ARTICLE pubs.acs.org/IECR
Conductive Monolithic Catalysts: Development and Industrial Pilot Tests for the Oxidation of o-Xylene to Phthalic Anhydride Gianpiero Groppi,† Enrico Tronconi,*,† Carlotta Cortelli,‡ and Roberto Leanza‡ † ‡
Laboratorio di Catalisi e Processi Catalitici, Dipartimento di Energia, Politecnico di Milano, 20133 Milano, Italy Polynt R&D, 24020 Scanzorosciate, Italy ABSTRACT: We present herein results from a campaign (>1500 h) of o-xylene oxidation runs in a 1 in. single-tube technical fixedbed pilot reactor (i.d. = 24.6 mm) loaded with 16 washcoated (V2O5/TiO2) monolithic catalysts with Al honeycomb supports (each 10 cm long) and operated at typical industrial conditions for PA (phthalic anhydride) production. The highly conductive monolithic supports afforded substantially reduced axial T-gradients in comparison with reference runs in the same pilot reactor loaded with conventional egg-shell ring catalyst pellets, the maximum T-difference with the salt bath being halved at the same hot spot temperature (∼440 °C) and the mean bed temperature being about 20 °C higher. Temperature gradients were still moderate at an o-xylene feed concentration of 80 g/Nm3 (at Qair = 4 N m3/h), which represents an upper limit for industrial PA packed-bed reactors loaded with the same V2O5/TiO2 catalyst coated onto ring pellets. Operation at higher o-xylene feed contents (up to 95 g/Nm3) was found feasible. The Al honeycomb catalysts were successfully unloaded at the end of the runs. The strong (∼2-fold) enhancement of radial heat transfer rates associated with the adoption of highly conductive monolith catalyst supports is thus herein demonstrated at a fully representative industrial scale for the first time. It can be exploited, for instance, either to increase the o-xylene feed concentration, possibly above 100 g/Nm3 (and the PA productivity accordingly), within a retrofitting strategy, or to design new reactors with larger tube diameters, resulting in reduced investment costs.
’ INTRODUCTION The operation of industrial multitubular packed-bed reactors with external cooling for strongly exothermic selective oxidations is typically limited by radial heat transfer, which relies on convection in the gas phase as the primary mechanism for heat exchange. Accordingly, the only practical way of enhancing heat transfer rates is to increase the flow velocity, but this is limited by the pressure drop, which grows more than linearly with flow rate. There is potential for significant enhancement of radial heat transfer rates, however, if the random packings of catalyst pellets are replaced by structured catalysts with highly conductive honeycomb supports, as heat conduction in the thermally connected solid monolith matrix becomes in this case the dominant transport mechanism. The idea of using conductive (metallic) monolith catalysts for chemicals production was pioneered by Cybulski and Moulijn1 in ke, r ¼
the 1990s. Their theoretical analysis, however, showed poor heat transfer performances of the commercially available monolith structures consisting of corrugated metal sheets. This can be rationalized by considering the dependence of the heat transfer properties of the substrates on their structural characteristics. The effective axial heat conductivity of monolith substrates ke,a is readily estimated as ke, a ¼ ks ð1 εÞ
with ks = intrinsic thermal conductivity of the support material and ε = monolith open frontal area, or void fraction. On the basis of a simple analysis of heat conduction in the unit cell of a honeycomb monolith with square channels according to an electrical network analogy, Groppi and Tronconi2,3 derived the following approximate equation for the effective radial thermal conductivity in washcoated monoliths, ke,r:
ks pffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi pffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffi ε þ ξ ε ε þ ð1 ε þ ξÞ þ pffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffi kw pffiffiffiffiffiffiffiffiffiffiffiffiffi kg pffiffiffi kw pffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi ð1 ε þ ξÞ þ ε þ ξ ð1 ε þ ξÞ þ ð ε þ ξ εÞ þ ε ks ks ks
