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KINETICS, CATALYSIS, AND REACTION ENGINEERING Experimental Study on the Oxidation of Butane to Maleic Anhydride in a Two-Zone Fluidized Bed Reactor Oscar Rubio, Reyes Mallada, Javier Herguido, and Miguel Mene´ ndez* Department of Chemical Engineering, University of Zaragoza, 50009 Zaragoza, Spain
The oxidation of butane to maleic anhydride in a new reactor is described. This reactor is a two-zone fluidized bed, where butane and oxygen are fed at different levels, providing separated zones for catalyst oxidation and for butane reaction/catalyst reduction. The solid mixing characteristic of fluidized beds provides the circulation of solid, in such way that the fed oxygen enters to the reducing zone as part of the solid structure; thus, the butane reacts with lattice oxygen in the catalyst. The effect of some operating variables on conversion and selectivity is described. High yields to maleic anhydride may be achieved by means of this operation system. The oxygen and butane concentrations at a point below the butane entry are also reported. This is to ensure the operation is outside flammability limits and to quantify the backmixing of the gas in the bed. Therefore, this system allows feeding butane concentrations higher than those industrially employed in fixed bed reactors but avoids the formation of explosive mixtures. The yield to maleic anhydride obtained with high butane concentrations is similar to that in other reactors where the catalyst operates in reduction-oxidation cycles but avoids an unsteadystate operation or the use of a circulating fluidized bed. Introduction The selective oxidation of saturated hydrocarbons is a difficult challenge. One of the main problems is that the reactants and partial oxidation products can undergo deep oxidation to carbon oxides, resulting in low selectivities toward the desired product. Another main concern is related to the safety of the process: in this kind of reaction the formation of explosive mixtures is possible, since the hydrocarbon and oxygen are usually mixed before entering the reaction vessel. One tool to avoid or at least to mitigate these problems is a suitable reactor design. For example, membrane reactors have been widely studied in several laboratories as a way to improve the performance and the safety in catalytic oxidations. Several recent reviews discuss the potentials and problems of these reactors.1-4 Another alternative reactor, proposed recently, is a two-zone fluidized bed reactor (TZFBR), characterized by the separation of the oxidizing and reducing zones,5-8 which could improve selectivity with catalysts that have a redox cycle. The operation procedure of such a reactor is shown in Figure 1: A stream containing oxygen and an inert gas is fed to the bottom of the reactor; as this stream rises, the oxygen is consumed, oxidizing the catalyst. Depending on the oxygen uptake of the catalyst and the oxygen concentration in the feed, all the oxygen could be depleted in this oxidizing zone, with only the inert gas reaching the point where the hydrocarbon is introduced, at some intermediate height, above the distributing plate. The hydrocarbon is oxidized to the * Corresponding author. Fax: +34-976 762142. E-mail:
[email protected].
desired product, in the reducing zone, by means of the oxygen existing in the catalyst lattice, and therefore the catalyst is reduced. To maintain the performance of the reactor, the circulation of the catalyst between the oxidizing and reducing zones is essential. This circulation is achieved thanks to the high solid mixing characteristic of fluidized beds. A similar system, where oxygen and hydrocarbon are fed separately to a fluidized bed was first described in an old patent,9 but no details of the system were given. This reactor system was first explored for the oxidative coupling of methane, using a Mn/P/SiO2 catalyst.5 In this reaction and with this catalyst, the possibility of achieving the separation of the oxidizing and reducing zones was demonstrated experimentally, but no improvement in yield to the desired products was observed. The TZFBR was also studied for the oxidative dehydrogenation of butane, using V/MgO catalyst.6-7 In this reaction, the yield to olefins achievable for a given butane conversion increases by using the TZFBR. The most remarkable change corresponds to the butadiene yield, that increases by a factor of 3 compared with the yield obtained at the same butane conversion level in a conventional fluidized bed reactor (FBR). Other catalysts have been studied, such as Mo/MgO,8 which also results in a clear separation of the two zones in the TZFBR and a higher selectivity to butadiene in the TZFBR than the FBR, although it was deactivated in some tens of hours. The feasibility of the TZFBR scaleup has been checked in reactors up to 9 cm diameter.10 Although oxidative dehydrogenation of butane is a big challenge for the catalytic research community, it is not economically viable for now. The only industrial process where butane is catalytically oxidized to more valuable
10.1021/ie020097t CCC: $22.00 © 2002 American Chemical Society Published on Web 09/20/2002
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Figure 1. Experimental setup. Reactor zone enlarged.
