Fluidized Bed Reactors with Two-Zones for Maleic Anhydride

The partial oxidation of n-butane to maleic anhydride over a vanadium−phosphorus oxide (VPO) catalyst has been studied in reactors with separation o...
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Ind. Eng. Chem. Res. 2005, 44, 8945-8951

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APPLIED CHEMISTRY Fluidized Bed Reactors with Two-Zones for Maleic Anhydride Production: Different Configurations and Effect of Scale Jorge Gasco´ n, Carlos Te´ llez, Javier Herguido, and Miguel Mene´ ndez* Department of Chemical and Environmental Engineering. University of Zaragoza. 50009 Zaragoza. Spain

The partial oxidation of n-butane to maleic anhydride over a vanadium-phosphorus oxide (VPO) catalyst has been studied in reactors with separation of the oxidizing and reducing zones. In the two-zone fluidized bed reactor (TZFBR) and in the internal circulating fluidized bed reactor (ICFBR), oxygen and n-butane are fed at different points of a single vessel, obtaining separated zones for catalyst oxidation and n-butane reaction. These configurations allow working at relatively high n-butane concentrations without being within the flammability range and permitting strict control of the catalyst performance, which will result in improvements in selectivity to the desired product. In this work, a detailed study of the main operating variables is presented, as well as an upscaling from laboratory to bench scale. 1. Introduction Maleic anhydride (MA) has become one of the most important chemical intermediates, with applications in a wide range of products, from acids to surfactants. Most of the MA is used for the production of unsaturated polyester resins, which represents about 60% of its total outlets. There has been a considerable interest in the selective oxidation of n-butane since Bergman and Frisch1 discovered that this reaction can be catalyzed by vanadiumphosphorus oxide (VPO) catalysts. The introduction of strict controls on the benzene emission from MA plants, especially in the United States, had a major influence on the switch to C4 feedstocks. The use of n-butane as a feedstock in the commercial production of maleic anhydride began in 1974 at Monsanto, using a fixed bed reactor system. By late 1985 there was no commercial manufacture of maleic anhydride in the United States by other than n-butane-based processes. Currently, all new processes to obtain maleic anhydride are based on n-butane oxidation. They differ only by the type of reactor used, the procedure for recovering the effluent (water or organic quenching), and the final purification system. Packed and fluidized beds have been the preferred reactors for developing the partial oxidation of n-butane. Fixed bed is a well-known technology where improvements in selectivity are possible only by improving the catalyst. There are other drawbacks associated with the use of packed beds in MA production, such as the formation of hot spots and the large butane dilution that must be used to avoid flammability limits (maximum concentration of butane in air around 1-2%). Fluidized beds are the preferred reactors for large-scale plants2-4 because they allow more efficient heat removal and * To whom correspondence should be addressed. Fax: +34976 762142. E-mail: [email protected].

better temperature control, and because higher feed concentrations are possible, up to 4% butane in air, since the fluidized catalyst acts as a flame arrester, quenching the free radicals.5 In recent years, to increase productivity, substantial research effort has been carried out to develop new promoted VPO catalysts or alternative processes based on new reactors. One approach takes advantage of the fact that the lattice of a catalytic oxide can act as an oxygen stock, transferring it in oxidation reactions under suitable conditions. This characteristic allows the development of new processes where the oxygen required for the oxidation is provided by the catalyst, which is reoxidized in a separate step. The best-known example of this way of operation is the circulating fluid bed reactor (CFBR), developed by Dupont.4,6 Membrane reactors have also been studied7-11 with the aim of improving selectivity by distributing oxygen. They can also provide a safer operation with high butane concentration by avoiding the explosion region and reducing the formation of hot spots. A system called two-zone fluid bed reactor (TZFBR; Figure 1. left panel), which is different from the CFBR but also uses the catalyst as an oxygen carrier, has been proposed for the first time in the oxidative coupling of methane.12 In the TZFBR only one vessel is employed, and oxygen is introduced as a part of the fluidizing gas, whereas the hydrocarbon flow is introduced at an intermediate point of the catalyst bed. This allows the hydrocarbon oxidation to take place with lattice oxygen in the reducing zone of the bed (above the hydrocarbon inlet). The oxygen-depleted (reduced) catalyst is then regenerated (reoxidized), after being transported by internal circulation to the oxidizing zone at the bottom part of the reactor, where oxygen is fed. This system has proved itself as very efficient for obtaining high yields and safer operation in the oxidative dehydrogenation of butane.13 A preliminary study showed that it