where ε and ξ are the monolith volume fractions of voids and washcoat, whereas ks, kg, and kw are the intrinsic thermal conductivities of solid support, gas phase and washcoat, respectively. Recently, Hayes and co-workers4 have validated eq 2 against numerical solutions of the temperature field in honeycomb structures, finding maximum deviations of less than 20% for a typical monolith void fraction of 75%. They derived also an alternative equation, based on a different (parallel) arrangement of the resistance network, which improved somewhat the prediction accuracy. r 2011 American Chemical Society
ð1Þ
ð2Þ
According to eq 2, the effective conductivity ke,r is directly proportional to the intrinsic thermal conductivity of the support material, ks: thus, the adoption of highly conductive materials is Special Issue: Russo Issue Received: September 21, 2011 Accepted: October 27, 2011 Revised: October 26, 2011 Published: October 27, 2011 7590
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Industrial & Engineering Chemistry Research expectedly very beneficial for the enhancement of radial heat transfer in monoliths. Most interestingly, in the case of highly conductive materials (e.g., Cu, Al) eq 2 predicts the estimates of ke,r to be 1 order of magnitude greater than the effective radial thermal conductivities in packed beds of catalyst pellets, which are typically in the range 2 - 5 W/m/K5,6 for selective oxidation processes. Equation 2 shows also that the radial effective conductivity is adversely affected by large monolith void fractions. Notably, even though eq 2 was derived under the assumption of stagnant fluid in the monolith channels, the static conductive contribution to radial heat transfer is by far dominant in conductive honeycombs, so that ke,r is also virtually independent of the flow velocity. These evaluations point out that heat exchange in monolithic structures can be made in principle more efficient than in random packings of pellets, but monolith supports with specific designs must be adopted, based on an ad hoc selection both of the monolith geometry and of the material aimed at minimizing resistances to conductive heat transfer. Notably, the existing commercial monolithic substrates, originally developed for the adiabatic applications of environmental catalysis, were not designed for that purpose. As a matter of fact, neither the construction material nor the geometry of such supports is optimized for heat conduction: the intrinsic conductivity of ceramic honeycombs is very low, whereas the commercially available metallic monolith structures are made of poorly conductive alloys (e.g., FeCrAlloy) and are assembled by piling up and rolling corrugated sheets which are in poor thermal contact one to each other, thus increasing the overall resistance to heat transfer. Finally, in commercial monoliths the open frontal area is kept very high, typically 0.70.8 for ceramic monoliths and 0.850.95 for metallic ones, so as to match the severe pressure drop constraints of environmental processes. On the basis of the above considerations, Groppi and Tronconi have explored by means of both modeling79 and experimental911 work the potential of heat conduction in the walls of monolithic structures as an effective mechanism to remove the heat of strongly exothermic selective oxidations. Simulation and test results confirmed the possibility to achieve almost flat radial temperature profiles using honeycombs with a relatively large solid volume fraction made of materials with high thermal conductivity. A major impact of the thermal contact at the interface between the monolith skin and the internal tube wall was also pointed out by simulations9 and experimentally investigated in refs 11 and 12. The results showed that to maximize the wall heat transfer coefficient the honeycomb should be packed with a minimum gap between the monolith outer and the tube wall inner diameter. A parallel investigation by Corning researchers13,14 essentially confirmed such conclusions, emphasizing the role of the “gap” as the controlling thermal resistance for radial heat transfer in highly conductive monoliths and the positive effect of differential thermal expansion between the honeycomb and the tube wall material (e.g., aluminum vs stainless steel) on the overall heat transfer coefficient. As summarized in ref 12, conductive heat exchange in honeycomb structures can be made much more effective then convective heat transfer in packed beds leading to near isothermal reactor operation under severe thermal loads, provided that dedicated monolith design specifications (including the selection of the material, the geometric configuration and the packing method) are chosen. Such monoliths would afford reduced hotspots in tubular reactors, resulting in reduced risks of thermal
