545623 Adoption of Highly Conductive Open Cell Foams As an Effective Solution to Enhance the Heat Transfer Performances of Packed-Bed Fischer-Tropsch Reactors

Thursday, June 6, 2019: 11:18 AM
Texas Ballroom EF (Grand Hyatt San Antonio)
Laura Fratalocchi1, Carlo Giorgio Visconti2, Luca Lietti2, Gianpiero Groppi3 and Enrico Tronconi4, (1)Energy, Politecnico di Milano, Milano, Italy, (2)Department of Energy, Politecnico di Milano, Milan, Italy, (3)Politecnico di Milano, Dipartimento di Energia, Via La Masa, Milan, Italy, (4)energy, politecnico di milano, milano, Italy

In the last decade, the interest in the Fischer-Tropsch synthesis (FTS) in compact reactors has been considerably renewed in view of exploiting both associated and remote natural gas fields and biomass. In view of the highly exothermic nature of the reaction, the heat removal is a key issue for the development of an intensified reactor [1]. The use of conventional packed-bed reactors (PBRs) is severely limited by heat removal causing a series of constraints which limit the catalyst performances. The poor heat transport properties in PBRs lead to non-isothermal operation of the reactor. This negatively affects the catalyst selectivity and, in the worst case, leads to the onset of thermal runaway [2].

In line with these premises, the efficient thermal management in the modular reactor technology suitable for the FTS on a compact scale (< 3000 BPD) is a key issue [2]. In this regard, Politecnico di Milano recently proposed the adoption of honeycomb catalysts with high-conductivity supports within externally cooled multitubular fixed-bed reactors [3]. In this way, the heat transfer is strongly enhanced because the primary radial heat exchange mechanism is changed from convection to conduction within the thermally connected solid matrix of the honeycomb monolith [3].

In this work, we propose to enhance the heat transport properties of a FTS packed bed reactor through the adoption of highly conductive open-cell metal foams packed with catalyst pellets [4]. These materials were recently proposed as a strategy to intensify heat transfer in strongly endo- and exo-thermic processes [5], but were never used for the FTS. They exploit the same conductive heat transfer mechanism of the monolithic substrates but, in addition, they have the advantage of enabling radial mixing within their structure, thus enhancing flow uniformity [5]. Our strength is also the adoption of “packed” open-cell foams, which can overcome the inherently limited catalyst inventory of washcoated structured reactors.

The open-cell Al-foam (40 PPI and efoam ≈ 0.906) with a cylindrical shape (dfoam= 2.78 cm and lfoam= 4 cm) is provided by ERG AEROSPACE. One axial through hole of 0.32 cm diameter is located at the centerline for the insertion of the stainless steel thermowell (1/8’’ O.D.), protecting a sliding J-type thermocouple.

Once the foam is loaded in the tubular reactor and the thermowell is positioned in the foam, the system is packed as follows. Initially, 5 g of α-Al2O3 pellets (dpellet= 300 μm) are poured into the foam, thus forming a 1 cm deep layer. Then, 7.2 g of Co/Pt/Al2O3 catalyst diluted with a very small amount of α-Al2O3 (catalyst:α-Al2O3 = 6:1 w/w) with the same particle size are poured into the foam, thus forming a catalyst layer of 1.89 cm. Eventually, 5 g of α-Al2O3 pellets (dpellet= 300 μm) are packed into the foam so to fill the last 1 cm of the structure. The resulting average catalyst volumetric density is 0.63 g/cm3, calculated as the ratio of the catalyst mass (7.2 g) to the reactor volume occupied by the catalyst bed (11.4 cm3).

In the case of the packed-bed reactor, 7.2 g of Co/Pt/Al2O3 catalyst are again randomly packed in the same tube used for the foam, but diluting with a large amount of α-Al2O3 (catalyst: α-Al2O3 = 1:1.7 w/w) pellets with the same particle size (dpellet= 300 μm), so to form a catalyst bed with length of 4 cm, equal to the length of the foam (≈4 cm). This leads to a catalyst volumetric density near 0.29 g/cm3, i.e. almost half of that corresponding to the packed-foam reactor. Accordingly in the packed bed, at the same CO conversion level, the volumetric heat duty (kW/m3) generated by the reaction is less than in the case of the packed foam.

The performances of a highly active Co/Pt/Al2O3 catalyst packed into the foam metallic structure have been assessed at industrially relevant process conditions (190-240 °C, 25 bar, H2/CO= 2 mol/mol, 6410 cm3(STP)/h/gcat) for more than 750 h and compared with those obtained in a conventional randomly packed fixed-bed reactor [4]. The structured reactor reaches outstanding performances (CO conversions >67.5%) with a remarkable temperature control. The volumetric heat duty (Q) calculated in the experiment with the packed-foam reactor increases with temperature, being a function of the CO conversion. It starts from 80.6 kW/m3 obtained at 180 °C, up to a remarkable value of 1360.4 kW/m3 at 240 °C. To our knowledge, this is the first time in the scientific literature that such high values are reported for the FTS reaction in a lab-scale apparatus.