10.1021/ie050638p CCC: $30.25 © 2005 American Chemical Society Published on Web 11/01/2005

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Figure 1. Reactor configurations used in this work: (left) TZFBR; (right) ICFBR; u1, u2, and u3 denote the different gas velocities in different parts of the bed.

can also be employed for the selective oxidation of butane to maleic anhydride.14 In addition, in reactions where the catalyst is deactivated by coke deposition, this system has also been proven as very efficient for the continuous removal of the coke formed, obtaining steadystate operation in butane15 and propane16 dehydrogenation. As a second alternative, another configuration called the interconnected circulating fluid bed reactor (ICFBR; Figure 1, right panel) has been tested. In this case an axial slot allows the partition of the reactor vessel into two beds connected at the bottom and the top. The advantage of this new configuration compared with the TZFBR lies in better control of catalyst circulation flow and, therefore, a better control of the degree of catalyst oxidation. The idea of using a system of connected fluid bed reactors (different from the CFBR) for a catalytic oxidation has been previously studied by our group for the oxidative dehydrogenation of butane,17 obtaining promising results. Obviously, there could be advantages in a system that keeps the oxidation and the reduction zone separated, like the CFBR (thus improving safety and selectivity), but that simplifies the circulation of solids, like the ICFBR. A detailed revision of the use of TZFBR and ICFBR has been published recently.18 This work provides a more detailed study of the oxidation of n-butane over a commercial VPO catalyst using reactors with separation of oxidizing and reducing zones. It includes a study of the main operating conditions on the performance of the TZFBR, and its upscaling from lab-scale (3 cm i.d.) to bench-scale reactors (6 and 9 cm i.d.), to ensure the absence of slug formation, which could result in spurious results. In addition, the results with the TZFBR will be compared with those obtained with the ICFBR. For each configuration the effect of the main operating variables was studied: bed temperature, gas velocity, W/F, and oxygen/n-butane ratio fed to the reactor.

Figure 2. Scheme of the experimental setup used in this work, in TZFBR configuration.

2. Experimental Section Figure 2 shows a scheme of the reaction system employed. All the streams were mass flow controlled (Brooks). Reactors of three different sizes have been used. The first (lab scale) was a 3 cm diameter, 20 cm long quartz reactor, while the others (bench scale) are two stainless steel tubes 100 cm long, of 6 and 9 cm diameter, respectively. For the TZFBR configuration, a mobile axial stainless steel probe was used to introduce the n-butane at different reactor heights (hb), and an O2-He mixture was fed into the bottom of the reactor (Figure 1, left panel). In the alternative configuration, the ICFBR (only experiments at bench scale have been performed for this configuration), an axial slot 0.6 m long in the reactor allowed the partition of the column into two regions of 1/3 and 2/3 of the total cross-sectional area of the column. Helium was fed into the bottom of

Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 8947 Table 1. Range of Studied Operating Conditions

temperature (°C) oxygen (%) n-butane (%) flow rate (STP cm3/min) catalyst weight (g) ur (uo/umf)

TZFBR (lab scale)

ICFBR and TZFBR (bench scale)

