Fischer-Tropsch synthesis intensification in foam structures

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Kinetics, Catalysis, and Reaction Engineering

Fischer-Tropsch synthesis intensification in foam structures Ane Egaña, Oihane Sanz, David Merino, Xabier Moriones, and Mario Montes Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01492 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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Industrial & Engineering Chemistry Research

Fischer-Tropsch synthesis intensification in foam structures Ane Egaña, Oihane Sanz*, David Merino, Xabier Moriones, Mario Montes Applied Chemistry Department, Chemistry Faculty of the University of the Basque Country (UPV/EHU), Donostia-San Sebastián, Spain Corresponding author: [email protected]

Graphical Abstract

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract This paper investigates the intensification of Fischer-Tropsch Synthesis (FTS) in open-cell foam structures. To check this point, the metallic foams with different alloy (aluminum and FeCrAl), pore density (10-40 ppi) and (20% Co–0.5% Re)/Al2O3 catalyst layer thicknesses were prepared. Catalytic results of aluminum foams show excellent thermal behavior but catalyst coatings thicker than ~70 µm suffer from catalyst effectiveness loss, increasing methane selectivity and decreasing C5+ hydrocarbons selectivity. Conversely, FeCrAl foams exhibited also a similar critical maximum catalyst layer thickness, but with diffusional and thermal limitations resulting in both temperature and selectivity runaways at higher thicknesses. Finally, the observed good thermal control and the tortuosity of the narrow porous structure for aluminum foams permitted to enhance the production of C5+ hydrocarbons to 87 kgC5+/m3·h at 250 ºC, 26 % higher than the productivity obtained with parallel channel aluminum monolith coated with similar catalyst amount.

Keywords: Fischer-Tropsch; Metallic foam; Diffusion limitations; Thermal conductivity


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1. Introduction Nowadays, crude oil (a natural source of hydrocarbons) is the main raw material for the production of liquid fuels and the majority of valuable chemical compounds. Nevertheless, other sources of hydrocarbons are being explored due to the exhaustion of known oil fields and/or an increase in oil price on the world market. In this sense, the Fischer-Tropsch synthesis (FTS) has been developed to obtain synthetic liquid fuels and valuable chemical compounds by the transformation of synthesis gas from various feed stocks including gas-to-liquid (GTL), coal-to-liquid (CTL) and biomass-to-liquid (BTL)

1,2

. In

addition to natural gas, coal and biomass can be converted to synthesis gas by partial oxidation, steam reforming or gasification processes. Among several applications, the use of synthesis gas from renewable feedstock has received particular interest with the increasing importance of decentralized and on-site fuel production reactors that can operate without depending on a fossil-fuel distribution network2. Moreover, current global energy scenario and the environmental deterioration aspects motivate substituting fossil fuel with a renewable energy resource – especially transport fuels. In response of this concern, research studies have accelerated towards the replacement of existing hydrocarbons with their renewable counterparts to comply with the health, safety and environment regulations. Therefore, FTS is viewed as a key technology to produce clean fuels from syngas derived from non-petroleum resources. As the design capacities of the FTS reactors to produce fuels from biomass are expected to be much lower than the minimum economic capacities of existing industrial-scale units, there has been a strong incentive towards exploring the use of intensified catalytic reactors that are able to transform hydrocarbons into synthetic fuels with notably improved efficiencies2-4. The 3 ACS Paragon Plus Environment


development of intensified catalytic reactors leads a substantially smaller, cleaner, safer and more energy efficient technology5. FTS is a highly exothermic reaction which causes many heat transfer issues for FTS intensification (∆HR= −165 kJ/mol). The efficient removal of heat from the reaction zone is vital due to the desired product distribution strongly depends on the reaction temperature that needs to be kept within a narrow control limit3. At this point, intensified reactors such as micro-channel, monolith or foam reactors become really relevant2-4,6,7. This type of devices provides large volumetric surface areas (two orders magnitude higher than conventional units like packed-bed reactors), low-pressure losses being also able to offer good thermal and chemical stability8. In addition to their benefits stated above, intensified reactors have operating capacities and compact volumes that are highly compatible with decentralized or space-limited operations aiming the valorization of synthetic fuels. Metallic micro-channel reactors, micro-structured reactors with channels with a dimension below 1 mm, offer potential advantages of improved heat and mass transfer, more precise control of reaction temperature within flammable composition regions leading to reduced hot spots, and higher surface area to volume ratio of micro-channels9. Almeida et al.7,10,11 and Arzamendi et al.12 reported that FTS micro-channel block with cross flow design (including an additional cooling line with pressurized water) allows excellent temperature control and therefore a highest C5+ selectivity compared to monoliths and foams. Utilization of micro-channel reactors FTS process can be intensified by a factor up to 1513 and it could be operated in wide conditions without large temperature gradient in the catalyst14. However, micro-channel reactors have as main limitation: the high cost of their manufacture. This drawback limits their application to cases where the use of


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conventional technologies is impossible or when the advantages of micro-technology compensate for the increase in manufacturing costs. Alternatively, another interesting and more economical strategy is to develop structured reactors, specifically in the form of monolithic reactors. Honeycomb-type monoliths are structures composed of parallel channels15. The use of monoliths is well established in environmental catalysis. In fact, high surface/volume ratios are obtained in comparison with conventional fixed bed reactors formed by pellets, thanks to the high void fraction of the structured substrate. There is also a decrease in the pressure drop (up to two orders of magnitude), thanks to the laminar flow in the monolith channels. The first approach to the use monoliths for FTS was with ceramic honeycomb-type monoliths16. However, the thermal insulating character of ceramic substrates hindered the radial heat removal from the monolithic system, making difficult to control the temperature and selectivity to desired product. Therefore, liquid product recirculation was proposed as a solution for ceramic monoliths, constituting the operation basis of monolithic loop reactors17. On the other hand, the group of Prof. Tronconi has studied the possibility of increasing heat transfer capacity of monolithic reactors by employing aluminium monoliths obtained by extrusion18. In the case of structured reactors made of highly conductive materials, heat conduction through the solid matrix is more effective than convective heat transfer in packedbeds19,20. Recently, Merino et al.3 studied the FTS intensification on corrugated metallic monoliths. They reported that there are two ways to favour a high catalyst hold-up and then, process intensification: using a highly conductive substrate material (like aluminium) or using monoliths with high cell density even if the substrate material has lower thermal conductivity.