Limited T-gradients along the catalyst bed are obtained at all the temperatures investigated, thus resulting in very small ΔTcat= Tmax – Tmin measured along the catalyst bed, even if in the presence of high volumetric heat duty. In this regard, the ΔTcat is negligible at 180 °C and 190 °C, in line with the very low catalyst activity. Also in the 195–205 °C T-range, it is small (ΔTcat ≈ 1 °C) although the CO conversion level increases up to 22%. Increasing the reaction temperature up to 210 °C and then to 215 °C, the T-gradients are only slightly affected, with a ΔTcat of 2 and 3 °C, respectively. The CO conversion values at these temperatures are 28% and 33%, respectively. At 220 °C and 225 °C, with significant CO conversions of 44% and 50%, the ΔTcat is still small and equals 4 °C and 4.5 °C, respectively. The most significant effect on the ΔTcat is obtained at 230 °C and 240 °C when the CO conversion reaches the highest values (55% and 67.5%). Accordingly, the ΔTcat becomes 5 °C and 6 °C, respectively [4].

Noteworthy, although the catalyst was tested for several hours (≈800 h), which is unusual for lab-scale runs, and frequently varying the process conditions, it was found to be very stable with Time on Stream (T.o.S.). In fact, the CO conversion measured by replicating the standard conditions (200 °C, 25 bar, H2/CO= 2 mol/mol, 6410 cm3(STP)/h/gcat) at different T.o.S. (200, 520 and 760 h) was always around 16%. This is particularly interesting since the catalyst worked under severe conditions (i.e. high PH2O/PH2 in particular in the second half of the catalyst bed [6]) for over 200 h. We believe that this is a further indication of the excellent heat transfer properties of the packed foam reactor, which prevent the deactivation of the catalyst by avoiding strong temperature gradients in the catalyst bed.

The results herein reported prove the excellent ability of the “highly conductive packed-foam reactor” concept to manage the strong exothermicity of the FTS reaction [4]. In contrast, when the same experiment was carried out over the same Co/Pt/Al2O3 catalyst just randomly packed in the reactor, an abrupt increase of the catalyst temperature occurred already at low temperature (duties <244 kW/m3 with CO conversions <12.1%) eventually leading to thermal runaway.

Our data confirm that, thanks to the adoption of the conductive foams, the mean temperature inside the reactor can be much better controlled providing new operating windows, which are not accessible using the conventional packed-bed reactor technology. The conductive packed-foams enable in fact running the FT reaction under severe conditions (i.e. high CO conversion and large heat duty) with an intensified temperature control. Indeed, in a crucial comparative experiment the conventional packed-bed reactor, although operated under milder conditions (i.e. the catalyst density was halved with respect to the foam), experienced thermal runaway already at very low temperatures and CO conversions, i.e. with limited release of reaction heat. These results are a direct indication that the heat exchange is significantly enhanced thanks to the structured conductive substrate of the foam.

Furthermore, the “packed-foam” configuration also represents an innovative solution to increase the catalyst inventory in structured tubular reactors, since the catalyst load which can be packed in the open-cell foam is much greater than the amount which can be loaded by washcoating the same foam. In this way, the productivity per reactor volume can be boosted. In addition, “packing” the foam means overcoming all the problems linked to the coating process, to the catalyst loading and unloading in the reactor, and to the replacement of the spent catalyst [4].

References:

[1] C. G. Visconti, E. Tronconi, G. Groppi, L. Lietti, M. Iovane, S. Rossini, R. Zennaro, Chem. Eng. J. 171 (2011) 1294-1307

[2] C. G. Visconti, E. Tronconi, L. Lietti, G. Groppi, P. Forzatti, C. Cristiani, R. Zennaro, S. Rossini, Appl. Catal. A-Gen. 370 (2009) 93-101

[3] M. Iovane, R. Zennaro, P. Forzatti, G. Groppi, L. Lietti, E. Tronconi, C. G. Visconti, S. Rossini, E. Mignone, Pat. Appl. WO/2010/130399

[4] L. Fratalocchi, C.G. Visconti, L. Lietti, G. Groppi, E. Tronconi, Chem. Eng. J. 349 (2018) 829-837

[5] G. Groppi. E. Tronconi, C.G. Visconti, A. Tasso, R. Zennaro, Pat. Appl. WO/2015/033266

[6] L. Fratalocchi, C.G. Visconti, L. Lietti, G. Groppi, E. Tronconi, E. Roccaro, R. Zennaro Catal. Sci. Technol., 6 (2016) 6431-6440

Acknowledgment:

This project has received funding from the European Research Council (694910/INTENT).


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