400-435 20-50 1-10 300-600 40-100 1.5-3

400-435 20-50 1-10 2400-4000 400-2500 2-4

the reactor, and n-butane and oxygen were fed separately at the sides of the reactor (Figure 1, right panel). In the ICFBR, the porosity difference produces the catalyst circulation between both reactor regions. In all the experiments performed, a commercial fixed bed VPO catalyst (Amoco) was used. The catalyst pellets were ground and sieved to a particle size of 160-250 µm and then calcined in air at 550 °C, and finally the catalyst was activated at 420 °C in 1.5% n-butane in air. The density of the catalyst bed before fluidization was 0.86 g/cm3, while the density of the solid was 1.018 g/cm3. Before each experiment, the activated catalyst was oxidized in 21% O2 (balance He) at 420 °C for 4 h. Preliminary experiments in a fluidized bed reactor, with an activated catalyst, were carried out in order to assess the stability of the system. Steady-state operation was achieved after the first few minutes of reaction; nevertheless, the reported results will be those obtained after 4 h of reaction. The fluidization conditions of the catalyst were determined by performing different experiments in the absence of reaction. Particle sizes below 150 µm produced cohesive behavior, resulting in the formation of channels through the bed, while good fluidization was found within the 160-250 µm size range. With He at 420 °C, a minimum fluidization velocity (umf) of 4.7 mm/s (STP) was calculated from the bed pressure drop vs flow rate curve. Table 1 shows the range of operating conditions studied for the different reactors used. The total feed flow (He + O2 and C4H10) was varied between 300 and 4000 cm3 (STP)/ min depending on the reactor scale. The n-butane flow rate was varied between 1% and 10% of the total feed. The mass of the catalyst in the bed was varied between 40 and 100 g, 400 and 600 g, and 2000 and 2500 g for each reactor scale. The temperature was varied from 400 to 435 °C. By use of a mobile thermocouple, axial temperature profiles were determined in most of the experiments, obtaining a nearly constant profile in all cases. Typically, the temperature difference between the top and the bottom of the bed in the TZFBR and ICFBR was lower than 10 °C. Also, a mobile axial stainless steel probe was used to check the oxygen concentration along the reactor in order to avoid working within flammability limits. The exit gases of the reactor were kept at 200 °C in order to avoid maleic anhydride deposition. By use of an absorption tower, MA is recovered and its concentration can be continuously measured by conductimetry. The formation of minor compounds was followed by HPLC analysis (Waters model 1515 with an UV 2487 detector, with an IC-Pack ion exclusion column). The rest of the compounds in the outlet of the reactor were analyzed by online gas chromatography (CE Instruments model GC-8000TOP, with TCD and FID detectors, with Chromosorb PAW 23% SP-1700 80/100 and Molecular sieve 10 A 80/100 columns). Butane conversion was defined as the ratio of the number of moles of

Figure 3. Influence of O2/butane ratio over n-butane conversion and MA selectivity in a TZFBR. Set 1: T ) 400 °C, ur ) 1.2, hb ) 6 cm; F ) 180 cm3 (STP)/min. Set 2: T ) 420 °C, ur ) 5, hb ) 6 cm; F ) 350 cm3 (STP)/min. Set 3: T ) 420 °C, ur ) 5, hb ) 4 cm; F ) 350 cm3 (STP)/min. Other conditions are the same: CC4H10 ) 2.5-8.3%; W ) 55 g, CO2 ) 25%, h0 ) 11 cm.

carbon in the reaction products to the total number of moles of carbon in the inlet streams. Selectivity to MA was obtained as the ratio of moles of MA formed to the moles of n-butane reacted. Carbon balances were always better than (5% and usually better than (3% for the steady-state experiments reported in this work. 3. Results and Discussion To study the effect of the main variables, several sets of experiments were performed at every reactor scale, varying the feed composition, temperature, and amount of catalyst in the reactor. The optimization of the main variables has been performed in the lab-scale reactor, although those conditions were checked later in the bench-scale reactors. Lab-Scale Results. The O2/n-butane ratio in the feed is one of the most important variables to be optimized in selective oxidations; this parameter is particularly critical in the selective oxidation of n-butane,19-22 where the performance is directly related to the oxidation/ reduction state of the catalyst. Several experiments have been performed in the lab-scale reactor in order to find out its influence. Figure 3 shows the effect of the oxygen/n-butane ratio on butane conversion and maleic anhydride selectivity for three sets of experiments performed, each under different conditions. Each set was carried out with constant partial pressure of oxygen while the n-butane partial pressure was varied. In all the sets, as expected, the n-butane conversion increases with increasing oxygen/n-butane ratio because the catalyst is more oxidized. Contractor19 reported that when n-butane is oxidized in the absence of oxygen gas, the surface layers of the catalyst are rapidly reduced and the catalyst activity decreases. This means that if the catalyst remains in a reducing atmosphere, reduced surface layers are built up on an oxidized core because the diffusion of the lattice oxygen to the surface is very slow. On the other hand, previous kinetic studies21,22 demonstrated that both adsorbed and lattice oxygen are selective for the n-butane partial oxidation. For each set of experiments, a maximum in MA selectivity was obtained for an oxygen/n-butane ratio