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runaway, in better thermal stability of the catalyst, and in improved selectivity: eventually, new reactors could be designed for increased throughput and/or with enlarged tube diameters, with significant economic benefits. The application of conductive monolith substrates to gasphase catalytic processes has been claimed in patents,1518 and recently investigated also in relation to the development of compact fixed-bed reactors for the FischerTropsch synthesis.19,20 Notably, while this concept, originally proposed on the basis of simulation studies, has been by now confirmed by heat transfer and reactive experiments at the lab scale, no proof-of-concept at an industrial scale has been published so far in the open literature to our knowledge. We report herein on a campaign of o-xylene oxidation runs in a technical tubular pilot reactor loaded with conductive (aluminum) honeycombs washcoated with a state-ofthe-art V2O5/TiO2 industrial catalyst and operated under representative conditions for the industrial production of phthalic anhydride (PA). The selective oxidation of o-xylene to PA is in fact one of the most exothermic processes of the industrial chemistry (ΔHR = 1500 kJ/mol): as such, it could thus benefit at best from improved radial heat transfer, but also represents a serious challenge for the application of the conductive monolith concept. Indeed, the significant potential benefits expected from the adoption of conductive honeycomb catalysts in this process have been already pointed out by simulation,14 and M€ulheims and Kraushaar-Czarnetzki21 have very recently reported reduction of the hot spot temperatures in the o-xylene oxidation in a tubular reactor loaded with washcoated sponge packings made of conductive SiC material. The purpose of the project discussed in the following was 2-fold: (i) to demonstrate the concept under representative industrial conditions, specifically focusing on the effect of the conductive honeycomb catalyst supports on the temperature profiles; (ii) to address a number of practical feasibility issues associated with the replacement of packed-beds with honeycomb catalysts in technical multitubular reactors.
’ MATERIALS AND METHODS Substrates. Small Al slabs as well as cordierite honeycomb samples were employed to develop the washcoating procedure during preliminary lab-scale work. The extruded aluminum honeycomb monoliths used in the pilot reactor runs were prototypes manufactured and supplied by Corning Inc. Cylindrical samples with different cell densities were extruded from 99.5% pure Al according to the methods described in ref 22; examples are displayed in Figure 1. The substrates selected for the o-xylene oxidation tests discussed in the following are those on the right of the figure, with the highest cell density (about 27 CPSI). For these samples the cell size was 4 mm and the OFA = 0.64. Segments 10 cm long were cut from the extruded bars. The diameter of the samples was originally about 1 in.; however, due to irregularities of the external surface, the honeycomb samples were machined to reduce their external diameter and then forced into Al “skins” with a precise external diameter of 24.4 mm and with very uniform and smooth external surfaces, as required to minimize the thermal resistance at the interface between the monolith and the internal tube wall. Catalysts. Structured supports (slabs and honeycombs) were cleaned (water/acetone/water, followed by drying in a ventilated oven at 110 °C for 1 h) and washcoated first with a primer (dispersible boehmite: Disperal, Sasol) and then with a V2O5/TiO27591
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Table 1. Washcoating of Structured Supports with Boehmite Primer + Polynt V2O5/TiO2-Based Catalyst Powder (H2O/ Powder = 1.75 w/w)a washcoat load,
washcoat
% weight loss
mg
thickness, μm
in US test
cordierite monolith
399
4446
2.2
Al slab (spiral)
458
41
3.5
a
Double dipping. Sample size: o.d. = 12 mm, length =30 mm. Cell density of monolith sample =200 cpsi. The OFA and the specific surface area of the Al spiral were similar to those of the monolith sample.