In general, parallel channel monoliths offer a narrow channel structure inside which laminar flow takes place, and this fact implies mass-transfer limitation from the reagents to the catalytic layer on the wall21,22. To increase the mass transfer to the wall, as well as the radial temperature and composition homogeneity, it can be increased the flow mixing between channels, which would lead to an improvement in the turbulence. This can be achieved by using foams of open porosity (85–90%). Although fewer applications of these structures in catalytic processes have been proposed in the literature: methane reforming23, oxidation of CO24, removal of volatile organic compounds25, soot and NOx removal26, 1-butadiene hydrogenation22, methanol synthesis27. Open-cell metal foams offer similar advantages to monoliths, including high porosity, high surface/volume, low pressure drop, and high mechanical strength. With regard to metallic monoliths, foams also have better heat and material transfers thanks to the tortuous flow inside the structure when adopting conductive substrate materials, due to the improved conduction of heat within the matrix, thus limiting the temperature gradients and hot/cold points in highly endo/exothermic processes28, but at the cost of a slight increase pressure drop29. The pore tortuosity increment can be able to improve catalyst effectiveness, requiring lower amount of catalyst28 and/or smaller reactor30. Recently, open-cell foams made of silicon carbide (SiC)31-33, aluminum7,34 and nickel35,36 have been reported as an efficient structured substrate for the FTS reaction allowing working at high CO conversion with high C5+ hydrocarbons selectivity. Philipe et al.31 developed a model to study the thermal properties of SiC foams structures for FTS. They reported that these reactors structure allows a wider range of operating conditions in terms of fluid velocity and tube diameter, but underlines the necessity to tune the void fraction of the foam structure to obtain the best compromise between its volumetric catalytic activity and a good thermal behavior. Lacroix et al.32 observed experimentally that the higher thermal conductivity of the SiC foam compared to that of 6 ACS Paragon Plus Environment

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the alumina foam allows higher efficiency of the structured support to evacuate the heat generated during the course of the reaction. The overall goal of FTS reactor design is to maximize productivity and selectivity to products desired (low methane and high selectivity to liquid hydrocarbons) while minimizing the pressure drop37. Structured reactors reported in literature are characterized by low reactor productivity due that most of the works analyzed structured substrates with low amount of catalyst16,38,39. Most authors adopted washcoat layers thinner than 50 μm based on Kapteijn et al.39 work, who observe that cordierite monoliths with catalyst coatings thicker than ~50 µm suffer from diffusional limitations. At a given temperature, the maximum thickness is given by beginning detrimental effects of pore diffusion on activity and more importantly, on selectivity. Thus, the exact value of this maximum thickness is a function of the catalyst activity, porosity of the support, temperature and arrangement of the catalyst. The cited value can be taken only as a guideline, and not as precise numbers. For instance, the observations of Kapteijn et al.39 for ceramic monoliths, where important temperature gradients are usually observed with poor conductive materials3,40. Nevertheless, Merino et al.3 working with aluminum monoliths did not observed important diffusion limitations on activity with 5-90 µm coating thicknesses, showing improved heat exchange capabilities and allowing higher catalyst inventory than that usually employed in cordierite monoliths. On the other hand, the problem linked with the effective catalyst weight per foam volume unit to achieve intensification limit in FTS has not been studied. Therefore, the main goal of this work is to study the FTS intensification in a foam structure to enhance the fuel production. The effects of operating parameters such as the foam porosity, the metallic alloy (aluminum and ferritic stainless steel), the catalyst thickness and reaction temperature in the reactor, and C5+ hydrocarbon productivity per unit volume were investigated. Metallic foams were coated with a Co-Re/Al2O3 catalyst to obtain a wide range of catalyst loadings and layer 7 ACS Paragon Plus Environment


thicknesses. The above samples were characterized by N2 physisorption, CO chemisorption, X-Ray diffraction and submitted to FTS test.

2. Experimental part 2.1. Metallic substrates and structured catalysts preparation Two different metallic foams were used, namely, aluminum (DUOCEL®, ERG Materials and Aerospace) and FeCrAl alloy (Selee Corporation). Foams were cut out from slabs using a hollow drill with a diamond saw border. In-house-fabricated metallic monoliths with parallel channels cell densities of ∼240 cpsi were prepared using a high-purity aluminum alloy (>99% (w/w), INASA). Monoliths were prepared by rolling alternating flat and corrugated sheets (thicknesses of 82 μm) into cylinders. The geometric characteristics of the investigated structured substrates are compiled in Table S1 (the supporting information). In order to improve the interaction between the catalyst coating and the metallic substrates, the surfaces were pretreated prior to coating. On the one hand, aluminum ones were introduced in a basic media (pH 10.5) for a few minutes and then, dried and calcined at 500 ºC for 2 h3. On the other hand, FeCrAl foams were thermally treated in air for 22 h at 900 ºC41. Structured substrates were pretreated prior to coating. On the one hand, aluminum ones were introduced in a basic medium (pH 10.5) for a few minutes and then, dried and calcined at 500 ºC for 2 h. On the other hand, FeCrAl foams were thermally treated in air for 22 h at 900 ºC. A washcoating procedure was used to deposit 20% Co – 0.5% Re/Al2O3 (w/w) on structured substrates by all-in-one slurry formulation42. In a typical synthesis, an aqueous suspension (pH 4) of 20 % (w/w) of total solids is prepared. This suspension contains 9.94 % Co(NO3)3·6H2O (SigmaAldrich), 0.06 % Re2O7 (Alfa Aesar), 8 % Al2O3 (Spheralite SCS 505, Procatalyse), and 2.01 % 8 ACS Paragon Plus Environment

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colloidal alumina suspension (Nyacol® Al20). During washcoating process structured substrates were dipped into the slurry, kept for 1 min and withdrawn at constant speed of 3 cm/min. After coating, the structured substrates were centrifuged to eliminate the excess slurry (400 rpm for 1 min). The coating was repeated several times using the same slurry with a drying step at 120 °C for 30 min between coatings. The nominal catalytic load varied from 300 to 1600 mg per structured substrate (6 cm3), which means average catalytic layer thicknesses (δ) between 30 and 90 μm, approx. Finally, washcoated structured substrates were calcined at 350 ºC for 6 h in air. Additionally, the catalytic slurry was dried and calcined at the same conditions than those of coated structured substrate to obtain the slurried catalyst. This powder catalyst is representative of the solid layer coating the metallic substrate walls, being both, the layer and the slurried catalyst, similar in composition and in the thermal treatments undergone. The average catalytic layer thickness was estimated using the expression δ = wcat · ρcat-1 · Sg-1, where wcat is the amount of catalyst deposited, ρcat is the catalyst coating density (1.47 g/cm3)43 and Sg is the geometric surface of the structured substrate (Table S1). Coated samples are referred to as AB_C_D: A is the type of structured substrate (F for foam and M for parallel channel monolith); B is the used metallic alloy (AL for aluminum and FEC for FeCrAl); C is the cell density of the structured substrate (10, 20, 40 or 240); and D is the nominal catalyst coating thickness (µm): FAL_40_70, MAL_240_30, …

2.2. Characterization techniques Textural properties of coated structured substrates and slurried catalyst were obtained by N2 physisorption in a Micromeritics ASAP 2020. For structured substrates a homemade cell was used allowing the analysis of entire samples. Samples were previously degassed at 180 ºC up to a 9 ACS Paragon Plus Environment


vacuum level of 10 μmHg for 8 hours and finally analyzed at -196 ºC. BET and BJH formalism of the desorption branch were used to obtain the surface area and the pore size distribution. The catalytic coating quality was evaluated by different techniques. On the one hand, the catalyst coating morphology was studied by optic microscopy (Leica Microscope M165C + DFC 420). For another, the pressure drop of the coated structured substrates at different gas velocities was measured with a Digitron 2080P Pressure Meter. And, finally, the adherence of the catalytic layer deposited onto the structured substrates was evaluated using an ultrasonic technique. The weight loss caused by the exposition of the sample to ultrasounds is measured. The structured substrate immersed in petroleum ether were submitted to an ultrasonic treatment for 30 min at room temperature. After that, the samples were dried and calcined. The weight loss was determined by the difference in the mass of the samples before and after the ultrasonic test. The results are presented in terms of the retained amount of coating on the structured substrate, expressed as percentage of the initial coating. Pulse CO chemisorption was also performed for coated structured substrates and slurried catalyst by an AutoChemII 2920 station from Micromeritics. The samples, placed in a U-shaped quartz structure (φ= 12 mm for powder and φ= 22 mm for structured substrate), were pretreated under H2 at 350 ºC for 10 h. Then, catalysts were cooled down to chemisorption temperature of 100 °C. The volume of the injection loop was 0.5 cm3. Pulses of CO were injected using H2 as carrier gas. The CO consumptions were measured by difference with the area of the pulses after saturation. When the CO chemisorption was finished, temperature programmed oxidation test (TPO) was performed to measure the reduced fraction rate. First, the sample was cooled down to 0 ºC with He. In addition, TPO was carried out using 5% O2 in He from 0 to 600 ºC (ramp= 10 ºC/min). The amount of CO adsorbed and the amount O2 for re-oxidation processes allows the


