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Kinetics, Catalysis, and Reaction Engineering

Catalytic combustion of low-concentration methane on structured catalyst supports Anna Gancarczyk, Marzena Iwaniszyn, Marcin Pi#tek, Mateusz Korpy#, Katarzyna Sindera, Przemys#aw J. Jod#owski, Joanna Lojewska, and Andrzej Kolodziej Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01987 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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Industrial & Engineering Chemistry Research

Catalytic combustion of low-concentration methane on structured catalyst supports Anna Gancarczyk*1, Marzena Iwaniszyn1, Marcin Piątek1, Mateusz Korpyś1, Katarzyna Sindera1, Przemysław J. Jodłowski2, Joanna Łojewska3 and Andrzej Kołodziej1,4 1

Institute of Chemical Engineering, Polish Academy of Sciences, Bałtycka 5, 44-100 Gliwice, Poland,

2

Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, 31-155

Kraków, Poland, 3

Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland,

4

Faculty of Civil Engineering and Architecture, Opole University of Technology, Katowicka 48, 45-061 Opole,

Poland.

ABSTRACT: Innovative structured catalyst supports, like solid foams, wire gauzes and shortchannel structures are considered in methane catalytic combustion. For comparison, classical supports such as packed beds and monoliths are also taken into account. Moreover, two catalysts, displaying “fast” and “slow” kinetics, are examined. The performance efficiency criterion is applied to account for a balance between the process conversion, mass transfer and flow resistance. Another “technological” approach compares reactor length and the corresponding pressure drop required to reach the desired conversion rate. Results indicate that wire gauze, solid foam and short-channel structure are highly promising catalyst supports due to their intense heat/mass transfer and moderate flow resistance, particularly for fast catalytic reactions. For slow reactions, monoliths seem to be the best as they exhibit the lowest flow resistance.

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INTRODUCTION In times of recurring energy crises, facing decreasing resources of fossil fuels, methane unquestionably appears as the most abundant of them. In relation to the energy supplied (combustion heat 55.5 MJ/kg or 39.8 MJ/m3), the lowest amount of carbon dioxide is released during combustion of methane due to the amazing C:H ratio. Therefore methane can be considered as an efficient and environmentally friendly fossil fuel and its consumption still increases. However, despite its undoubted advantages, methane is the cause of serious environmental problems, mainly related to global warming. Methane is a harmful greenhouse gas with a Global Warming Potential (GWP) of 21.1 This means methane causes a greenhouse effect of an intensity 21 times greater than the same amount of CO2 released into the atmosphere. The current concentration of methane in the atmosphere is 1754 ppb, an increase of 150% compared to the pre-industrial period.2 The share of methane in the greenhouse effect is still growing. Annually, 0.45 Gt of methane is emitted to the atmosphere. This is, seemingly little compared to the 24 Gt of CO2, but, multiplication by the methane GWP of 21 gives CO2 equivalent of 9.45 Gt, almost half of the whole anthropogenic emission of carbon dioxide.3 The sources of methane can be divided into scattered (e.g. agriculture and breeding, transport and waste storage) and local (industry). Coal mining is an important source of methane is the. Methane concentrations in the ventilation air of coal mines may reach as high as 1%, although they are usually lower, commonly below 0.5%. Simultaneously, the streams of the ventilation air are very large.4 The problem is even more urgent as EU new regulations banning methane emissions are due as early as 2021. Methane emissions are of particular interest for major coal producers such as China, India and the USA, and remain a pressing problem for science and economy.


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There are different ways to reduce methane emissions. Catalytic combustion is currently the most effective one, although this process faces several serious problems including: •

Usually low methane concentrations in the air. This involves problems with the autothermy process, initial gas heating, reaction heat recovery, and sufficiently efficient transport of reagents towards the catalyst surface.

•

High exothermy (enthalpy) of the methane oxidation reaction. This causes local overheating of the catalyst and, in consequence, sintering and thermal deactivation. Diverse pollutants contained in the reactive gases, e.g. compounds of sulphur or halogens, are an additional factor causing loss of catalyst activity.

•

Very large streams of the gas to be purified. This entails high energy costs for the gas pumping through the reactor, so the reactor design requires the least possible flow resistance.

Therefore, the catalytic reactor design has to provide: •

A highly efficient and selective catalyst.

•

Appropriate management of the energy/temperature.

•

Mass transport of reactants to the catalyst active centres intense enough to place no limit on the reaction kinetics and thus on the process yield.