Figure 1. Prototype extruded Al honeycombs with different cell densities (Corning Inc.).22
based precursor powder for industrial o-xylene oxidation catalysts provided by Polynt. Both steps involved dipping the support into a slurry with suitable rheological properties, removing the excess slurry either by gravity (slabs, controlled withdrawal speed = 3 cm/min) or by blowing compressed air (honeycombs, P = 3 barg for 5 s), and flash heating the resulting catalyst structure at 280 °C in a ventilated oven for 5 min. The washcoat adhesion was checked by measuring the weight loss after 30 min in an ultrasound bath (petroleum ether). More details on the adopted washcoating methods are given in ref 23. Pilot Reactor Runs. Sixteen Al honeycomb samples (27 cpsi, o.d. = 24.4 mm, length =10 cm) washcoated with a total catalyst mass of 47 g were loaded in the upper part of an industrial pilot reactor consisting of a single jacketed tube (i.d. = 24.6 mm, length = 3 m) cooled with a molten salt bath, located at the Polynt site of Scanzorosciate, Italy. Spacers consisting of small springs (3 mm) were inserted between the honeycombs. The tube loading was completed with inert rings, since we were interested in characterizing the thermal and catalytic behavior of the monolithic catalysts only. In industrial applications this second section of the reactor could be eventually loaded with catalytic rings as well. Axial temperature profiles were recorded by means of a thermocouple sliding in a 3 mm o.d. thermowell inserted in the central channel of the honeycombs. The temperature readings were taken with a typical spatial resolution of 1 cm. The outlet gases were analyzed online by a GC equipped with a TCD for determination of COx. A side stream was then absorbed in acetone for analysis of the organic compounds by an off-line GC equipped with a FID. Carbon balances closed within (5% in all the tests. Comparative experiments at the same conditions were also run in the same pilot reactor packed with industrial egg shell catalyst pellets consisting of steatite rings (9 5 1.5 mm) coated with the same V2O5/TiO2-based catalyst precursor powder used for washcoating the Al honeycombs.
’ RESULTS AND DISCUSSION 1. Preliminary Laboratory Work. 1.1. Washcoating of Structured Supports. A recipe for the deposition of stable layers of the
V2O5/TiO2-based precursor powder supplied by Polynt onto
structured substrates was developed by trial-and-error in the laboratories of Politecnico di Milano, starting from previous experience on Al2O3 coatings.23 For this purpose, a variety of supports were tested in order to verify the effects both of the support material and of the support geometry on the coating methods. In terms of active phase load, the target was a washcoat thickness of at least about 50 μm: in the case of 100 cpsi honeycomb substrates, in fact, this would result in matching the catalyst density in the tubes of modern PA industrial reactors, which is in excess of 70 kg/m3.14 As shown for the selected results in Table 1, second column, this target was indeed achieved for different structures (honeycombs and spirals) made of different materials (cordierite, aluminum) thanks to (i) the adoption of the boehmite primer, (ii) the identification of an appropriate H2O/ powder ratio in the slurry composition resulting in optimized slurry rheological properties, and (iii) a double-dipping procedure. For all the selected sample geometries, the overall catalyst load was about 400 mg (see first column in Table 1), as suitable for the lab-scale catalytic activity tests discussed below. Notably, the final coatings were found stable in ultrasound bath adhesion tests, their weight loss being typically limited to about 5% or less. 1.2 Catalytic Tests. Prior to the preparation of the coated honeycomb samples for the pilot runs, the washcoated Al spirals were thoroughly evaluated in lab-scale catalytic activity tests of o-xylene oxidation in a microflow reactor. The purpose of these tests was to verify that the washcoating procedure, the primer, and the support material would not interfere with the catalytic behavior of the original catalyst powders. Accordingly, the V2O5/TiO2-based precursor powders supplied by Polynt were also tested as such to establish reference activity data. A close match of conversion and selectivity data was found in all cases, proving that the method adopted for deposition of active layers on Al supports did not affect the catalytic properties of the active phase. 2. Pilot Reactor Runs. 2.1. Preparation of Conductive Monolith Catalysts and Reactor Loading. The bar plot in Figure 2 shows the distribution of the vanadia/titania Polynt catalyst washcoat thickness for the 16 honeycomb samples used in the pilot runs. The average thickness was around 85 μm, but it is apparent that we did not obtain a perfectly uniform distribution of washcoat load. We decided therefore to load the reactor with samples in growing order of washcoat load in order to achieve a graded distribution of catalytic activity. The total catalyst inventory was 47 g, corresponding to 62 kg/m3. This is only somewhat less than the catalyst load with the conventional ring packing at typical industrial reactor conditions (= 7080 kg/m3). As stated above, the 16 washcoated honeycombs were loaded in the upper (inlet) section of the pilot reactor for an overall length of 1.65 m out of the total 3 m. This is of course the most 7592
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Figure 2. Distribution of the average washcoat thickness in the 16 Al honeycomb monoliths loaded in the Polynt tubular pilot reactor.