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calculation of Co dispersion (D), reduction fraction (R), and the average size of the Co0 particles (Dp(Co0)), as obtained from the expression Dp(Co0) (nm) = 0.96 · R(%)/D(%). In general, it is preferable to use H2 than CO in chemisorption since it gives rise to a more accurate stoichiometry that in addition is less dependent on factors such as the size or orientation of the active phase crystals. However, in the dynamic method, it is extremely difficult to avoid O2 contamination in the gases used or in the different lines, valves, fittings and controllers of the equipment. And few ppm of O2 are enough to cobalt re-oxidation. If H2 is used as carrier gas in the dynamic measurement with CO, the proper reduction of the sample is ensured at all times44. On the other hand, CO is adsorbed much more strongly than H2, displacing it in a quantitative way, which gives rise to very reliable measurements45. In this work, the use of the pulse method is conditioned by the format of the samples, since it is the only equipment that allows us to measure monoliths and foams without the need to crush or destroy them. Finally, X-Ray Diffraction (XRD) measurements of the slurried catalyst was performed in a Bruker D8 Advanced Diffractometer with CuKα radiation (λ = 0.154 nm) and a graphite monochromator, working at 40 kV and 30 mA. Samples were scanned within a 2θ range of 5º – 85º at steps of 0.05 º and 5 s per pass. Average cobalt crystallite size was determined using the most intense pick (36.8 º) and the Scherrer´s equation.

2.2. Catalytic tests Structured substrates were tested in a tubular FTS reactor (i.d. 17 mm) of Hastelloy® at 220 ºC, 20 bar and syngas space velocities of 3 and 6 LN/gcat·h. The tubular reactor was placed inside a commercial microreaction system from PID Eng&Tech. Since it is impossible to achieve a perfect fit between the foam (ϕ=16 mm) and the reactor wall (i.d. 17 mm), the gap was minimized using wrapping the foams with domestic aluminum foil. Three 1.5 mm thermocouples were introduced 11 ACS Paragon Plus Environment


in order to measure the radial temperature profiles inside the structured substrate. The thermocouples were placed at positions r = 0, r = R/2 and r = R (Figure 1).

Figure 1. Optic image of holes made into FAL_40 structure to introduce thermocouples

Previously to the reaction, the structured catalysts were reduced in-situ at 350 °C by a pureH2 flow of 54 mLN/min for 10 h. Then, the structured catalyst were cooled down to 180 °C in a H2 flow, and purged for 10 min with N2. Finally, the syngas was introduced and the reactor was heated up to 220 °C at 1 °C/min. A molar H2/CO ratio of 2 was employed as feeding gas contained a 5 % (v/v) of N2 as internal standard for gas chromatography (GC). Gaseous products were analyzed online with a GC Agilent 6890N coupled to the reactor. This online gas chromatograph was comprised of three lines. The first line was used to analyses light hydrocarbon (9 Foot Hayesep Q 80/100) and permanent gases (N2, CO and H2) (10 Foot Molecular Sieve 13X 45/60). The second line was used to quantify hydrogen (10 Foot Molecular Sieve 13X 45/60) and the third line was used to quantify hydrocarbons (60 m x 0.25 mm x 1.0 µm DB-1 capillary column). The first and the second line were connected to a TCD detector and the third one to a FID detector. Liquid and solid products (waxes) fraction were collected separately and analyzed off-line after reaction in another gas chromatograph (Agilent 7890A). The offline gas chromatograph had a


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column injection system, a high temperature capillary column (VF-5 UltiMetalTM) and a FID detector. The samples were dissolved in CS2 before its injection in the chromatograph.

3. Results 3.1. Characterization of foam structures Four different nominal catalytic thicknesses were deposited on metallic substrates by washcoating method: 30, 50, 70 and 90 µm. The number of coatings necessary to obtain the target thickness decreased with foam ppi (Table S2). This is due to the higher geometric surface of the former (see Table S1). In addition, it can be observed that the foams presented a higher specific load than the parallel channel monoliths. This result could be explained by the geometry difference with respect to monoliths: foams present large tortuosity where accumulations could be produced. Optical images of the foams with 70 µm catalyst layer show that the washcoating method gave homogeneous layers without plugging pores, as observed by optical microscope (Figures 2 A and B). However, to evaluate the homogeneity inside foams is very difficult. In our previous work pressure drop measurement before and after coating-process was proposed for coating quality evaluation46: catalyst coating on structured substrates can seriously increase the pressure drop by production of heterogeneities and pore plugging. Figure 3 presents, as a representative example, the pressure drop of FAL_40 foam coated with different catalyst layer thicknesses. It can be observed that the pressure drop increment due to catalytic coatings was moderate, less than 60 % at the highest gas velocities. Finally, after foams washcoating and calcination, the adherence test was done. The adherence in all of them was excellent, showing values higher than 95 % (Table S2).



Figure 2. Optic image of coated foam structures: (A) FAL_40_70 and (B) FFEC_40_70 500 FAL_40 400

FAL_40_50 FAL_40_70

300

Pa/m

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FAL_40_90

200

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0.1

0.15

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Figure 3. Pressure drops of the 40 ppi aluminum foams with and without catalyst coating

Textural parameters of the coating were measured by N2 adsorption and compared to that of the slurried catalyst (Table S3). It can be seen that specific surface area of the catalytic coating on structured substrates is similar to that of the slurried catalyst, without alterations due to the coating process. On the other hand, CO chemisorption and O2 re-oxidation experiments for the structured catalysts with different coating thicknesses was performed (Table S4). The results reveal similar parameters in slurried and structured catalysts for loadings down to 90-µm thickness. Nevertheless, the chemisorbed CO amount decreases for higher catalyst coating leading to a lower cobalt dispersion.


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The XRD pattern of a calcined slurried catalyst is shown in Figure 4, with the observed peaks matching those of cubic Co3O4 (JCPDS no. 00-001-1152) and γ-Al2O3 (JCPDS no. 00-004-0875). As can be seen, there is significant overlap among the peaks making it difficult to de-convolute all of the peaks observed in the XRD profiles measured for each sample. Therefore, the average crystallite size of Co3O4 particles was determined by applying the Scherrer equation to the most intense peak (36.8°). Subsequently, this value was corrected using the factor 0.75 to obtain the average diameter of reduced Co particles (Dp(Co0)XRD = 11 nm), reflecting the molar volume reduction upon the reduction of Co3O4 to Co0 47. Co O 3

Intensity (a.u.)