•

Satisfactory low flow resistance. Moreover, from the catalytic point of view, methane molecules react difficulty due to

their high symmetry. Methane places high demands on catalysis. For the proper design and operation of the catalytic reactor, adequate knowledge of the chemical properties, process parameters and geometry of catalyst supports is essential, enabling control of the thermo-fluid dynamic phenomena. The influence of the kinetic, flow and transfer characteristics of the


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catalytic structure on the reactor performance (including conversion, yield, selectivity, pumping cost, etc.) has been of interest to researchers for many years. The most popular type of catalytic reactor, the packed bed of catalytic grains, is widely used in the chemical industry due to its simplicity, low cost and ease of replacement when the catalyst is deactivated. However, such packing reactors packing are unattractive due to numerous shortcomings. Their major drawback is large flow resistance depending on the grain diameter (the smaller the diameter the higher the pressure drop). Another disadvantage is low catalyst effectiveness, especially in the case of fast chemical reactions; the catalyst efficiency decreases with grain diameter due to the resistance of internal diffusion. Usually, the diameter of grains is within 2 ÷ 10 mm. Larger diameters are often purposeless, due to lower catalyst effectiveness, while smaller grains increase flow resistance, and thus the cost of fluid pumping. The outline of this type of reactor can be found in many publications.5-6 Classic monolithic structure is constructed of many parallel channels; the catalyst is applied in the form of a thin coating (typically below 50 µm) on the inner walls of the ducts, which brings about low internal diffusion resistance and amazing catalyst efficiency. Additionally, monoliths offer a satisfactory large surface area for the fluid-catalyst contact, and low flow resistance. The main design parameter given by the manufacturers is the number of channels per square inch (cpsi). For reducing car exhaust emissions, densities of 400-900 cpsi are of interest;7-8 catalytic combustion or emission reduction in power plants requires 10 ÷ 400 cpsi.8 Specific surface area of a typical monolith of 200 cpsi corresponds to 1.5 mm grains, while that of 900 cpsi corresponds to 0.7 mm, and has very low flow resistance. Monoliths, initially used as catalytic automotive afterburners (car catalysts), have subsequently been used in many industrial processes such as hydrogenation, hydrodesulphurisation, oxidation, bioremediation and the


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Fischer-Tropsch process.8-10 Monolithic reactors are typically made of ceramics (cordierite, alumina), metal alloys and graphite.7,

9

Apart from their advantages, classical monoliths have

significant drawbacks. Monoliths are sensitive to coking which can plug the channels; moreover, replacement of the deactivated catalyst layer (usually including noble metals) is very difficult.11 In addition, monoliths show low intensity of mass transport to the channel surface. This may limit the overall process rate, especially for fast reactions. An overview of this type of reactors and examples of its numerous applications can be found in many publications and monographs.710, 12-13

One of the ways to intensify heat/mass transport under laminar flow conditions in the monolithic reactors is to operate in the entry region of the channels, where the laminar flow develops.11,

14

Usually, the gas stream reaches the monolithic reactor through a tube of larger

diameter, where turbulent flow causes the velocity, temperature and concentration profiles to be smoothed (despite local eddies/turbulences). Therefore, all the profiles (considered flat at the monolith channel inlet) develop simultaneously within the inlet section of the monolith where the laminar flow exists as a result of a much smaller channel diameter. There, the heat, mass and momentum transfer are much more intensive compared to the developed laminar flow. Short monoliths, called short-channel structures,11, 15 enable the operation of the whole reactor under conditions of developing laminar flow, thus intensifying the transport properties. They are considered by the automotive industry to be best placed before the turbocharger in vehicle engines (just behind the exhaust valves), where gas velocity achieves 100 m/s and total conversion of exhaust gases reaches 50-70%.16-18 Other advantages of short-channel structures are their high specific surface area, good efficiency of layered catalysts, and increasing resistance to hot-spots due to intense heat transfer. Moreover, an interesting and promising feature of these


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structures is the possibility to control the transport and friction properties within broad ranges by channel length regulation alone.19-20 However, they display somewhat higher flow resistance when compared with long-channel monoliths. Woven and knitted wire gauzes are another type of promising catalyst support, owing to their large contact surface area and intensive heat/mass transfer. They are arranged into a stack consisting of many layers. A choice between mesh sizes and wire diameters allows structures of appropriate void volume and specific surface area to be designed. Gauzes are characterized by regular mesh dimensions and shapes. The ability to produce them of different materials makes them more resistant to adverse conditions such as contact with corrosive chemicals or high temperatures. Another advantage of the gauzes is their low price. Wire gauzes are used in the catalytic oxidation of ammonia,21 but they are also considered in catalytic combustion, high efficiency heat exchangers, heat storage, porous fins, solar-receiving devices, structures in heat pipes, regenerators, and as electronics coolers.22-24 Solid foams are relatively new solutions. Their mechanical and thermal properties are advantageous. Solid foams can be manufactured of various materials such as metals and alloys (aluminium, nickel, copper, nickelchromium, fecralalloy), ceramics (Al2O3, ZrO2, mullite, SiC), polymers (polystyrene, polyethylene, PVC), and glass and carbon (graphite or amorphous vitreous carbon).25-26 Convective heat transfer is enhanced by strong mixing of the flowing fluid and continuous destruction of boundary layers provided by foam ligaments normal to the flow direction. Other advantageous features are large specific surface area and extremely high void fraction (even above 90%), which makes foams attractive as catalyst supports and results in lower pressure drop compared to packed beds. Solid foams have been widely used in different fields of industry, such as liquid metal filters, shock/energy absorbers, electrodes, etc.12, 27-29