products on a large scale is in maleic anhydride production. The catalyst employed for this reaction is VPO, and the reactors used in the industry are fixed beds,11 fluidized bed reactors, e.g., the ALMA process,12-14 and the circulating fluidized bed reactor (CFBR), recently developed.15-17 In the last case there are two zones, oxidizing and reducing, that are physically separated; the reaction takes place in a riser reactor, and then the catalyst is transported to a fluidized bed reactor where it is oxidized. Different models have been developed for this kind of reactor.18-19 Although maleic anhydride production by butane oxidation is a well-established process, improvements in yield and other operational advantages are still possible, and thus the exploration of a new reactor for this reaction is of interest. In the proposed gas-solid contacting scheme the oxidation state of the catalyst in the two zones, and then the selectivity of the process to maleic anhydride, can be controlled by modifying the working variables (relative height of each zone, butane and oxygen flow rates, ...). Otherwise, the conventional reactors, catalytic fixed bed or fluidized bed with cofeeding of reactants, suffer from the limit in the butane concentration in the feed imposed by the need to avoid explosive mixtures. The feed is butane diluted in air, with a butane concentration smaller than 1.8% in fixed beds or about 4% in fluidized bed reactors.20 Due to this high dilution, the maleic anhydride concentration in the exit gases is only around 1% and large volumes of gases have to be processed. Therefore, any system that allows the obtaining of a more concentrated product stream would be advantageous, since it would reduce the costs of compression and the separation system for MA recovery, and the volume of the reactor. This paper focuses on the use of the TZFBR for butane oxidation to maleic anhydride. Experimental System The reaction system is similar to that one employed in a previous work7 for the oxidative dehydrogenation
of butane and is shown in Figure 1. The fluidized bed reactor is quartz-made, with a quartz porous plate as gas distributor and with an entry of butane at a central point (h ) hc) of the catalyst bed. Oxygen and helium are fed to the bottom of the reactor (h ) 0), providing the required fluidizing gas. A gas probe is employed to analyze the gas composition at a point located 0.5 cm below the butane entry point (h ) h*). This is useful as a measure of the amount of oxygen that is being transported by the catalyst, and also to check the extent of the butane backmixing. The bed diameter is 3 cm, the weight of catalyst, unless otherwise indicated, is 53 g, and this results in a bed height, during fluidization, of ca. h0 ) 10 cm. The VPO (mainly vanadyl pyrophosphate) catalyst has a 74 µm mean particle diameter and a particle density of 1.9 g‚cm-3. In the catalyst preparation, silica has been used to improve its attrition resistance. The feed flow (He + O2) is typically 83 cm3 (STP)/min and corresponds to 1.5 times the minimum fluidization conditions. In some experiments a thermocouple was moved along the bed, to check its isothermicity. A near-constant temperature was found in the entire bed, despite the highly exothermal reaction. This showing that a good fluidization was obtained. The butane entry point (hc) is 5 cm above the porous plate. Exit gases are analyzed by on-line gas chromatography using an HP 5890 Series II gas chromatograph with FID and TCD detectors, equipped with three capillary columns: Poraplot U, 53 µm diameter and 25 m long, Molecular Sieve 5 Å, 53 µm diameter and 30 m long, and HP-Plot/Al2O3, 53 µm diameter and 50 m long. The analysis was carried out using a 30 cm3 (STP)‚min-1 flow of He as carrier. The temperature program starts at 50 °C, during 6 min, then rises at 8 °C‚min-1 up to 190 °C. The lines from the reactor to the gas chromatograph are heated to avoid the condensation of maleic anhydride. The exit gas flow rate was measured, which allowed the calculation of carbon mass balances, which
Ind. Eng. Chem. Res., Vol. 41, No. 21, 2002 5183 Table 1
a
variable
ref value
T, °C ur Pox, % Roh h0, cm hc, cm
400 1.5 20 5 10a 5
other studied values 375, 425 2.0, 3.5 2.5, 3.3, 4.2, 10.0 14b 8
W0 ) 53 g of catalyst. b W0 ) 73 g of catalyst.