critical portion of the reactor tube, where the hot spot is usually located. At room temperature the gap between the Al honeycombs and the inner reactor tube wall was approximately 100 μm, which enabled easy and smooth insertion of the Al honeycombs. Notably, the gap was greatly reduced at the higher reaction temperatures, however, due to the greater thermal expansion coefficient of Al (24 106 K1) as compared to steel (16 106 K1), thus minimizing the radial heat transfer resistances.12,13 This is another important favorable feature associated with the selection of aluminum as the construction material for conductive monolith substrates. Before starting the o-xylene feed, two blank heat transfer runs were performed setting the molten salt bath temperature at 380 °C, feeding air (flow rates = 4 and 5.5 Nm3/h) and measuring the steady-state axial T-profiles along the honeycombs. The data were eventually analyzed to extract estimates of the overall heat transfer coefficient, which turned out to be largely controlled by gassolid heat transfer, accounting for about 2/3 of the overall radial heat transfer resistance. Nevertheless, the residual 1/3 resistance was consistent with “gap” heat transfer coefficients in the 450550 W/(m2K) range, in line with expectations based on the gap size of 100 μm,11,12 thus lending confidence for the eventual startup of the reactor. Noteworthy, the gassolid heat transfer resistance should not play any significant role in reactive tests, when heat is generated by the catalytic reaction at the solid wall. 2.2. Pilot Runs over Al Honeycomb Catalysts. After the dosing of the o-xylene to the feed stream was started, the pilot reactor was operated almost continuously for over 1600 h. During the campaign the air feed flow was set to 4 Nm3h, and the o-xylene feed concentration was stepwise increased from 30 to ∼95 g/Nm3 (i.e., o-xylene feed loads from 120 up to ∼380 g/h). Correspondingly, the salt bath temperature was progressively decreased so as to keep the measured hot spot temperature around a rather conservative level of 440 °C. Notably, the o-xylene feed load is about 320 g/h in state-of-the-art technical multitubular packed-bed reactors for PA production, with maximum admissible hot-spot temperatures sometimes exceeding 450 °C.14 Figure 3 shows the measured cooling salt bath temperature (Tsb), the mean bed temperature (Tave), and the hot spot temperature (Ths) plotted versus the o-xylene feed concentration: the coolant temperature had to be reduced by 12 °C (from 400 to 388 °C) in order to accommodate the incremented heat load associated with the 3-fold increase of the o-xylene feed
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Figure 3. o-Xylene oxidation runs in an industrial tubular pilot reactor loaded with washcoated Al honeycombs: cooling salt bath temperature (Tsb), mean bed temperature (Tave) and hot spot temperature (Ths) versus o-xylene feed load. Feed: o-xylene in air, air flow rate = 4 Nm3h.
Figure 4. o-Xylene oxidation runs in an industrial tubular pilot reactor loaded with V2O5/TiO2 washcoated Al honeycombs: Phthalic anhydride (PA) productivity, o-xylene conversion, and PA selectivity versus o-xylene feed load. Feed: o-xylene in air, air flow rate = 4 Nm3h.