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The aim of the study is the deep and comprehensive analysis of possible structured catalyst supports to find the best solution for such a demanding process as catalytic combustion of low-concentration methane. In addition to the packed bed or monolith, three structured catalyst support were considered. These were nickelchromium solid foam, knitted wire gauze and metal short-channel structure. The analysis included two different methods of depositing the catalysts (leading to different kinetics) to evaluate the tested catalyst support for “fast” and “slow” reactions. This will yield an answer to the key question of which reactor arrangement is the best for the given process conditions, in particular the reaction kinetics. The amazing feature of the structured catalyst supports is their heat/mass transfer rate, which may be regulated within wide ranges by the appropriate structure design. Although the flow resistance is always somewhat proportional to the heat/mass transport due to the heat and momentum transport analogy, it is possible to find some optimum specific process assumptions, in particular the intrinsic reaction kinetics, inlet methane concentration, process temperature and the final conversion required. This optimum includes the support design (thus its flow and transfer characteristics) and the process conditions (thus temperature and flow rate).

MATERIALS AND METHODS Characterization of the catalyst support All of the catalyst supports tested were selected to have comparable specific surface area available for active catalyst deposition. Wire gauze and triangular short-channel structure were made from Kanthal, a steel containing approximately 20 wt% Cr, 3 wt% Al and less than 1 wt% Co. The gauze void fraction and specific surface area were measured using a weighing


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technique: by weighing a piece of wire of known (measured) length and diameter and comparing the results with the weight of a piece of a gauze sheet of known dimensions, the surface area and the void fraction were calculated. The short-channel structure is composed from flat and zig-zag strips stacked alternately and joined by micro-welding. In order to obtain similar electric resistance for both strips, the zig-zag strip (approximately twice as long as the flat one) was twice as thick as the flat strip (0.1 and 0.05 mm, respectively). Specific surface area and porosity for the short-channel structure were determined in the same way as for the wire gauze. The foam studied was nickelchromium (NC 0610) foam (delivered by Recemat B.V.) with the pore density (pores per inch – PPI) within 6 ÷ 10 (according to the producer’s specifications). The specific surface area and porosity of the foam were determined using computer microtomography (µCT; SkyScan 1172). The foam sample (cylinder, 10 mm in diameter and high) was scanned with the voxel size of 10 µm. The image processing was performed using the global thresholding method assisted by iMorph software; the details can be found in our previous works.30-32 The cells, windows and strut dimensions were determined using optical microscopy methods based on about 100 measurements for each of them.

Heat transfer and pressure drop measurement The heat transfer studies were conducted in a test reactor of rectangular cross-section (45 x 30 mm), as shown in Fig. 1.A. The metal structures were heated by the electric current flowing directly through them; the electrical wires were soldered to the structures. Temperatures of the structure surface and flowing gas (air and ambient conditions) were measured by a dozen or so small thermocouples. Finally, the heat transfer coefficients and Nusselt numbers were calculated.20 The same rectangular reactor was also used to determine flow resistance for the


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wire gauze and the short-channel structure using the Recknagel micromanometer. Pressure drop experiments for solid foam were carried out in a 0.057 m ID test column filled with the foam discs (Fig. 1.B). Because it was found that the height of stacked foam discs does not affect the unit pressure drop,31 a height of 0.10 m was used.

Figure 1. Experimental set-up for (A) heat transfer and (B) pressure drop for solid foam studies: 1 – blower; 2 – flowmeter 3 – thermocouples, 4- electric power generation system, 5 – structure studied, 6 – reactor, DP – differential pressure gauge.

Kinetic tests The first palladium catalyst (Pd/Al2O3) was prepared by the incipient wetness (IW) method (for details see22, 33), and the second one (Pd/ZrO2) by the sonochemical (SC) method (cf.33-34). The kinetic tests for Pd/Al2O3 were performed using a CSTR (Continuous Stirred Tank Reactor) gradientless reactor within a temperature range of 100 ÷ 550oC under atmospheric pressure with a constant flow rate, while for the Pd/ZrO2 catalyst the tests were carried out using a Catlab (Hiden Analytical) fixed bed quartz tubular test reactor. For a specific temperature, in stabilized conditions, the concentrations of the gaseous reactants were measured using an FTIR photoacoustic gas analyzer (Gasera PA101).22,

35

The reaction rate and the kinetic parameters


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such as activation energy and kinetic rate constant were calculated using mass balance for the CSTR (for Pd/Al2O3) and tubular reactor (for Pd/ZrO2). The kinetic rate constant was related to the geometric, external surface area of the catalyst support. Detailed information about the procedure of the catalytic experiments can be found in our previous studies.34-35

