if, in addition to B and A entering the tube as fresh feed Ats,0, A is distributed via the membrane (Ass), the decrease of the concentration profile of A that is caused by the reaction can be reduced. Consequently, a higher reaction rate (r1) and, therefore, a higher conversion of A can be expected, respectively. 12896
dx.doi.org/10.1021/ie2001692 |Ind. Eng. Chem. Res. 2011, 50, 12895–12903
Industrial & Engineering Chemistry Research
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Figure 3. Typical molar and total flow rates for FBR and PBMR. Parameters: Ats,0 = Bss,0 = 1.0 mol/s, Ass = 0 or 0.5 mol/s; k1 = k2; k3 = 0.5k1; R1 = R3 = γ = β1 = 1; β2 = β3 = 2.
Figure 5. Continuous dosing in a PBMR and discrete dosing in a FBR of Ass after recycling.
Figure 4. Differential selectivity for three cases of distributed dosing of A and B via the membrane. (Parameters: Ats,0 = Bss,0 = 1 mol/s, Ass = 0, 0.5, and 1.0 mol/s; k1 = k2; k3 = 0.5k1; R1 = R3 = γ = β1 = 1; β2 = β3 = 2.)
The differential selectivity (dS) of the considered parallelseries reaction, with respect to D, is
dSD ¼
r1 r2 r1 þ r3
γ1 ! k2 x D β2 β1 r2 1 R1 xB 1 k x 1 A r1 ¼ ¼ r3 k3 R2 R1 β3 β1 1þ 1þ xB x r1 k1 A ð10Þ
Based on the obtained local concentration of B (see Figure 3), a high selectivity of intermediate D can be expected for the parameter constellation β1 < β2. When A is dosed via the membrane, the higher concentration of A leads to an enhanced differential selectivity of intermediate D. Figure 4 depicts this result of the multicomponent dosing clearly. Thus, as the amount of Ass additionally distributed via the membrane increases, the differential selectivity of D is significantly enhanced and almost constant along the reactor for the simulation conditions selected (Ats = B, Ass = 0, 0.5, and 1.0 mol/s). These were chosen
arbitrarily and do not correspond to an optimum, with respect to the recycling ratio. Next, the potential of a fixed-bed reactor with recycling of A was considered. For a fair comparison, the amount of Ass recycled in both cases is maintained the same. This was investigated by simulations using the simplified reactor model and the boundary conditions given in eqs 5 and 6. The only difference is the feeding of Ass is performed continuously in a membrane reactor and discretely at the reactor inlet in the case of the fixed-bed reactor, as illustrated in Figure 5. The amount of Ass additionally dosed is the same in both concepts (_nrecycle,i = jrecycle,iAMembrane). The simulation results for the performance parameters (i.e., the conversion of A, the selectivity and yield of the desired intermediate D for the continuous and discrete dosing of Ass) are summarized in Table 1. As can be recognized from Table 1, the membrane reactor with a distributed dosing of B (PBMR-1B) outperforms the conventional FBR. The yield of D could be enhanced from 53% to 67%, based on a higher conversion of A and higher selectivity of D. In contrast to the single-component dosing via the membrane (only B), a binary-component mixture of B and A through recycling of Ass revealed a significant increase in the selectivity of B (from 88% to 95%), the conversion (from 76% to 91%), and the yield of the desired D (from 67% to 86%) for multicomponent dosing with Ass = 1.0 mol/s. Thus, the process could be significantly intensified. A similar trend can be observed for the FBR with an increasing amount of Ass. Here, for Ass = 1.0 mol/s, a maximal yield of D was only 63%. Thus, under the same conditions (Ass = 1.0 mol/s), the membrane reactor with a multicomponent distributed dosing of A and B results in an increase in the yield of the desired intermediate D (from 63% to 86%). This corresponds to an enhancement of a factor of ∼1.4. 12897
dx.doi.org/10.1021/ie2001692 |Ind. Eng. Chem. Res. 2011, 50, 12895–12903
Industrial & Engineering Chemistry Research
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Table 1. Performance Parameters for a Continuous Dosing in a PBMR and Discrete Dosing in a FBR of Assa conversion
selectivity
yield
of A (%)
of D (%)
of D (%)
Continuous Dosing PBMR PBMR — Ass/Ats,0 = 0 PBMR — Ass/Ats,0 = 0.5
76 86
88 93
67 80
PBMR — Ass/Ats,0 = 1.0
91
95
86
Discrete Dosing FBR FBR — Ass/Ats,0 = 0
75
70
53
FBR — Ass/Ats,0 = 0.5
80
75
60
FBR — Ass/Ats,0 = 1.0
81
77
63
Figure 6. Reaction network for the oxidative dehydrogenation (ODH) of propane to propylene.
a Simulation parameters: Ats,0 = Bss,0 = 1 mol/s; Ass = 0, 0.5, and 1.0 mol/ s; k1 = k2; k3 = 0.5k1; R1 = R3 = γ = β1 = 1; β2 = β3 = 2.
The results of the theoretical study performed indicate a significant potential for a multicomponent distributed dosing with an internal reactant recycling in membrane reactors, compared to a fixed-bed reactor. This was verified by the results of the simulations carried out on a simplified parallel-series reaction network. Based on these results, the oxidative dehydrogenation (ODH) of propane to propylene (limited intermediate) was investigated by means of the present dosing strategies, to validate the results obtained in the preliminary feasibility study.
3. EXPERIMENTAL ANALYSIS: OXIDATIVE DEHYDROGENATION (ODH) OF PROPANE In the second part of this contribution, an evaluation of the trends identified in the theoretical studies is done using selected experiments in a pilot-scale fixed-bed reactor and a membrane reactor. The industrially relevant oxidative dehydrogenation (ODH) of propane to propylene was chosen for the investigations. This reaction satisfies the required conditions on the reaction orders6,7 motivating a distributed dosing of oxygen. Catalyst. For the experimental investigation, a VOx/ γ-Al2O3 catalyst was used, which was prepared via the wet impregnation of 1-mm γ-Al2O3 spheres with vanadyl acetylacetonate in acetone.16 The vanadium content of the catalyst was 1.4%, and its specific surface area was 158 m2/g (BET). The color of the fresh catalyst was yellow, indicating that vanadium was mainly in the pentavalent (þ5) oxidation state.17 After the measurements, the catalyst color had changed to a light blue-green color. This can be attributed to a significant reduction of V5þ to V4þ, which is the species responsible for high ODH selectivity.17,18 Reaction Network and Kinetics. The reaction network postulated for the ODH of propane to propylene is illustrated in Figure 6. The ODH of propane is advantageous because the reaction is not equilibrium-limited. It is also preferable from the point of view of energy consumption, in contrast to nonoxidative processes. The reaction network consists of several parallel and series reactions, comparable to the theoretical network summarized in Figure 2 and eqs 1115. The reaction can be realized at temperatures