were better than (5% for all the experiments reported in this work, and usually better than (2%. The reactor was surrounded by a shell in order to protect the operator if an explosion was produced, since in some experiments with cofeeding of reactants the feed was inside the flamability limits. Anyway, the amount of butane inside the reactor at a given time is very small, less than 10 mg.
Figure 2. Conversion of butane and selectivity to maleic anhydride as a function of time. Solid symbols correspond to cofeeding and open symbols to separated feed. Roh ) 10.
Experimental Results Preliminary tests in the absence of reaction were used to determine the fluidization characteristics of the catalyst used. Its minimum fluidization velocity, umf, was determined with He at 400 °C from the bed pressure drop vs flow rate curve on decreasing the gas flow rate, the intercept of this line with the maximum theoretical pressure drop gives umf. A minimum fluidization flow rate of 100 cm3 (STP)‚min-1 was estimated in a 4 cm diameter quartz reactor, that supposes umf ) 19 cm/min under bed conditions. In this work, the gas velocity u was calculated as the ratio of the actual flows of He plus O2 at the distributor plate (measured at the reaction temperature) to the cross-sectional area of the reactor. In addition to the bed temperature T, and the relative velocity ur, defined as u/umf, the main reactor-related variables were the ratio of the molar feed rates of oxygen and hydrocarbon, Roh, the percentage of oxygen in the reactor feed Pox (irrespective of whether a conventional or a two-zone fluidized bed was used), the total bed height, h0, and the height at which the hydrocarbon was introduced, hc. Table 1 shows the reference values used in this study for each of these variables. Initial experiments were carried out in order to know the stability of the catalyst in the system. Figure 2 shows the experimental values of butane conversion and maleic anhydride selectivity for cofeed (solid symbols) and separated feed (open symbols) over time for a given set of operating conditions (reference conditions except Roh ) 10) with a catalyst previously activated and used in different previous conditions. In both cases the selectivity and conversion levels reach a steady state after 1 h of reaction. From now on the results reported will be those obtained after 4 h of reaction. The results obtained in a conventional fluidized bed reactor, i.e., cofeeding the reactants, are compared in Figure 3 with those obtained in the TZFBR, over a wide range of oxygen/hydrocarbon ratios. The performance of the TZFBR is superior in all the cases studied, giving larger selectivity to maleic anhydride (MA). Although the butane conversion is smaller, the resulting yield is higher in the TZFBR. The selectivities in the TZFBR are higher since, in this case, the catalyst previously oxidized in the lower zone moves to the upper part of the bed, the reduction zone, where butane reacts with the oxygen in the catalyst lattice, with a lower oxygen concentration in
Figure 3. Conversions and selectivity to MA vs ration oxygen/ hydrocarbon in a TZFBR and in a conventional fluidized bed reactor with cofeeding.
the gas phase. The reaction with the lattice oxygen provides high selectivity to MA, according to the results obtained in the circulating fluidized bed reactor15-17and pulse experiments.21-22 The consumption of oxygen by the catalyst is never complete. This means that there is always a minimum oxygen concentration necessary for maintaining an optimum oxidation state.23 To explain the results in the conventional fluidized bed reactor, with cofeeding, the different oxygen/butane ratios should be distinguished. In the case of a butane concentration of 6% (Roh ) 3.3), there is a total consumption of oxygen. However, the butane conversion is not complete which denotes a highly reducing atmosphere in the upper part of the bed. This implies a low selectivity to MA, only 3%, in agreement with the findings of several authors when total oxygen consumption was produced in the reactor.22-24 The analysis of the products reveals a high concentration of cracking products, mainly ethylene, with a selectivity (not shown in the graph) around 4%.24-26 With the following oxygen/ hydrocarbon ratio (Roh ) 5), corresponding to a 4% butane concentration, the selectivity to maleic anhydride increases and oxygen and butane conversions are
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Figure 4. Concentrations of butane and oxygen at a height 0.5 cm below the feed of butane vs oxygen/hydrocarbon ratio. Key: solid symbols, W0 ) 53 g; open symbols, W0 ) 73 g.