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4

* Al2O3

* 10

20

30

*

40

50

*

60

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2θ θ Figure 4. XRD pattern of the slurried 20% Co – 0.5% Re/Al2O3 catalyst used.

3.2. Catalytic results First of all, blank tests were performed to discard the possible activity provided by the metallic substrate. The result confirmed that pretreated substrates do not exhibit FTS activity at the conditions employed in the present work (220 °C and 20 bar).



3.2.1. The effect of catalyst layer thickness At given reaction conditions FTS the selectivity depends on pore size and catalyst particle size, which in the case of the structured catalyst is related to the thickness of the catalyst layer. Therefore, the results must be analyzed as a function of catalyst layer thickness on the foams surface. Different catalyst layer thicknesses were obtained with increasing catalyst loading (see Table S2). Figure 5 represents CO conversion and CH4 and C5+ hydrocarbons selectivities versus catalytic layer thickness for FAL_20 foams. CO conversion was around 51 % at space velocity of 3 LN/gcat·h and 28 % at 6 LN/gcat·h, except for FAL_20_90, whose CO conversion decreased for both space velocities. Methane selectivity increased with increasing catalyst layer thickness and space velocity. The opposite behavior was observed for C5+ hydrocarbons selectivity. On the other hand, the volumetric hydrocarbon productivity increased with catalyst layer thickness (catalyst loading) from 13 to 32 kgC5+/m3·h (Table S5). However, for FAL_20_90 foams the productivity decreased slightly.

3 LN/gcath 6 LN/g h cat

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CO

(%)

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20

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Figure 5. Effect of catalyst thickness on CO conversion (A) and selectivity (B) using FAL_20 foams 16 ACS Paragon Plus Environment

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Table S5 shows the temperature profiles of 20 ppi aluminum foams coated with different catalyst thicknesses at the onset of the reaction, with internal and external temperature gradients recorded when the desired temperature was reached. The internal temperature gradient (∆TR,int) is the difference between the reading of central thermocouple and that of the one on the outer wall of foam structure (inner wall of the reactor tube). While the external temperature gradient (∆TR,ext) is the difference between the reading of the central thermocouple and that of the one attached to the outer wall of the reactor tube. The results reveal that ∆TR,ext increased with increasing the catalyst thickness (increasing the coated catalyst amount, Table S2) due to the higher released reaction heat increased. However, FAL_20 foams did not show significant internal temperature profiles (∆TR,int≈0).

3.2.2. The effect of foam pore density Figure 6 represents CO conversion and CH4 and C5+ hydrocarbons selectivities versus pore density (10, 20 and 40 ppi) of aluminum foams coated with around 70-µm catalyst thickness. CO conversion slightly increased with foam pore density: from 49 to 52% for a space velocity of 3 LN/gcat·h; and from 28 to 34 for 6 LN/gcat·h. Methane selectivity also slightly increased with increasing foam pore density and space velocity: from 12 to 16 % at 3 LN/gcat·h; and from 17 to 20 % at 6 LN/gcat·h). C5+ hydrocarbons selectivity showed the opposite behavior with pore density and space velocity. However, the productivity (kgC5+/m3·h) increased with foam pore density (Table S5).



S

CH4

3 LN/g h

3 LN/g h

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cat

C5+

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60

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Selectivity (%)

CO

(%)

40 30

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20

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ppi

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Figure 6. Effect of foam pore density coated with 70 µm of catalyst thickness on CO conversion (A) and selectivity (B)

On the other hand, comparing the temperature profiles of aluminum foams with different pore density coated with 70 µm catalyst thicknesses at the onset of the reaction (Table S5), it can be observed that internal temperature profile was practically flat for all foam pore density. However, ∆TR,ext increased with increasing foam pore density. Increasing the foam pore density, the coated catalyst amount was higher to obtain the same catalyst thickness (Table S2). Then, requiring more reagents flow to maintain constant space velocity producing an increment in released reaction heat (Table S5).

3.2.3. The effect of foam metallic alloy Figure 7 represents the effect of foam metal alloy on CO conversion and CH4 and C5+ hydrocarbons selectivities using different catalyst thicknesses (50-90 µm) but the same foam pore density (40 ppi). CO conversion slightly decreased using FeCrAl foams instead of aluminum foams, except for FFEC_40_90. In this case, a thermal runaway was observed reaching temperatures


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values above 290 °C and achieved 100% of CO conversion producing only CO2, CH4, and H2O, with CH4 being the main product. In addition, FAL_40 foams presented the same CO conversion behavior at increasing catalyst thickness observed for FAL_20 foams (Figure 5A). Moreover, FeCrAl foams showed a marked increase in methane selectivity with the thicker catalyst layer. For 90-µm layer, CH4 selectivity equaled ≈ 75 %, with the C5+ hydrocarbons selectivity being negligible (Figure 7B). On the other hand, FAL_40 foams exhibited linear trend in the whole range of thickness tested achieving a maximum CH4 selectivity of 21%.

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Figure 7. Effect of foam alloy and catalyst thickness on CO conversion (A) and selectivity (B) at 3LN/gCat·h using 40 ppi foam structures

Figure 8 and Table S5 show the temperature profiles of 40 ppi aluminum and FeCrAl foams with different catalyst thicknesses at the onset of the reaction. The peak of temperature observed in Figure 8 after 20 minutes of data acquisition corresponds to the feeding of the reagents, which slightly destabilizes the temperature because it begins to react. In general, the results show that while the ∆TR,int was negligible for aluminum foams, internal temperature profile was not flat with



FeCrAl foams (Table S5). Moreover, the temperature control was not good enough increasing the catalyst thickness from 50 µm and a thermal runaway was observed for FFEC_40_90 foam (Figure 8F). In this case, the reaction temperature reached values above 290 °C achieving 100 % conversion (Figure 8A) and showing an external temperature gradient of 28 °C and producing only CH4, CO2 and H2O.

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210 205

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Figure 8. Reaction startup temperature profile for aluminum (A, C and E) and FeCrAl foams (B, D and E) of 40 ppi coated with different catalyst thicknesses: 50 µm (A and B), 70 µm (C and D), and 90 µm (E and F). 20 ACS Paragon Plus Environment

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3.2.5. Foam vs. parallel channel monolith For comparisons purpose, the CO conversion and CH4 and C5+ hydrocarbons selectivity values with parallel channel aluminum monolith (MAL_240_30) and aluminum foam (FAL_40_70) coated with the same catalyst amount (~1 g) and space velocity has been included in Figure 9. Results show that CO conversion is ~32 % higher for the foam structure than for the monolith. Additionally, methane selectivity was slightly higher for foam even the catalyst layer is 40 µm thicker.

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The study of the catalytic behavior as a function of reaction temperature (220-250 ºC) was studied with FAL_40_70 and MAL_240_30 structured catalyst (Figure 9). The conversion increased with temperature for both samples, while the methane selectivity increased from 20 to 24 % for the foam and from 19 to 22 % for the monolith. On the other hand, the selectivity to C5+ hydrocarbons slightly decreased with temperature, but CO2 and C2-C4 hydrocarbon selectivities increased. Consequently, hydrocarbon productivity increased from 42 to 87 kgC5+/m3·h for FAL_40_70 and from 29 to 68 kgC5+/m3·h for MAL_240_30 (Table S5). Finally, the internal 21 ACS Paragon Plus Environment


temperature profiles were around 2 ºC, except in the case of the highest temperature, at 250 ºC was around 5 ºC for the foam and 3 ºC for the monolith (Table S5). In addition, the external temperature profiles increased with reaction temperature with both structured substrates (Table S5).