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in the range 5-6. At low oxygen/n-butane ratios, the oxygen conversion is high and the oxygen concentration is below a minimum value that is needed in order to maintain a selective catalyst. Mallada et al.7,20 have found a decrease in maleic selectivity at low oxygen/nbutane ratios (1-2.5) and suggest that if the oxygen concentration is depleted beyond a certain limit, maleic anhydride is oxidized to carbon oxides, causing a large decrease in selectivity. Rubio et al.14 found similar results by cofeeding oxygen and n-butane at low ratios, obtaining small MA selectivities for experiments where a highly reduced atmosphere is present in the upper part of the reactor (total oxygen conversion). This trend was also corroborated by other kinetic studies.23,8 At high oxygen/n-butane ratios (more than the optimum value), the catalyst is too oxidized and therefore less selective. The overall trend of MA selectivity is in agreement with several works; for example, Rodemerck et al.24 investigated the behavior of VPO catalysts with an average degree of oxidation for vanadium ranging from +3.2 to +4.9. Catalysts with an average oxidation state of +3.7 or lower were not able to form MA, although subsequently Volta25 demonstrated that V3+ sites are never observed by NMR spin-echo mapping. Independently of the exact value of the oxidation degree, the fact that an oxygen-depleted catalyst is not selective to MA is consistent with most of the published works. On the other hand, an extremely oxidized VPO catalyst produces only CO2. However, it is interesting to note that these samples could be reduced under reaction conditions, and MA started to appear in the product stream when the average oxidation state of the catalyst fell below a value of V4.6+. For intermediate oxidation states these authors found an increase in the yield of MA with increasing valence state of V (up to a maximum of +4.6). Thus, it seems that a limited amount of V5+ is necessary to obtain a high selectivity to MA, but excessive V oxidation leads to complete oxidation of the products to form predominantly CO and CO2. In Figure 3, sets 2 and 3 have the same catalyst weight in the reactor but different n-butane entry points (hb), 6 and 4 cm, respectively, that is, different Wr/F ratios. It is clear from Figure 3 that in these conditions more catalyst in contact with n-butane produces more n-butane conversion and also more MA selectivity. These results in selectivity can be explained with the help of the influence of the oxygen/n-butane ratio, since in the experiments with a shorter reaction zone, the oxidation zone is larger and the oxidation state of the catalyst and oxygen/n-butane ratio in the reaction zone change. Set 3 has a shorter oxidizing zone than set 2, and thus the oxygen/n-butane ratio arriving at the reaction zone is higher and both conversion and selectivity can increase (see Figure 3 for oxygen/n-butane ratio lower than 5). In Figure 3, the main difference in operating conditions between sets 1 and 2 is the temperature, 400 and 420 °C, respectively. When the reaction temperature increases, butane conversion increases, but the selectivity to MA decreases. A similar effect has been reported in kinetic studies26 and has been attributed to the degradation of MA at high n-butane conversion. The TZFBR and a conventional fluidized bed reactor (FBR) have also been compared. In the TZFBR, different experimental results have been obtained by increasing the catalyst in the reaction zone with a constant oxidizing zone. Experiments in FBR have been per-

Figure 4. Comparison between TZFBR and FBR; CC4H10 ) 2.3%; Wr ) 30-45 g; T ) 400 °C. F ) 675 cm3 (STP)/min.