complete, and since there is no butane, the catalyst is not as reduced as in the previous case but does not reach the optimum oxidation state ca. 4-4.2. With the last oxygen/butane ratio (Roh ) 10), the catalyst is always in an oxidizing atmosphere. Since in the FBR the reaction takes place from the entrance of the bed, there is a higher residence time in the FBR compared with the TZFBR, and the products undergo deep oxidation to CO and CO2, decreasing the selectivity to MA. From the above results the superiority of the TZFBR vs the conventional fluidized bed reactor is clear. The following paragraphs will discuss the effect of different operating conditions in the two-zone fluidized bed reactor.
Figure 5. Selectivity to maleic anhydride vs butane conversion for two different heights of bed (h0) and different points of butane feed (hc). T ) 400 °C; ur ) 1.5.
Effect of Operation Variables in the TZFBR Oxygen/Butane Ratio, Roh. The oxygen/butane ratio was varied from 2.5 to 10, by varying the butane concentration from 8 to 2% and maintaining the oxygen concentration constant at 20%. The results are shown in Figure 3 (open symbols), together with those using the FBR under the same operating conditions. It is worth remarking that, throughout this article, concentration values of the TZFBR feed are those that would be obtained if the feed was premixed. As may be expected, the butane conversion increases and oxygen conversion decreases as the Roh increases. The selectivity to MA shows a maximum at an oxygen-to-butane ratio of 4. Looking at the production of maleic anhydride, measured by space time yield (STY), a maximum (1.17 × 10-3 mmol MA/min‚g) was obtained for Roh ) 3.33, corresponding to a MA concentration of 1.83%. At a high butane concentration, 8% (Roh ) 2.5), the selectivity decreases since oxygen conversion is almost 100%, and there is a small concentration of oxygen, ca. 1%, that is below the minimum amount necessary in order to keep the surface oxidized.23 In this case the catalyst is in a highly reducing atmosphere, detrimental for the selectivity as is discussed above. Zone Separation, Effect of hc. To check the separation of the two zones, reduction and oxidation, the concentrations of oxygen and butane were measured, using a gas probe located at a point 0.5 cm below the introduction of the hydrocarbon. Figure 4 shows the results for two different sets of experiments, with different locations of the butane entry point and total amount of catalyst (i.e., varying h0 and hc). In both cases the concentration of butane below the point of its introduction is low, less than 10% of the butane concentration in the feed for all the operating conditions, indicating a low degree of gas backmixing. The concen-
Figure 6. Conversion of butane and selectivity to maleic anhydride vs temperature, for different Roh. h0 ) 14 cm, hc ) 8 cm. Keys: molar butane/oxygen/helium ratio in the feed.
tration of oxygen at this point gives a measure of the amount of oxygen transported by the catalyst: taking into account that the inlet concentration is 20%, the catalyst transports 20-35% of the oxygen fed. By increasing the height of the butane entry point (hc) and the amount of catalyst, the amount of oxygen captured by the catalyst increases. Figure 5 shows the selectivity to maleic anhydride as a function of the butane conversion for two different sets of experiments, for different catalyst weights and butane entry points. In the first set of experiments 53 g of catalyst were used, which resulted in a bed height around 10 cm, and the hydrocarbon was fed at 5 cm above the distributor plate. In the second case, with 73 g of catalyst and a bed height of 14 cm, the butane feed point was 8 cm above the distributor plate. In the experiment with a higher oxidation zone, hc ) 8 cm, the total oxygen uptake is higher (Figure 4). On the other hand in the experiments with h0 ) 14 cm, the reaction zone is 17% larger, and the selectivity to maleic anhydride decreases. We can explain this by assuming that in both cases the catalyst is oxidized when it reaches the reaction zone, and in the experiments with a larger reaction zone, the catalyst is more exhausted, meaning that it reaches a lower oxidation state, resulting in lower selectivity. Effect of Temperature. The effect of temperature is shown in Figure 6, where it is shown that by increasing the temperature the butane conversion increases. From the data in Figure 6 it may be inferred that for the same level of conversion the selectivity to
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similar trend to those with the TZFBR. However is necessary to take into account that this system requires two reactors, with a complex circulation system, as opposed to one single vessel in the case of the TZFBR, where oxidation and reduction take place in the same reactor. Conclusions
Figure 7. Effect of the gas velocity on the reactor performance.