4. Discussion 4.1. Influence of catalyst layer thickness In this work, foam structures with different pore density (surface geometry) and catalyst loading were prepared corresponding to average layer thicknesses of 30-90 µm. Figures 5 and 7 shows the catalytic results of coated aluminum foam substrates as a function of catalyst layer thickness. For thicknesses of 30-70 µm, the CO conversion remained constant. However, for thicker catalyst layer the CO conversion decreased. In addition, the selectivity to methane linearly increased in all cases (from 14 to 23% at 6 LN/gcat·h). In washcoated structured reactors, increasing the catalyst loading results in thicker layers, which leads to an increase in the length of reactant and product diffusion pathway. Long pores filled with hydrocarbons limit the diffusion of CO and heavy products, favoring that of lighter ones, especially hydrogen and methane48,49. The internal diffusional limitations result in lower reaction rate (decrease in catalyst effectiveness) and decrease in selectivity to liquid hydrocarbons37,39. Indeed, hydrogen molecular diffusivity in the liquid waxes is twice as high as that of carbon monoxide50. Reactants penetrate catalyst layer slowly and cannot satisfy the kinetic rate of chemical reactions37; this lowers the catalyst effectiveness and frequently changes product selectivity. Accordingly, the local H2/CO ratio in close proximity of the catalyst active centers may be significantly higher than in the bulk gas phase37. 22 ACS Paragon Plus Environment

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The effect of catalyst thickness in catalyst effectiveness was studied using ceramic and metallic monoliths3,16,18,39,40,43, and by numerical simulations37. While it was observed in monoliths made of cordierite that catalyst coatings thicker than ~50 µm suffer from catalyst effectiveness39, using structured catalysts with uniform temperature distribution (isothermal conditions) is possible increase to ~100 µm3,37,43. However, the results of this study demonstrate that diffusion limitations play a role for catalyst coats up to ~70 µm in this foam structure (Figures 5 and 7). Rickenbach et al.51 determined by simulations that the effect of catalyst layer thickness is more pronounced in foam structures compared to parallel channel monoliths, since external mass transfer coefficients in foams are significantly higher than in monolith. The small reduction in the amount of CO adsorbed by the sample with a coating of 90 μm in thickness is surprising at first sight (Table S4). It seems difficult to admit that by increasing the thickness of the coating, adding more coating steps, the dispersion of Co begins to decrease just from 70 μm. The conventional washcoating process is carried out with a suspension of the preformed catalyst. That is, the powdered catalyst is prepared by a conventional technique that includes up to the calcination and with which the suspension is prepared. In this way, the dispersion of the active phase is already stabilized and changes during coating are not expected. So it would be difficult to explain the loss of dispersion with the thickness of the coating. But we must remember that in our preparation method, the coating and impregnation is carried out in a single stage since the suspension is prepared with the catalyst precursors, alumina and Co and Re salts (the method we call “all in one”). Therefore, it is also possible that as the thickness of the catalytic layer increases, which increases the porosity of said layer, during the drying process the capillary forces favor the concentration of the solution with the salts of the active phase in the deep zones of the porosity. This would result in a certain accumulation of Co that could lead to a decrease in dispersion as shown by the CO chemisorption data (Table S4). 23 ACS Paragon Plus Environment


In order to evaluate the influence of internal diffusion on the reaction rate (CO consumption) calculations were made of the extended Weisz-Prater criterion, which is based on observable magnitudes. The details of the calculation are indicated in a previous article in which we studied the kinetics of FT with the same catalyst11. The values of the Weisz-Prater module obtained for the experiments carried out at 220° C show values ranging from 0.028 to 0.25 when passing from 30 to 90 μm in thickness of the catalytic layer. The values can be considered low indicating a null or low influence of intraparaticular diffusion in these experiments. In the series of experiments carried out at increasing temperatures (up to 250 ° C) with substrates all coated with a catalyst layer of around 70 μm (Figure 9), the values of the WeiszPrater module reach values of 0.15 which also suggest the absence of diffusional control in the pores over the CO consumption rate. However, FTS is an extremely complex reaction with numerous series-parallel steps that therefore give rise to a large number of different products, with selectivity being the most important parameter. Unfortunately, neither the Weisz-Prater criterion nor the efficiency factor based on the Thiele module can be applied to these complex situations to quantify all possible influences of diffusion in selectivity. On the other hand, catalyst layer on FeCrAl foams presented different behavior (Figure 7). When the foams are coated with a catalyst layer thinner than 90 µm, the conversion of CO is slightly lower with the steel foams than with the aluminum ones. It was also observed that ∆TR,int for aluminum foam structure is flat, while internal temperature profile was not flat with FeCrAl foams (Figure 7 and Table S5). These temperature profiles are most likely influenced by thermal conductivity differences of structured substrates3,52. The effective thermal conductivity of FeCrAl foams is 10 times lower than that of aluminum foams53,54. This difference makes heat dissipation 24 ACS Paragon Plus Environment

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less efficient in steel foam structures. Being FTS an exothermic reaction, the average temperature (the system controller uses the temperature of the foam central point for the set point) decreases with decreasing thermal conductivity of the foam alloy, that should therefore result in a lower CO conversion. In our previous work studying the effect of the thermal conductivity of metallic monoliths on methanol steam reforming52, we observed that the thermal conductivity of the structured substrates play an important role in the reactor average temperature, and therefore in the catalytic activity of the structured catalyst.

Moreover, no differences in selectivity have been observed between aluminum and FeCrAl foams with similar catalyst thicknesses less than 90 µm (Figure 7B); thicker catalyst layers on FFEC_40 sample showed increased CH4 selectivity, due to the thermal runway observed (Table S5, Figure 7). It could be concluded that when the heat produced during the FTS reaction is efficiently eliminated (using highly conductive structured substrates and/or low catalyst loading), selectivity depends only on the diffusion of reactants and products inside catalytic coating3. However, when heat removal is more difficult in poor conductive materials with high catalyst loading, a considerably increase in CH4 selectivity is observed3,54. Thus, we can conclude that highly conductive structured substrates allow to study diffusion limitations decoupled from thermal effects.

4.2. Influence of foam pore density The catalytic activity results obtained with different pore densities indicate slight influence of pore size in the investigated ranges (Figure 6). Some authors have observed that foam pore size produces a strong influence on performance of foam structure in reaction such as selective hydrogenation of 1,3-butadiene22, VOCs combustion29, WGS reaction55 and biodiesel production56. 25 ACS Paragon Plus Environment


The smaller the pore is, the higher mass-transfer coefficient becomes57,58. Sanz et al.29 observed that the increase in pore density of the foam (decrease in pore size) required lower amount of noble metal. However, Lang et al.55 observed that influence of foam pore density in WGS reaction depends on the used gas hourly space velocity. At low gas hourly space velocity, no difference in activity has been observed. Therefore, the higher gas hourly space velocity is, the more active is the foam structure with the largest ppi. This could explain the small differences in activity observed by varying the pore-density of the foam at the used flow conditions in this work.