concentration without altering significantly the hot spot temperature. As a result of the adopted strategy, to a first approximation the mean bed temperature remained constant as well at about 420 °C, thus only 20 °C below the maximum temperature. The PA productivities, the o-xylene conversions, and the PA selectivities measured during the experimental pilot campaign are plotted versus the o-xylene feed content in Figure 4. Expectedly, the o-xylene conversion decreased somewhat upon incrementing the reactant feed concentration, in line with an o-xylene kinetic order less than 1.24 On the other hand the PA selectivity was almost unaffected by the incremented o-xylene feed load, remaining stable around 70%. On the whole, the PA productivity grew significantly upon increasing the o-xylene load, eventually exceeding 250 g/h as the o-xylene feed concentration approached 100 g/Nm3. When comparing this figure with typical industrial PA productivities (up to 340 g/h per reactor tube14), one should consider that in our tests the pilot reactor configuration was fully representative of a single tube in industrial multitubular reactors, 7593
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Figure 5. o-Xylene oxidation runs in an industrial tubular pilot reactor loaded with V2O5/TiO2 washcoated Al honeycombs: measured axial temperature profiles at different o-xylene feed loads. Feed: o-xylene in air, air flow rate = 4 Nm3/h; o-xylene feed = 200360 g/h. Salt bath temperatures = 400388 °C.
but for the length of the catalyst bed (1.65 m versus 3 m). Considering the additional reactant conversion expected if also the second part of the reactor tube had been filled with catalyst (either washcoated monoliths or pellets), we can conclude that the data in Figure 4, reflecting the performances of our monolithic pilot reactor, are very closely in line with those of typical industrial packed-bed PA reactors. Throughout the runs the observed pressure drop was roughly half of that measured at the same conditions when the pilot reactor was completely packed with ring pellets: this indicates that the pressure drop across the section filled with the Al honeycomb monoliths was essentially negligible, since all of the measured pressure drop was due to gas flow through the bottom section of the tube (1.35 m) filled with inert rings. The crucial results of the present investigation are shown in Figure 5, where we have plotted the axial T-profiles measured in the pilot reactor during the runs at different o-xylene feed loads. The unusually irregular “segmented” shape of the experimental profiles is explained both by the piecewise nature of the catalyst bed, consisting of 16 individual monolithic sections, and by the nonuniform washcoat load of the different honeycomb catalysts. Altogether, however, it is clearly apparent from Figure 5 that the temperature gradients along the reactor were quite moderate during the pilot runs with Al monoliths, and the smooth T-distributions were only slightly affected upon incrementing the o-xylene feed load. In other words, there was hardly any evidence of parametric sensitivity in the operation of the pilot reactor at these conditions, which are the same operating conditions of industrial PA reactors, as discussed before. It is worth emphasizing that T-gradients were still moderate at the highest investigated o-xylene feed concentration (about 95 g/Nm3, i.e., 380 g/h), which is in line with or beyond the upper limit for current industrial PA packed-bed reactors. Thus, there is potential for using greater o-xylene feed loads, which is quite promising in view of enhancing further the productivity of the process. At the end of the nine weeks campaign, the Al honeycombs were successfully unloaded from the pilot reactor. The total weight loss of the washcoat was about 20%, which however was
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Figure 6. o-Xylene oxidation runs in an industrial tubular pilot reactor loaded with V2O5/TiO2 eggshell ring catalyst pellets: measured axial temperature profiles at different o-xylene feed loads. Feed: o-xylene in air, air flow rate = 4 Nm3/h; o-xylene feed = 180320 g/h. Salt bath temperatures = 364358 °C.
Figure 7. o-Xylene oxidation runs in industrial tubular pilot reactor. Axial temperature profiles: comparison of washcoated Al honeycomb monoliths versus eggshell ring catalyst pellets at reference industrial conditions. Air flow rate = 4 Nm3/h, o-xylene feed = 320 g/h. Salt bath temperature = 391 °C (honeycombs), = 358 °C (pellets).
mainly attributed to mechanical stresses during unloading since variations of the pilot reactor performances were hardly observed during the experimental campaign. 2.3. Pilot Runs over Pellet Catalysts, and Comparison with Conductive Honeycombs. Figure 6 shows axial T-profiles measured during o-xylene oxidation runs in the same pilot reactor packed with the industrial ring catalyst pellets and operated at the same feed conditions of the runs over the Al honeycomb catalysts. In these runs the specific load of V2O5/TiO2 active phase was about 70 kg/m3. In contrast to that in Figure 5, the shape of the profiles was more regular in this case due to the more uniform catalyst distribution (here the spatial resolution of the T-readings was 5 cm), but the axial T-gradients were definitely more important, 7594
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Figure 8. o-Xylene oxidation runs in an industrial tubular pilot reactor. Effect of the o-xylene feed load on the average bed temperature: comparison of washcoated Al honeycomb monoliths versus eggshell ring catalyst pellets. Conditions as in Figures 5 and 6.