Reactor modelling The reactor modelling, checked for n-hexane catalytic combustion,36 was performed for the one-dimensional (1D) plug flow reactor model using Matlab R2011a software.22, 33 The material balance was expressed as: ௗሺ஼ಲ ௪ሻ ௗ௭

+ ܵ௩ ݇௖ ሺ‫ܥ‬஺ − ‫ܥ‬஺ௌ ሻ = 0

(1)

where w was calculated by dividing the total flow rate (in cubic meters per second) to the transversal section of the reactor (in square meters). Assuming: (i) boundary condition (BC): z = 0 CA = CA0; (ii) influence of the axial dispersion on the final conversion can be neglected,36-37 (iii) mass transfer of methane from the gas stream to the catalyst surface is balanced by chemical reaction according to the equation: ݇௖ ሺ‫ܥ‬஺ − ‫ܥ‬஺ௌ ሻ = ߟሺ−ܴ஺ ሻ = ߟ݇௥ ‫ܥ‬஺ௌ

(2)

where the efficiency factor, η, can be expressed as: ߟ=

୲ୟ୬୦ሺథሻ థ

(3)

and the Thiele modulus, φ, is given as: ߶ = ݈ට

௞ೝ

஽ಲ೔

(4)

l (ell)

where l is the catalyst layer thickness. The Thiele modulus varies along the reactor, therefore its value (thus, its efficiency factor) was calculated for the local reaction rate.


10

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The heat generated due to catalytic combustion is transferred to the gas stream, thus the energy balance is: ை

ௗ்

‫ܿߩݓ‬௣ ௗ௭ + ܵ௩ ℎሺܶ − ܶௌ ሻ + ஺ ݇ு ሺܶ − ܶ௪ ሻ = 0 ೎

(5)

O (oh)

with boundary conditions z = 0; T = T0. The last term in Eq. (5) describes heat losses. In the case of an externally well-insulated reactor, heat losses can be ignored. The heat generated at the catalyst is balanced by the heat transfer from the catalytic layer to the gas stream: ℎሺܶௌ − ܶሻ = −Δ‫ܪ‬ோ ߟሺ−ܴ஺ ሻ

(6)

During the reactor modelling, all the physical and chemical parameters were calculated for the local temperature. For modelling, the following assumptions were made: •

Inlet gas temperature T0 = 723K

•

Inlet CH4 concentration CA0 = 2000 ppm (0.2 vol%, typical methane concentration in the coal mines ventilation air)

•

Reaction enthalpy, ∆HoR = - 803 kJ/mol

•

1st order reaction

•

Gas superficial velocity, w, is equal 1 m/s, 2 m/s and 3 m/s

•

Heat losses are ignored.

RESULTS AND DISCUSSION Characterization of the catalyst supports Comparison of the specific surface area and porosity of the tested structured catalyst supports (wire gauzes, short-channel structures and solid foams) with those for packed beds and monoliths is presented in Fig. 2. Structured catalyst supports offer a much wider range of


11


specific surface area and porosity than classic packing. Moreover, it is evident that they display much higher porosity compared to classical packed beds. Therefore, their flow resistances are expected to be lower. 9000 specific surface area [m 2/m 3]

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

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wire gauzes

8000

solid foams

7000 6000

short-channel structures

5000 4000

packed beds

3000 2000 1000

monoliths

0 0,3

0,4

0,5

0,6 0,7 porosity

0,8

0,9

1

Figure 2. Overview of the studied catalysts supports: structured versus classic supports. For the study, catalyst supports displaying similar external specific surface area were chosen. Therefore, the knitted wire gauze (Fig. 3.A), triangular short-channel structure (Fig. 3.B) and the nickelchromium (NC 0610) foam (Fig. 3.C) were studied. Their geometrical parameters are presented in Table 1.

Figure 3. Reactor packings tested: A – wire gauze, B – triangular short-channel structure, C – nickelchromium foam.


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Table 1. Parameters of the catalyst supports studied. Parameter

Wire gauze

mesh/cell per inch

17.45

gauze thickness, mm

0.66

Triangular short-channel structure

NC 0610 foam

Monolith

Packed-bed

100

channel dimensions: base/height, mm

5.5/4.5

channel length, mm

10

200

grain diameter, mm

3

cell/pore/window diameter, mm

3.60/1.89/1.06

wire/strut diameter, mm

0.098

porosity

0.97

0.95

0.89

0.72

0.38

1

1355

1314

1298

1339

1240

hydraulic diameter, mm

2.85

2.88

2.71

2.15

1.23

specific surface area, m-

0.529

Flow resistance The flow resistance is presented in terms of pressure drop vs. gas velocity (Fig. 4.A) and the Fanning friction factor vs. Reynolds number (Fig. 4.B). The Fanning friction factor, f, is defined by the Darcy-Weisbach equation: ∆௉ ௅

= 2݂

ఘ௪ మ

ఌ మ ஽೓

(7)


13


A

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B

Figure 4. (A) flow resistance vs. gas velocity and (B) Fanning friction factor vs. Reynolds number for tested catalyst supports. As the catalyst supports tested differed considerably, the hydraulic diameter, Dh, was chosen as the characteristic dimension, and consequently this diameter was used in all the dimensionless numbers as well as in the Fanning friction factor definition. The solid foam, knitted wire gauze and short-channel structures display flow resistance between that of a monolith and packed bed, and they are arranged in the following order: packed bed > wire gauze > solid foam > short-channel structure > monolith; the results for the solid foam and the wire gauze are close. Based on the experimental results, correlation equations describing the Fanning friction factor of solid foam, knitted wire gauze38 and short-channel structure19 have been derived; as presented in Table 2.