4.3. Process intensification on foam structures In order to intensify foam structure in FTS, the optimization of catalyst amount inside the reactor is necessary to maximize reactor volumetric productivity. Comparing the results as a function of catalyst loading, it can be observed that the most interesting foam is the one with higher pore density (Table S5). Due to the existence of a limitation of catalyst layer thickness of ~70 µm (Figure 5), it is necessary that the foam structure presents the larger geometric surface area (Tables S1 and S2) and, thus, allows the larger amount of catalyst without CO diffusion limitation (Figures 5 and 7, Table S5). Therefore, the maximum volumetric productivity in our experiments was achieved loading around 1 g of catalyst in the 40 ppi aluminum foam substrate of 6 cm3. On the other hand, the catalyst activity of foam structures were compared with parallel channel monoliths coated with the same catalyst, showing an improvement in activity of 45 % (Figure 9, and Table S5). The internal temperature gradient in both structures increases slightly with temperature. By increasing the conversion of CO increases the heat produced, making it more difficult to eliminate. This also makes the heat provided by the oven lower, that is to say higher ∆TR,ext. However, although the effective radial conductivity of the foams is lower according 26 ACS Paragon Plus Environment

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to the bibliography (7 W/m·k for foam53,54 and 10 W/m·k for monolith3), not important difference in the ∆TR,int were observed at the studied conditions. Therefore, the better activity of the foam structure as compared with the monoliths could be related to a better mixing of reactants due to the tortuosity of the narrow porous substrate structure, resulting in a turbulent flow as opposed to the laminar flow characteristic of the monoliths channels. Turbulence implies better heat and mass transfer properties58,59. The special orientation of the cells assists in promoting the motion of the fluid either radially or axially4. Similar improvement in activity using foams has been previously reported for toluene combustion25, selective hydrogenation of 1,3-butadiene22, CO oxidation58 and Fischer-Tropsch synthesis34. Moreover, foams afford marked reductions of loaded catalyst amount, with respect to parallel channel monoliths. Nevertheless, the longitudinal channel monoliths allow higher catalyst loads without internal diffusion problems due to the larger geometric surface for high cell density3,42. On the other hand, the greater activity that the foams usually present in front of the monoliths of parallel longitudinal channels means that the latter suffer external diffusional limitations, in the boundary layer. This is not a problem in itself, many industrial reactors work under diffusional control regime, but it has to be taken into account when interpreting experimental results to avoid erroneous conclusions. Considering the promising results obtained for washcoated foam structure, FAL_40_70 was selected to test its thermal control capability at a total catalyst loading of 1 g and temperatures of 220–250 °C, with the space velocity equaling 6 LN/gcat·h. The results presented in Figure 9 show that conversion increased from 47 to 87 %, with reduction in C5+ hydrocarbons selectivity (66-50%) and by a slight increment in methane selectivity (20-24 %). Other authors also



observed that methane selectivity varies very little with temperature in structured reactors when practically isothermal behavior is observed3,60. To understand the change in C5+ selectivity, it must be considered C2-C4 hydrocarbons selectivity increment (from 12 to 16 %) and CO2 selectivity increment (from 1 to 9 %) with temperature. The changes in CO2 selectivity could be related with the water-gas-shift (WGS) reaction. At higher temperature (higher CO conversion), the produced water promotes the WGS reaction resulting in greater selectivity to CO261. As a consequence of increased CO conversion at elevated temperatures, the total C5+ hydrocarbon volumetric productivity increased from 47 to 87 kgC5+/m3· h1, with the latter value being 26 % higher than obtained by the parallel channel monolith coated with similar amount of catalyst.

5. Conclusion Metallic open-cell foam made of different alloys were washcoated with a Co-Re/Al2O3 catalyst and tested in FTS at 220 °C and 20 bar, revealing loss of catalyst effectiveness only for the higher catalyst thickness. In general, CH4 selectivities slightly increased and C5+ ones slightly decreased with increasing catalyst layer thickness. All coated foams showed similar trend in selectivities at δ < 70 μm. However, for thicker layers, the exact dependences were different for each of the employed alloys. For instance, aluminum foams exhibited a linear dependence for layer thicknesses of up to 90 μm. Conversely, a FeCrAl foam with δ = 90 μm showed an increased CH4 selectivity resulting from a thermal runaway. The important differences observed between these two types of foams alloys are related to their different effective thermal conductivities. These 28 ACS Paragon Plus Environment

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results indicated the importance of high effective thermal conductivity of substrates for adequate temperature control in structured FTS catalysts. The advantages of the use of foam substrates for FTS have been evidenced in this study. The catalytic results show that aluminum foams present better performance than the parallel channel monoliths. This result suggests that under the conditions studied, the transfer of reactants and products from the bulk gas phase to the catalyst surface, most favored in foams, is the controlling step in parallel channel monoliths. In addition to that, the increase in foam pore density did not improve catalytic effectiveness, provably due to the low flow conditions used in this work. Finally, the practically isothermal behavior of aluminum foams allowed the C5+ productivity to be increased in 85 % by increasing the reaction temperature from 220 to 250 °C, 26 % higher than the productivity obtained with parallel channel aluminum monolith coated with similar catalyst amount. It can be concluded that controlling the coating process (specific load and thickness) and the substrate geometry and material are the key parameters for an efficient structured reactor design with the purpose of the FTS.

Acknowledgments The authors acknowledge the Basque Government (IT1069-16) and the Spanish MINECO/FEDER (ENE2015-66975-C3-3-R and CTQ2015-73901-JIN) for the financial support and Micromeritics Instruments Corp. for the AutoChem II 2920 awarded. D. Merino acknowledges the University of



the Basque Country (UPV/EHU 2012) for the PhD grant received. A. Egaña acknowledges the Spanish MINECO (ENE2015-66975-C3-3-R) for the PhD grant received.

Supporting Information Tables of characteristics of the used structured catalysts, and radial temperature gradients and hydrocarbon productivity in FTS.

REFERENCE

(1) Khodakov, A. Y.; Chu, W.; Fongarland, P. Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels. Chem. Rev. 2007, 107, 1692.

(2) Delparish, A.; Avci, A.K. Intensified catalytic reactors for Fischer-Tropsch synthesis and for reforming of renewable fuels to hydrogen and synthesis gas. Fuel Process. Tech. 2016, 151, 72.

(3) Merino, D.; Sanz, O.; Montes, M. Effect of the thermal conductivity and catalyst layer thickness on the Fischer-Tropsch synthesis selectivity using structured catalysts. Chem. Eng. J. 2017,327, 1033.

(4) Saeidi, S.; Nikoo, M. K.; Mirvakili, A.; Bahrani, S.; Amin, N. A. S.; Rahimpour, M. R. A review of methanol production from methane oxidation via non-thermal plasma reactor. Rev. Chem. Eng. 2015, 31, 209.

(5) Stankiewcz, A. I.; Moulijn, J.A. Process Intensification: transforming chemical engineering. Chem. Eng. Prog. 2000, 22. 30 ACS Paragon Plus Environment

Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Holmen, A.; Venvik, H.J.; Myrstad, R.; Zhu, J.; Chen, D. Monolithic, microchannel and carbon nanofibers/carbon felt reactors for syngas conversion by Fischer-Tropsch synthesis. Cat. Today 2013, 216, 150.