formed by feeding oxygen/n-butane ratios of 6 and 10, varying in both cases the quantity of catalyst. In both TZFBR and FBR reactors, the larger the amount of catalyst, the larger the butane conversion, but the MA selectivity decreases mainly due to the oxidation of MA to COx. Regarding Figure 4, we can conclude that it is possible to increase the maleic anhydride selectivity by 10 percentage points for the same conversion by using a TZFBR system. It is also important to emphasize that with a FBR, no maximum in selectivity is found by varying the oxygen/n-butane ratio, similar results being obtained for both 6 and 10 ratios. Results for the conversion-selectivity fitting are in agreement with most of the published works,4 which report a MA selectivity that decreases dramatically when n-butane conversion is over 80%. It should also be noted that as the oxygen conversion approaches 100%, the MA selectivity decreases drastically down to 32% in agreement with previous findings.27,28 From the results presented, the superior selectivity-conversion behavior of the TZFBR over conventional fluidized bed reactors is clear. In the following section, trends obtained at lab scale will be checked when a larger reactor is used. Bench-Scale Results. In the bench-scale reactor (up to 9 cm i.d.), both the TZFBR and the ICFBR configurations have been studied. The TZFBR bench-scale reactor shows similar trends to the lab-scale reactor: an optimum oxygen/n-butane ratio of around 6 (not shown), a substantial decrease in maleic anhydride selectivity at 100% n-butane or oxygen conversion, and selectivities higher than conventional fluidized bed reactors (Figure 5). Furthermore, the optimal temperature for the bench-scale reactor was clearly 400 °C (not shown), instead of 400-420 °C as described above for the lab-scale reactor. Figure 5 compares the best results obtained for the TZFBR bench-scale reactor with the best results presented above for the same reactor configuration at lab scale and with the results obtained for the fluidized bed reactor with cofeeding of both reactants (FBR). It is important to notice that, for conversions up to 70%, the results obtained in the bench-scale reactor are better than those for the lab scale, but conversions greater than 70% produce a high decrease in selectivity, which is not so pronounced in the lab-scale reactor. It must be taken into account that the size of the bubbles increases along

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Figure 5. Selectivity-conversion behavior in TZFBR bench-scale reactor vs TZFBR lab scale and FBR. Table 2. Influence of the Relative Size for Oxidizing and Reducing Zones in the ICFBR Reactora

a

Figure 6. Effect of the presence of gas-phase oxygen in the reducing zone in the ICFBR. CC4H10 ) 4%; XC4H10 ) 68%; T ) 400 °C.

W ) 1500 G, Ft ) 6600 cm3 (STP)/min.

the bed, which means that bubbles are larger at the top of the highest reactor, producing a different concentration profile and therefore different behavior. This problem could be solved by adding internals in the upscaling. Two kinds of experiment have been performed with the ICFBR configuration: in each of them butane has been fed to a different reactor compartment, that is, if the axial slot allows the partition of the reactor vessel into two beds with different sizes, connected at the bottom and the top, in this study butane was fed in some experiments at the smallest bed and in other cases at the largest bed, while a mixture of He-O2 was fed to the opposite bed. Results for both kinds of experiment are shown in Table 2. As expected, the relative size of both zones has a large influence on the result: the reducing zone must be smaller than the oxidizing zone, which suggests that the catalyst oxidation is slower than the catalyst reduction, in agreement with several studies.21-23 Therefore, high residence times for the catalyst under reducing conditions produce its quick deactivation, contributing to the formation of undesired products. The best results for the ICFBR show behavior and performance similar to that of TZFBR. A new set of experiments has been designed in order to assess the importance of the presence of small quantities of oxygen in the reducing zone: different amounts of oxygen have been added together with the n-butane flow in the ICFBR reactor (always outside flammability limits), while the total flow of oxygen was

Figure 7. Oxygen concentration at 3 cm below the n-butane inlet for different n-butane concentrations and inlet concentrations found if a conventional fluidized bed reactor were used. ur ) 3; W ) 600 g; T ) 400 °C; O2/butane ) 6; h0 ) 15-35 cm.

kept constant, with a butane/O2 ratio of 6 maintained. A maximum in selectivity (Figure 6) is found for an intermediate value of oxygen flow, which demonstrates that some quantity of gas-phase oxygen in the reducing zone helps to achieve high maleic anhydride selectivities. Finally, it is important to notice that some of the experiments carried out in this work would be within the flammability range if a fixed or fluidized bed reactor with the same overall feed composition had been used. This problem is avoided in the TZFBR and ICFBR by the use of separated oxygen and n-butane feeds. To ensure that the TZFBR works outside flammability limits, even at high n-butane concentrations, measurements of oxygen concentration at 3 cm below the hydrocarbon inlet (h ) hb - 3 cm) have been performed for different experiments; using different heights of n-butane feed (hb). Figure 7 represents the concentrations of n-butane and oxygen at 3 cm below hb and the concentrations that would be found if a conventional fluidized bed reactor had been used. Experimental data are represented together with the n-butane and oxygen flammability diagram for butane-oxygen-inert mixture at atmospheric pressure. The flammability limits have been taken from the work by Alonso et al.29 The