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Table 2. Correlations describing heat and mass transfer and flow resistance properties for considered catalyst supports Catalyst support NC 0610 (this study)

Correlations

݂ = 79.9/ܴ݁ + 0.445 for 1 < Re < 2700

ܰ‫ = ݑ‬0.96ܴ݁ ଴.ହଷ ܲ‫ݎ‬ଵ/ଷ for 1 < Re < 2700 ܵℎ = 0.96ܴ݁ ଴.ହଷ ܵܿ ଵ/ଷ for 1 < Re < 2700 ሺ݂ܴ݁ሻ = 13.33 + 11.59ሺ‫ܮ‬ା ሻି଴.ହଵସ

short-channel structure19

ܰ‫ = ݑ‬ሾ3.11 + 0.45ሺ‫ ∗ܮ‬ሻି଴.଺ଵ ሿሾ0.55ሺPr‫ ∗ܮ‬ሻି଴.ଵହ ሿ ି଴.଺ଵ

ܵℎ = ቂ3.11 + 0.45൫‫ ୑∗ܮ‬൯ knitted wire gauze38-39

ି଴.ଵହ

ቃ ቂ0.55൫Sc‫ ୑∗ܮ‬൯

ቃ

݂ = 118.09/Re + 0.836 for 1 < Re < 2700 ܰ‫ = ݑ‬2.19ܴ݁ ଴.଺ଷ଺ ܲ‫ݎ‬ଵ/ଷ for 1 < Re < 2700 ܵℎ = 2.19ܴ݁ ଴.଺ଷ଺ ܵܿଵ/ଷ for 1 < Re < 2700 ሺ݂ܴ݁ሻ = 14.23ሺ1 + 0.045/‫ܮ‬ା ሻ଴.ହ

monolith40

ܰ‫ = ݑ‬3.608ሺ1 + 0.095/‫ ∗ܮ‬ሻ଴.ସହ

଴.ସହ

ܵℎ = 3.608൫1 + 0.095/L∗୑ ൯

packed bed41-42

߂ܲ ߤw ሺ1 − єሻଶ ߩw ଶ ሺ1 − єሻ = 150 ଶ + 1.75 ‫ܮ‬ єଷ ‫ܦ‬௛ єଷ ‫ܦ‬௛ ܰ‫ = ݑ‬2 + 1.1ܴ݁ ଴.଺ ܲ‫ݎ‬ଵ/ଷ

ܵℎ = 2 + 1.1ܴ݁ ଴.଺ ܵܿ ଵ/ଷ

The hydrodynamic dimensionless channel length is defined as: ‫ܮ‬ା = ஽

௅

(8)

௅

(9)

೓ ோ௘

The thermal dimensionless channel length is: ‫= ∗ܮ‬ ஽

೓ ோ௘௉௥

The mass dimensionless channel length is:


15


‫∗ܮ‬ெ = ஽

௅

೓ ோ௘ௌ௖

Page 16 of 33

(10)

where L is the channel length.

Heat and mass transfer Heat transfer results for all the packings studied are presented in terms of the Nusselt vs. Reynolds numbers (Fig. 5).

Figure 5. Nusselt number vs. Reynolds number for the catalyst supports studied.

As evident from Fig. 5, Nusselt numbers for the considered structures are arranged in the order: wire gauze > packed bed > solid foam > short-channel structure > monolith. When comparing to Fig. 4, the most striking difference is that the knitted wire gauze displays higher heat transfer intensity (Nusselt number) than the packed bed, but lower flow resistance (cf. Fig. 4). Taking into account Fig. 4 and Fig. 5, wire gauze displays promising friction and transfer characteristics. However, there is some deviation from the transfer-friction analogy for the


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considered set of reactor internals: the higher the flow resistance, the more intensive the transfer properties, and vice versa. Moreover, heat transfer of wire gauze is even higher than that of packed bed. The correlation equations based on the experimentally determined heat transfer19, 39 are presented in Table 2. The mass transfer coefficients for catalyst supports were derived using the Chilton – Colburn43 analogy: ݆=

ே௨

ோ௘௉௥ భ/య

ௌ௛

= ோ௘ௌ௖ భ/య

(11)

This analogy, commonly applied for different reactor internals, was experimentally confirmed for wire gauzes,39,

44

as well as for solid foams.45-46 The mass transfer results are

presented in Table 2.