(7) Almeida, L.C.; Echave, F.J.; Sanz, O.; Centeno, M.A.; Arzamendi, G.; Gandía, L.M.; SousaAguiar, E.F.; Odriozola, J.A.; Montes, M. Fischer-Tropsch synthesis in microchannels. Chem. Eng. J. 2011, 167, 536.

(8) Cybulski, A.; Moulijn, J.A. The Present and The Future of Structured Catalysts: An Overview,in A. Cybulski, J.A Moulijn (Eds.), Structured Catalyst and Reactors, CRC Press, Boca Raton, FL 2006, pp. 1−17

(9) Kolb, G.; Hessel, V. Review: micro-structured reactors for gas-phase reactions. Chem. Eng. J. 2004, 98, 1.

(10)Almeida, L. C.; Sanz, O.; D’olhaberriague, J.; Yunes, S.; Montes, M. Microchannel reactor for Fischer-Tropsch synthesis: Adaptation of a commercial unit for testing microchannel blocks. Fuel 2013, 110, 171.

(11)Almeida, L.C.; Sanz, O.; Merino, D.; Arzamendi, G.; Montes, M.; Gandía, L.M. Kinetic analysis and microstructured reactors modeling for the Fischer-Tropsch synthesis over a CoRe/Al2O3catalyst. Catal. Today 2010, 180, 915.

(12) Arzamendi, G.; Diéguez P.M.; Montes, M.; Odriozola, J.A.; Falabella Sousa-Aguiard, E.; Gandía, L.M. Computational fluid dynamics study of heat transfer in a microchannel reactor for low-temperature Fischer–Tropsch synthesis. Chem. Eng. J. 2005, 109, 11.

(13) Cao, C.; Wang, Y.; Jones, S. B.; Hu, J.; Li, X. S.; Elliot, D.; Jones, S.; Stevens, D. Microchannel Catalytic Processes for Converting Biomass-Derived Syngas to Transportation Fuels, ACS Symp. Ser. 2005, Chapter 17, pp. 273-284 31 ACS Paragon Plus Environment


(14) Myrstad, R.; Eri, S.; Pfeifer, P.; Rytter, E.; Holmen, A. Fischer-Tropsch Synthesis in a Microstructured Reactor. Catal. Today 2009, 147, S301.

(15) Avila, P.; Montes, M.; Miró, E. E. Monolithic reactors for environmental applications. A review on preparation techniques. Chem. Eng. J. 2005, 109, 11.

(16) Hilmen, A.M.; Bergene, E.; Lindvåg, O.A.; Schanke, D.; Eri, S.; Holmen, A. Fischer-Tropsch synthesis on monolithic catalysts of different materials. Catal. Today 2001, 69, 227.

(17) Guettel, R.; Knochen, J.; Kunz, U.; Kassing, M.; Turek T. Preparation and catalytic evaluation of cobalt-based monolithic and powder catalysts for Fischer−Tropsch synthesis. Ind. Eng. Chem. Res. 2008, 47, 6589.

(18) Visconti, C.G.; Tronconi, E.; Groppi, G.; Lietti, L.; Iovane, M.; Rossini, S.; Zennaro, R.; Monolithic catalysts with high thermal conductivity for the Fischer-Tropsch synthesis in tubular reactors. Chem. Eng. J. 2011, 171, 1294.

(19) Groppi, G.; Tronconi, E. Design of novel monolith honeycomb catalyst support for gas/solid reaction with heat exchange. Chem. Eng. Sci. 2000 55, 2161.

(20) Groppi, G.; Tronconi, E. Honeycomb supports with high thermal conductivity for gas/solid chemical processes. Catal. Today 2005,105, 297.

(21) Voecks, G.E.; Cybulski, A.; Moulijn J.A., Structured Catalysts and Reactors; Marcel Dekker Inc.: New York, 1998.

(22) Méndez, F.J.; Sanz, O.; Montes, M.; Guerra, J.; Olivera-Fuentes, C.; Curbelo, S.; Brito, J. L. Selective hydrogenation of 1,3-butadiene in the presence of 1-butene under liquid phase conditions using structured catalysts. Catal. Today 2017, 289, 151.


Page 32 of 37

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Catillon, S.; Louis, C.; Rouget, R. Development of new Cu0-ZnO/Al2O3 catalyst supported on copper metallic foam for the production of hydrogen by methanol steam reforming. Top. Catal. 2004, 30/31, 463.

(24) Sirijaruphan, A.; Goodwin Jr, J. G.; Rice, R. W.; Wei, D.; Butcher, K. R.; Roberts, G. W.; Spivey, J. J. Effect of metal foam supports on the selective oxidation of CO on Fe-promoted Pt/γAl2O3. Appl. Catal. 2005, A 281, 1.

(25) Sanz, O.; Almeida, L. C.; Zamaro, J. M.; Ulla, M. A.; Miró, E. E.; Montes, M. Washcoating of Pt-ZSM5 onto aluminium foams. Appl. Catal. B: Environ. 2008, 78, 166.

(26) Banus, E. D.; Milt, V. G.; Miro, E. E.; Ulla, M. A. Structured Catalysts for Soot Combustion for Diesel. Appl. Catal. A: Gen. 2010, 379, 95.

(27) Montebelli, A; Visconti, C. G.; Groppi, G.; Tronconi, E.; Ferreira, C.; Kohler, S. Enabling smallscale methanol synthesis reactors through the adoption of highly conductive structured catalysts. Catal. Today 2013, 215, 176.

(28) Montebelli, A; Visconti, C. G.; Groppi, G.; Tronconi, E.; Cristiani, C.; Ferreira, C.; Kohler, S. Methods for the catalytic activation of metallic structured substrates, Catal. Sci. Technol. 2014, 4, 2846.

(29) Sanz, O., Echave, F. J., Sánchez, M., Monzón, A., Montes, M. Aluminium foams as structured supports for volatile organic compounds (VOCs) oxidation. Appl. Catal. A: Gen. 2008, 340, 125.

(30) Łojewska, J.; Kołodziej, A.; Łojewski, T.; Kapica, R.; Tyczkowski, J. Structured Cobalt Oxide Catalyst for VOC. Combustion. Part I: Catalytic and Engineering Correlations. Appl. Cat. A: Gen. 2009, 366, 206.



(31) Philippe, R.; Lacroix, M.; Dreibine, L.; Pham-Huu, C.; Edouard, D.; Savin, S.; Luck, F.; Schweich, D.High thermal conductive β-SiC for selective oxidation of H2S: A new support for exothermal reactions. Cat. Today 2009, 147S, S305.

(32) Lacroix, M.; Dreibine, L.; de Tymowski, B.; Vigneron, F.; Edouard, D.; Bégin, D.; Nguyen, P.; Pham, C.; Savin-Poncet, S.; Luck, F.; Ledoux, M.-J.; Pham-Huu, C. Silicon carbide foam composite containing cobalt as a highly selective and re-usable Fischer–Tropsch synthesis catalyst. Appl. Catal. A: Gen. 2011, 397, 62.

(33) Liu, Y.; Edouard, D.; Nguyen, L.D.; Begin, D.; Nguyen, P.; Pham, C.; Pham-Huu, C. The effective thermal properties of solid foam beds: Experimental and estimated temperature profiles. Chem. Eng. J. 2013, 222, 273.