Figure 9. o-Xylene oxidation runs in an industrial tubular pilot reactor. Effect of the o-xylene feed load on the maximum temperature difference (Ths Tsb); comparison of washcoated Al honeycomb monoliths versus eggshell ring catalyst pellets. Conditions as in Figures 7 and 8.
too, and became more and more evident upon increasing the o-xylene feed load, pointing out a markedly greater sensitivity as compared to the Al honeycomb runs. A key aspect in this comparison is also the fact that in the case of the ring pellets, in order to control the hot spot temperature below the threshold of 440 °C, the molten salt bath temperature had to be lowered significantly, as further discussed below. Noteworthy it was not possible to run the reactor with an o-xylene feed concentration greater than 80 g/Nm3 because runaway occurred despite lowering the salt-bath temperature according to conventional control strategies. Temperature profiles measured over the Al honeycomb catalysts and over the packed egg-shell ring catalysts are compared directly in Figure 7 for the case of reference industrial reactor conditions, namely o-xylene feed content = 80 g/Nm3 corresponding to an o-xylene load of 320 g/h. Notice that the hot spot temperatures measured in the two runs were similar, but the corresponding salt bath temperatures were 392 °C for the Al honeycombs versus 358 °C for the pellets: this is patent direct evidence that the removal of the reaction heat was much more efficient with the conductive honeycombs, thus requiring a much less driving force to achieve the same cooling effect. In fact the axial T-profile of the ring packing is evidently sharper than over the Al honeycombs, indicating the possible onset of an inflection point in the inlet region. Inspection of Figure 7 points out also that, because of the differences in the two T-profiles, the mean catalyst bed temperature was lower over the ring pellets. Figure 8, comparing the mean bed temperatures in the runs over the two catalyst systems, confirms that this was a general result observed also at all the other investigated o-xylene feed contents, the mean catalyst temperature being at least 20 °C higher over the Al honeycomb catalysts for the same hot spot temperature. The associated advantages in the effective utilization of the catalyst mass are obvious. Thus, the direct comparison of the thermal behaviors of the same pilot reactor loaded with the conventional (rings) and with the innovative (conductive honeycombs) catalyst supports undoubtedly emphasizes improved radial heat transfer rates in
the latter case. A rough estimate of the extent of improvement can be extracted from Figure 9, where we have plotted the maximum observed temperature difference, i.e. hot spot temperature salt bath temperature, Ths Tsb, versus the o-xylene feed concentration for honeycombs and for rings. Notice, for example, that at reference industrial conditions (o-xylene feed concentration = 80 g/Nm3) the maximum T-difference was about 90 °C in the packed bed, but only approximately half that when the reactor was loaded with Al honeycombs. We can assume accordingly that the efficiency of the radial heat transfer process was virtually doubled by replacing the ring pellets with the conductive structured catalyst supports. This conclusion, based on direct experimental evidence, is in excellent agreement with independent engineering estimates: the overall radial heat transfer coefficient for the rings packing, computed according to standard literature correlations,25 was 240 W/(m2K), whereas the “gap” heat transfer coefficient for the conductive monoliths was about 500 W/(m2K), as discussed in a previous section.