Performance criteria Considering the potential industrial applications of the structured catalyst supports, possibly the highest process yield (including reaction selectivity, mass transport, etc.) and, at the same time, the lowest flow resistance (pumping cost) are expected. It is not easy to compare such different structures and to decide arbitrarily about the best one for a given process. Therefore, different criteria are proposed in the literature to estimate the efficiencies of the structures. Such criteria include performance efficiency (the trade-off between mass transfer and pressure drop),11,

47

or “technological” factor (comparison of necessary reactor length and resulting

pressure drop33). All the criteria allow an optimal reactor type (catalyst support) and operating conditions (temperature and flow rate) to be found.


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As discussed, catalytic combustion of low-concentration methane is a highly demanding process and both the heat/mass transfer and flow resistance are crucial for its efficient realization. Therefore two different methods of palladium catalyst preparation, namely the incipient wetness (IW) method and the sonochemical (SC) method, were applied. The details are summarized in Table 3.

Table 3. Catalyst preparation and kinetic data. Catalyst

Preparation method

Metal content, wt %

k∞, m/s

Activation energy, Ea, kJ/mol

φ[a]

η[a]

Pd/ZrO234-35

wetness

3.5

252.49

62.79

0.03

1

Pd/Al2O3

sonication

3.43 ± 0.9

1.07x1010

110.4

3.24

0.32

[a] calculated for 723 K and l = 20 µm

As found in our former studies,34 the sonochemical (SC) method of catalyst preparation gives higher catalyst activity in comparison to preparation using the incipient wetness (IW) method. Therefore, the process carried out on the SC catalyst can be treated as “fast” reaction, and for the IW catalyst as “slow” reaction (cf. Table 3). Based on the SEM analysis, it was found that the catalyst layer thickness is approximately 20 µm for catalysts deposited on the steel surface, independent of the method of catalyst preparation. For comparison, the same catalyst layer thickness was assumed for all the catalyst supports studied, including a packed bed of eggshell type grains. Parameters describing reactor performance such as reactor length, conversion and flow resistance have been calculated using the one-dimensional (1D) plug flow reactor model, kinetic data from Table 3, and experimental correlations describing heat, mass and momentum transfer


18

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for structured catalyst supports (Table 2). For packed beds and monoliths classic literature expressions describing heat and mass transfer as well as flow resistance have been adopted: for packed beds, Ergun41 and Wakao and Kaguei42 equations, and for monoliths, Hawthorn40 equations, as presented in Table 2.

Criterion 1 – “technological” criterion The modelling results for the SC catalyst are presented in Fig. 6 for all the catalyst supports considered. The conversion reached as well as the resulting pressure drop vs. reactor length are shown in Fig. 6.A. The reactor length required to reach 90% conversion and resulting flow resistance vs. gas velocity are presented in Fig. 6.B and in Table 4.


19


Page 20 of 33

Figure 6. Modelling results obtained for SC catalyst: (A) – conversion and pressure drop profiles along the reactor for w = 1m/s, inlet CH4 concentration: 2000 ppm, 723 K; (B) – required reactor length and corresponding pressure drop for assumed conversion rate X = 90% vs. gas velocity.

Table 4. Reactor performance for assumed final conversion for SC and IW catalysts. (Inlet conditions: 2000 ppm CH4, 723 K, SC catalysts). SC catalyst X=0.9 w m/s 1

2

3

Reactor support

IW catalyst X=0.8

L mm

∆P kPa

Mcat kg/m2

L mm

∆P kPa

Mcat kg/m2

wire gauze

1.70

0.008

0.038

320

1.474

6.938

NC 0610

20.0

0.070

0.415

320

1.110

6.646

short-channel str.

20.0

0.037

0.420

333

0.608

6.996

monolith

16.0

0.015

0.358

260

0.225

5.820

packed bed

1.40

0.159

0.028

177

19.792

3.511

wire gauze

2.20

0.029

0.048

600

7.833

13.008

NC 0610

24.0

0.230

0.498

600

5.700

12.461

short-channel str.

28.0

0.129

0.589

640

2.936

13.455

monolith

32.0

0.061

0.716

480

0.835

10.744

packed bed

2.10

0.686

0.042

354

114.169

7.016

wire gauze

2.40

0.064

0.052

880

22.308

19.078

NC 0610

32.0

0.584

0.665

840

15.172

17.445

short-channel str.