(34) Almeida, L.C.; González, O.; Sanz, O.; Paul, A.; Centeno, M.A.; Odriozola, J.A.; Montes, M. Fischer-tropsch catalyst deposition on metallic structured supports. Stud. Surf. Sci. Catal. 2007, 167, 79.

(35) Park, J.C.; Toh, N.S.; Chun, D.H.; Jung, H.; Yang, J.I. Cobalt catalyst coated metallic foam and heat-exchanger type reactor for Fischer–Tropsch synthesis. Fuel Process. Tech. 2014, 119, 60.

(36) Yang, J.-I.; Yang, J. H.; Kim, H.-J.; Jung, H.; Chun, D. H.; Lee, H.-T. Highly effective cobalt catalyst for wax production in Fischer-Tropsch synthesis. Fuel 2010, 89, 237.

(37) Mandic, M.; Todiz, B.; Zivanic, L.; Nikacevic, N.; Bukur, D. B. Effects of Catalyst Activity, Particle Size and Shape, and Process Conditions on Catalyst Effectiveness and Methane Selectivity for Fischer–Tropsch Reaction: A Modeling Study. Ind. Eng. Chem. Res. 2017, 56, 2733.

(38) Visconti, C.G.; Tronconi, E.; Lietti, L.; Groppi, G.; Forzatti, P.; Cristiani, C.; Zennaro, R.; Rossini, S. Fischer-Tropsch synthesis on a Co/Al2O3catalyst with CO2containing syngas. Appl. Cat. A: Gen. 2009, 370, 93. 34 ACS Paragon Plus Environment

Page 34 of 37

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(39) Kapteijn, F.; de Deugd, R.M.; Moulijn, J.A. Fischer-Tropsch synthesis using monolithic catalysts. Catal. Today 2005, 105, 350.

(40) Hilmen, A.M.; Bergene, E.; Lindvag, O.A.; Shanke, D.; Eri, S.; Holmen, A. Fischer-Tropsch synthesis on monolithic catalysts with oil circulation. Cat. Today 2005, 105, 357.

(41) Champman, L.R. US 4318828 (1982)

(42) Merino, D.; Pérez-Miqueo, I.; Sanz, O.; Montes, M. On the Way to a More Open Porous Network of a Co-Re/Al2O3Catalyst for Fischer-Tropsch Synthesis: Pore Size and Particle Size Effects on Its Performance. Top. Catal. 2016, 59, 207.

(43) Merino, D.; Sanz, O.; Montes, M. Effect of catalyst layer macroporosity in high-thermalconductivity monolithic Fischer-Tropsch catalysts. Fuel 2017, 210, 49.

(44) Scholten, J.J.; Pijpers, A.P.; Hustings, A.M.L. Surface Characterization of supported and nonsupported hydrogen catalysts, Catalysis Reviews: Science and Engineering. 1985, 27, 151. (45)

McKee, D. W. Interaction of hydrogen and carbon monoxide on platinum group metals. Journal of Catalysis 1967, 8(3), 240.

(46)Sanz, O.; Banús, E.D.; Goya, A.; Larumbe, H.; Delgado, J.J.; Monzón, A.; Montes, M. Stacked wire-mesh monoliths for VOCs combustion: Effect of the mesh-opening in the catalytic performance. Cat. Today 2017, 296, 76.

(47) Schanke, D.; Vada, S.; Blekkan, E.A.; Hilmen, A.M.; Hoff, A.; Holmen, A. Study of Pt-promoted cobalt CO hydrogenation catalysts. J. Catal. 1995, 156, 85.

(48) Post, M.F.M.; van't Hoog, A.C.; Minderhoud, J.K.; Sie, S.T.; Diffusion limitations in fischertropsch catalysts. AIChE J. 1989, 35, 1107.



(49) Iglesia, E.; Reyes, S.C.; Madon, R.J.; Soled, S.L. Selectivity control and catalyst design in the Fischer-Tropsch synthesis: sites, pellets, and reactors. Adv. Catal. 1993, 39, 221.

(50)Jung, A.; Kern, C.; Jess, A.; Davis, B.H. Advances in Fischer–Tropsch Synthesis, Catalysts, and Catalysis. M.L. Occelli (Eds.), CRC, Press, Boca Raton, 2010.

(51) von Rickenbach, J.; Lucci, F.; Narayanan, Ch.; Eggenschwiler, P. D.; Poulikakos, D. Effect of washcoat diffusion resistance in foam based catalytic reactors. Chem. Eng. J. 2015, 276, 388.

(52)Sanz, O.; Velasco, I.; Reyero, I.; Legorburu, I.; Arzamendi, G.; Gandía, L.M.; Montes, M. Effect of the thermal conductivity of metallic monoliths on methanol steam reforming. Catal. Today 2016, 273, 131.

(53) Bianchi, E.; Heidig, T.; Visconti, C.G.; G. Groppi, H. Freund, E. Tronconi, Heat transfer properties of metal foam supports for structured catalysts: Wall heat transfer coefficient. Catal. Today 2013, 216,121.

(54) Bianchi, E.; Heidig, T.; Visconti, C.G.; Groppi, G.; Freund, H.; Tronconi, E. An appraisal of the heat transfer properties of metallic open-cell foams for strongly exo-/endo-thermic catalytic processes in tubular reactors. Chem. Eng. J. 2012, 198-199, 512.

(55) Lang, C; Sércordel, X.; Kiennemann, A.; Courson, C. Water gas shift catalysts for hydrogen production from biomass steam gasification. Fuel Process. Tech. 2017, 156, 246.

(56) Yu, X.; Wen, Z.; Lin, Y.; Tu, S.-R.; Wang, Z.; Yan, J. Intensification of biodiesel synthesis using metal foam reactors. Fuel 2010, 89, 3450.

(57) Stemmet, C.P.; van der Schaff, J.; Kuster, B.F.M.; Schouten, J.C. Solid Foam Packings for Multiphase Reactors: Modelling of Liquid Holdup and Mass Transfer. Chem. Eng. Res. Des. 2006, 84 (A12), 1134.


Page 36 of 37

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(58) Giani, L.; Groppi, G.; Tronconi, E. Mass-transfer characterization of metallic foams as supports for structured catalysts. Ind. Eng. Chem. Res. 2005, 44, 4993.

(59)Giani, L.; Groppi, G.; Tronconi, E. Heat transfer characterization of metallic foams. Ind. Eng. Chem. Res. 2005, 44 (24), 9078.

(60) Sheng, M.; Yang, H.; Cahela, D.R.; Yantz Jr, W.R.; Gonzalez, C.F.; Tatarchuk, B.J. High

conductivity catalyst structures for applications in exothermic reactions. Appl. Catal. A, 2012, 445–446, 143. (61) Riedel, T.; Claeys, M.; Schulz, H.; Schaub, G.; Nam, S.-S.; Jun, K.-W.; Choi, M.-J.; G. Kishan, Lee, K.-W. Comparative study of Fischer–Tropsch synthesis with H2/CO and H2/CO2 syngas using Fe- and Co-based catalysts. Appl. Cat. A: Gen. 1999, 186, 201.


Fischer-Tropsch synthesis intensification in foam structures

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