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Figure 8. Comparison of the results obtained in this work with reactors from the literature.

diagram is useful for illustrating the ability of the TZFBR to obtain a safe operation under conditions that would be explosive if n-butane and oxygen were fed together in a FBR. The only way to avoid a flammable mixture in a FBR is to change the n-butane and/or oxygen concentration in the reactor feed. In contrast, the TZFBR allows a significant increase in the n-butane and oxygen concentrations without entering the explosive region. Figure 8 shows a comparison between our results and results obtained in previously published works with several kinds of reactors and high butane concentrations (i.e., higher than 1.5%). Although differences must exist among the catalysts used in each work and results from different laboratories are often difficult to compare because of the different experimental conditions, it would be expected that if large differences exist in the performances of different reactors, these will be due to their intrinsic properties, rather than to small changes in the catalyst composition. In fact, it was pointed out early on with fixed bed reactors,34 and confirmed recently with fluidized bed reactors,35 that in reactors with cofeeding of butane and oxygen the selectivity drops as the butane concentration increases. As can be observed in Figure 8, the TZFBR has far exceeded the results obtained for other reactor configurations, such as membrane or fixed bed reactors for high concentrations of hydrocarbon. Only the performance of some CFBR4 is slightly better than the TZFBR, obtaining yields around 7 percentage points higher. On the other hand, as has been explained above, the performance of the ICFBR is poorer than the TZFBR but better than the fixed bed. Selectivities are in the same range as with membrane reactors, while the conversions obtained are greater. From these considerations and taking into account the easier operational performance of the TZFBR and the ICFBR than the CFBR, we can conclude that our reactors could provide a new and competitive way of performing the partial oxidation of n-butane. Conclusions This work demonstrates that the TZFBR and the ICFBR represent a good way of developing the partial

oxidation of n-butane to maleic anhydride. By use of these reactors, butane with high concentrations can be selectively oxidized to MA outside the flammability range. Although a significant part of the oxygen fed to the reactor can be transported to the reaction zone in the catalyst lattice, it has been found that some oxygen in gas phase is necessary in the reaction zone to increase the maleic anhydride yield, which could corroborate that oxygen adsorbed over the catalyst surface plays a key role in the n-butane oxidation. The performance of these reactors has been compared with other reactors under high butane concentration conditions. Yields to the desired product by using the TZFBR or ICFBR are better than reactor configurations where only one vessel is used, such as fixed bed or membrane reactors, and otherwise problems associated with the movement of solids between different reactors are avoided. The trends observed at laboratory scale with the TZFBR have been checked at larger scale, up to 9 cm diameter, which constitutes a step toward further large-scale tests. Although the effect of most of the operation variables has been studied, further work is necessary in order to optimize these systems. The use of mathematical models, such as that developed recently for TZFBR reactors,17,29 would be advantageous. Acknowledgment We thank DGI (Spain) for financial support for Projects PPQ-2001-2519-CO2-01 and CTQ2004-01721PPQ. Nomenclature h0 ) bed height, cm hb ) height of the hydrocarbon feed point, cm u ) gas velocity, cm (STP)/min. umf ) minimum fluidization velocity, cm (STP)/min ur ) relative gas velocity (u/umf) W ) amount of catalyst in the reactor, g Wr ) amount of catalyst in contact with n-butane, i.e., in the reacting zone, g F ) total gas feed flow, cm3 (STP)/min

Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 8951 TZFBR ) two-zone fluidized bed reactor ICFBR ) internal circulating fluidized bed reactor FBR ) fluidized bed reactor.

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Received for review June 1, 2005 Revised manuscript received September 1, 2005 Accepted September 28, 2005 IE050638P