36.0

0.292

0.757

920

7.391

19.342

monolith

44.0

0.131

0.985

720

1.889

16.116

packed bed

2.80

1.795

0.056

480

303.479

9.523


20

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For the SC catalyst, independent of the gas velocity the supports are divided into two groups distinctly different in their performances (Fig. 6.A). The lowest reactor lengths for nearly complete conversion of methane are found for packed bed and wire gauze. The foam, shortchannel structure and monolith display a little worse performance, requiring a somewhat longer reactor to get the same conversion rate. Flow resistance of the wire gauze per unit reactor length is higher than that of solid foam and short-channel structure (c.f. Fig. 6.A and Fig. 4). However, wire gauze, with the SC catalyst deposited on it, requires a much shorter reactor length to reach assumed 90% conversion; this results in lower flow resistance (Fig. 6.B and Table 4). Higher gas velocity results in increased reactor length and thus in catalyst amount (Table 4). While for wire gauze and packed bed the increase in the length is moderate, for other catalyst supports it is more significant (Fig. 6.B, Table 4). It is especially visible for monolith: for w = 1 m/s, monolith displays shorter reactor length than short-channel structure and solid foam, but for higher gas velocity it becomes the longest one. This result from the weak dependence of the transport characteristics of the monolith on the gas velocity; for other supports, the relationship is more pronounced (cf. Fig. 5). However, when comparing the reactor length required and the resulting pressure drop, the best performance, i.e. the lowest reactor length and the lowest pressure drop, is found for wire gauze. The modelling results evaluated for IW catalyst are shown in Fig. 7 and given in Table 4 as well.


21


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Figure 7. Modelling results obtained for IW catalyst: (A) – conversion and pressure drop profiles along the reactor length for w = 1m/s, inlet CH4 concentration: 2000 ppm, 723 K; (B) – reactor length and corresponding pressure drop obtained for conversion rate X = 80% vs. gas velocity.

In this case, conversion equal to 80% was chosen due to slower kinetics compared to SC catalyst. For IW catalyst, the shortest reactor length, thus the lowest catalyst amount, was found for packed bed, and next for the monolith (Fig. 7.A). The structured supports display comparable reactor length to achieve the assumed conversion rate, X=80%, with a small advantage for solid foam (Fig. 7.B). The tendency agrees with the results presented by Jodłowski et al.35 When comparing the pressure drop, the catalyst supports are arranged in the order: packed bed > wire


22

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gauze > solid foam > short-channel structure > monolith. The IW catalyst offers a slower reaction rate, so a much longer reactor is required than for SC catalyst. The greatest difference was obtained for the knitted wire gauze: the same conversion for IW catalyst was obtained for a reactor at least two orders of magnitude longer than for an SC one (cf. Fig. 6 and 7 as well as Table 4).

Criterion 2 – Performance Efficiency Criterion (PEC) Another way to examine the reactor efficiency is verification of the balance between pressure drop and mass transfer. Generally, this criterion is a ratio of the reactor yield to the flow resistance. It means the criterion considers the effects of reactor operation (chemical reaction limited by mass transfer and resistance), thus profits, and the operational cost (flow resistance). Such a criterion type was presented by Kołodziej and Łojewska,11, 48 to compare short-channel structures with classical monoliths. The criterion is: ܲ‫= ܥܧ‬

௟௡ሺ஼ಲబ /஼ಲಽ ሻ ∆௉/ሺఘ௪ మ ሻ

=

ୈ௞಴ ௌೡ

(12)

ೖ ଶ୵௙ቀଵା ೎ ቁ ೖೝ

Assuming the process is strictly limited by the diffusional transport (mass transfer limited conditions), i.e. when kr >> kc, the criterion can be written as: ܲ‫= ܥܧ‬

ି௟௡ሺଵିଡ଼ሻ

௱௉/ሺఘ୵మ ሻ

=

௟௡ሺ஼ಲబ /஼ಲಽ ሻ ∆௉/ሺఘ௪ మ ሻ

=

ୈ௞಴ ௌೡ ଶ୵௙

ୈௌ ௌ௛ఌ

ೡ = ଶ௙ோ௘ௌ௖

(13)

Criterion similar to Eq. (13) was also proposed by Giani et al.47 to compare the metal foams with packed beds and monoliths. Some formal differences in comparison to PEC definition presented by Kołodziej and Łojewska11, 48 appear in the dimensionless numbers definitions. The higher the PEC, the better the catalyst supports, due to the higher conversion and smaller pressure drop. As can be seen from Eq. (12) and (13), the efficiency of the catalytic reactor is influenced by the mass transfer coefficient, kC, the Fanning friction factor, f, and


23


Page 24 of 33

specific surface area, Sv. The efficiency increases with the kinetics and mass transfer intensity, and decreases with the flow resistance. The comparison of PEC for catalyst supports is presented in Fig. 8.A and Fig. 8.B for SC and IW catalysts, respectively. A

B

Figure 8. PEC for tested catalyst supports (eq. (12)) vs. Reynolds number for (A) SC catalyst and (B) IW catalyst; CH4 inlet concentration: 2000 ppm at 723 K. For SC catalyst, the highest PEC was obtained for the knitted wire gauze (in the Reynolds range within 50 ÷ 1000) due to high mass transfer and low pressure drop (Fig. 8.A). High efficiency was obtained also for the monolith, as the results of the lowest flow resistance. On the other hand, the least efficiency of the packed bed reactor results from the highest pressure drop (low porosity). Almost constant criterion index observed for monolith is the result of almost constant momentum, heat and mass transfer (cf. Figs 4 and 5). A similar trend – a slight increase of PEC – is shown for the short-channel structure (Fig. 8.A). For other supports, a certain PEC increase was observed up to about Re=200, where the mass transfer is the limiting factor. For higher Reynolds numbers, the impact of the flow resistance prevails, and PEC decreases.