Figure 8. Comparison of our results with other reactors from the literature.
MA decreases by increasing the temperature. This suggests that the activation energy for the main reaction, butane to maleic anhydride, is lower than the activation energy for formation of carbon oxides, in agreement with several kinetic studies.26-29 Effect of Relative Velocity. A higher ur increases the solid recirculation rate in the wakes associated with bubbles, as a larger flow of gas rises in the bubbles. In this way, the catalyst could provide more oxygen in the reaction zone, as lattice oxygen, increasing the butane conversion and decreasing the oxygen concentration at the reactor exit, as shown in Figure 7. There is some decrease in the MA selectivity, as is usually observed when the conversion increases in this kind of reaction, but the MA yield remains almost constant. Comparison with Results in the Literature. Finally, we compare our results with results obtained in previously published works17,30-33 with several types of reactors, using high butane concentrations (Figure 8). Although differences may exist between the catalysts employed in each work, it may be expected that if large differences exist between the performance of different reactors, these will be due to their intrinsic properties, rather than to small changes in the catalyst composition. It is clear that the performances (both measured by the selectivity to MA or by the yield) obtained in this work are superior to those obtained by different authors in fixed bed reactors, and the conversions are higher than those obtained in the membrane reactor. The performance of the circulating fluidized bed reactor is slightly better, with selectivities around 10 percentage points higher. In some cases the results obtained with circulating fluid bed or with a pulse system follow a
The TZFBR is a promising device for the oxidation of butane to maleic anhydride. The performance of this reactor is clearly higher than the fluidized bed with cofeeding of reactants under the same operating conditions. A significant part of the oxygen fed to the reactor can be transported to the reaction zone in the catalyst lattice, thus reducing the gas-phase oxygen concentration and the risk of explosion. However, it is convenient to maintain some oxygen in the gas phase, because if the catalyst becomes too reduced, the selectivity drops dramatically. Under the operating conditions studied, the backmixing of butane is small. The performance of this reactor has been compared with other reactors under conditions of high butane concentration. Better yields to MA than those reported in the literature for fixed bed reactors with cofeeding of reactants or membrane reactors have been obtained. The results follow similar trends to those reported for a circulating fluidized bed or employing a pulses system, i.e., systems where the catalyst operates under oxidation/reduction cycles. In some cases, the circulating fluidized bed reactor provides higher selectivity, but this is counterbalanced by the more complex system. Given the possibilities that the TZFBR offers for the oxidation of butane to MA, further work is necessary to obtain a detailed analysis of the effect of the operation variables on the catalyst oxidation state and reaction activity and also to analyze its industrial usefulness. The use of a mathematical model of the TZFBR, such as the one developed recently for the oxidative dehydrogenation of butane, would be advantageous.34 Acknowledgment Financial support from DGICYT, Spain, Projects QUI98-0592 and PPQ2001-2519 is gratefully acknowledged. Nomenclature h ) bed height, cm. hc ) height of the butane feed point, cm. h0 ) total bed height, cm. h* ) hc - 0.5, cm. Pox ) percentage of oxygen in the feed, %. Roh ) oxygen/butane ratio in the feed. SMA ) selectivity to maleic anhydride, %. T ) bed temperature, °C. u ) gas velocity, cm‚min-1. umf ) minimum fluidization velocity, cm‚min-1. ur ) relative gas velocity (u/umf). W0 ) Catalyst weight, g Xi ) Conversion of the i compound (i ) oxygen or butane), %. YMA ) yield to maleic anhydride, %.
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Received for review February 4, 2002 Revised manuscript received July 18, 2002 Accepted July 18, 2002 IE020097T