’ CONCLUSIONS The 1500 h experimental campaign using washcoated aluminum honeycombs in an industrial single tube pilot reactor for o-xylene oxidation to phtalic anhydride has clearly verified the applicability of this new technology to the industrial processes. Washcoated honeycombs have been easily loaded in the industrial reactor tube at room temperature with a 100 μm diameter gap, have granted stable operation, and have been successfully unloaded at the end of the campaign. A strong enhancement of radial heat transfer rates associated with the use of such novel monolithic catalysts with high thermal conductivity has been herein demonstrated at an industrial scale for the first time. This performance is clearly superior (≈2) to what could be potentially achieved at best in the case of conventional packed-bed reactors. Such a unique improvement can be exploited for intensification of the PA process in a number of ways, for example, to increase the o-xylene feed load above 100 g/Nm3 (and the PA productivity accordingly) in existing 7595
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’ AUTHOR INFORMATION Corresponding Author
*Tel.: +39 02 2399 3264. Fax: +39 02 2399 3318. E-mail:
[email protected].
’ ACKNOWLEDGMENT Corning Inc., USA, and Dr. Serge Marseau, Corning Europe Technology Center, Avon, France, are gratefully acknowledged for supplying the Al honeycomb substrates. ’ REFERENCES (1) Cybulski, A.; Moulijn, J. A. Modeling of heat transfer in metallic monoliths consisting of sinusoidal cells. Chem. Eng. Sci. 1994, 49, 19–27. (2) Groppi, G.; Tronconi, E. Continuous versus discrete models of nonadiabatic monolith catalysts. AIChE J. 1996, 42, 2382–2387. (3) Tronconi, E.; Groppi, G. High conductivity” monolith catalysts for gas/solid exothermic reactions. Chem. Eng. Technol. 2002, 25, 743–750. (4) Hayes, R. E.; Rojas, A.; Mmbaga, J. The effective thermal conductivity of honeycomb monolith structures. Catal. Today 2009, 147S, S113–S119. (5) Doraiswamy, L. K. Sharma, M. M. Heterogeneous Reactions: Analysis, Examples and Reactor Design; Wiley: New York, 1984; Vol. 1. (6) Eigenberger, G. Reaction Engineering. In Handbook of Heterogeneous Catalysis; Ertl, G., Knozinger, H. Weitkamp, J. Eds.; J.Wiley-VCH: Weinheim, 1997; Vol. 3, pp 13991425. (7) Groppi, G.; Tronconi, E. Design of novel monolith catalyst support for gas/solid reactions with heat exchange. Chem. Eng. Sci. 2000, 55, 2161–2171. (8) Groppi, G.; Tronconi, E. Simulations of structured catalytic reactors with enhanced thermal conductivity for selective oxidation reactions. Catal. Today 2001, 69, 63–73. (9) Tronconi, E.; Groppi, G. High conductivity monolith catalysts for gas/solid exothermic reactions. Chem. Eng. Technol. 2002, 25, 743–750. (10) Tronconi, E.; Groppi, G. A study on the thermal behavior of structured plate-type catalysts with metallic supports for gas/solid exothermic reactions. Chem. Eng. Sci. 2000, 55, 6021–6036. (11) Tronconi, E.; Groppi, G.; Boger, T.; Heibel, A. Monolithic catalysts with “high conductivity” honeycomb supports for gas/solid exothermic reactions: Characterization of the heat transfer properties. Chem. Eng. Sci. 2004, 59, 4941–4949. (12) Groppi, G.; Tronconi, E. Honeycomb supports with high thermal conductivity for gas/solid chemical processes. Catal. Today 2005, 105, 297–304. (13) Boger, T.; Heibel, A. K. Heat transfer in conductive monolith structures. Chem. Eng. Sci. 2005, 60, 1823–1835. (14) Boger, T.; Menegola, M. Monolithic catalysts with high thermal conductivity for improved operation and economics in the production of phthalic anhydride. Ind. Eng. Chem. Res. 2005, 44, 30–40 and references therein contained. (15) Carmello, D., Marsella, A., Forzatti, P., Tronconi, E. Groppi, G. Metallic monolith catalyst supports for selective gas phase reactions in tubular fixed bed reactors. EP 1110605 (2008). (16) Carmello, D., Marsella, A., Forzatti, P., Tronconi, E. Groppi, G. Metallic monolith catalyst supports for selective gas phase reactions in tubular fixed bed reactors. US Patent 7,678,343 (2008).
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