24

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For the IW catalyst the best PEC is found for monolith as a catalyst support, and the worst for packed bed (Fig. 8.B). The structured supports lie between those two limiting packings and are arranged in the order: knitted gauze < solid foam < short-cannel structure. Moreover, for IW catalysts, only the descending branch of the criterion curve was found. This means that the flow resistance is the limiting factor across the whole considered range of Reynolds number. The impact of the mass transfer is weak because of slow reaction rate, thus the mass of reactants transported to the catalyst is rather low.

CONCLUSIONS Catalytic combustion of low-concentration methane is still a challenge for catalysis and engineering. From the catalysis point of view, symmetric methane molecules are difficult for oxidation. It is not easy to elaborate a catalyst displaying high activity (i.e. fast reaction rate), especially in low temperatures, resistance to thermal deactivation and poisons as well as limited consumption of noble metals. For engineering, intense mass and heat transfer are required due to low methane concentration and high reaction enthalpy, respectively, to avoid limiting the reaction rate (by insufficient mass transfer) and prevent local hot-spots. A low pressure drop is expected due to the large gas streams to be processed. In the study, diverse structured supports such as knitted wire gauze, short-channel structure and metal foam were compared. Two differently prepared catalysts were considered the SC and IW ones showing “fast” and “slow” reaction kinetics, respectively. Two criteria, referred to as “technological” and PEC (Performing Efficiency Criterion), were applied to evaluate the effectiveness of the catalysts supports.


25


Page 26 of 33

The results show that the structured catalyst supports studied can compete with, or even have an advantage over the commonly used packed beds and monoliths. Both criteria showed that for “fast” reaction (SC catalyst) the best performance is offered by the knitted wire gauze: the shortest reactor length thus the smallest catalyst amount are required for 90% conversion accompanied by the lowest flow resistance. According to PEC, wire gauze is rated higher than others for Reynolds numbers within 50 ÷ 1000. For lower and higher Reynolds numbers, monolith displays higher PEC criterion. For “slow” reaction (IW catalyst) monolith displays the best results according to both the criteria applied. An amazing feature of structured catalyst supports is their ability to regulate the intensity of transport and flow resistance. Therefore, for fast kinetics a structure of intense transport properties may be designed to avoid limiting the possible process yield and secure acceptably low flow resistance (pumping costs). For slow kinetics, the transport may be less intense. As always, transport is proportional to the flow resistance, and excessively intensive transport entails also excessive flow resistance. Appropriate structure design may offer smooth regulation between highly intense heat, mass and momentum transfer (e.g. wire gauze) towards lower intensity (e.g. monolith). This means that it is possible to select the structure that provides the optimal combination of satisfactory low flow resistance and intense mass transfer rate.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions


26

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Polish National Science Centre (Project No. DEC2011/03/B/ST8/05455, DEC-2016/21/B/ST8/00496 and 2015/17/D/ST8/01252) and the National Centre for Research and Development (LIDER/204/L-6/14/ NCBR/2015). ABBREVIATIONS AC – cross-sectional area of reactor CA – mean reactant A concentration in bulk gas phase, mol/m3 cp – gas specific heat, J/(kgK) DA – diffusivity, m2/s Dh – hydraulic diameter, = 4ε/Sv, m f – Fanning friction factor ∆HR – standard heat of reaction, J/mol h – heat transfer coefficient, W/m2·K kC – mass transfer coefficient, m/s kH – heat transfer coefficient of reactor wall, W/m2·K kr – reaction rate constant k ஶ - pre-exponential coefficient in Arrhenius equation, m/s


27


Page 28 of 33

L – bed or channel length, m L+ – dimensionless length for the hydrodynamic entrance region, = L/(DhRe) L* – dimensionless length for the thermal entrance region, = L/(DhRePr) L*M – dimensionless length for the mass transfer entrance region, = L/(DhReSc) l – catalyst layer thickness, m Mcat – mass of catalyst, kg/m2 Nu – Nusselt number, = αD/λ O – reactor circumference, m Pr – Prandtl number, = µcp/λ (-RA) – reaction rate, mol/(m2·s) Re – Reynolds number, = wgDhρ/(ηε) Sc – Schmidt number, = µ/(ρDA) Sh – Sherwood number, = kCDh/DA Sv – specific surface area, m2/m3 w – superficial velocity, m/s ∆P – pressure drop, Pa T – temperature, K X – conversion z – reactor axis, m ε – porosity η - effectiveness factor for catalyst λ – thermal conductivity of fluid, W/(mK) φ – Thiele modulus


28

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µ – dynamic viscosity, Pas ρ – density, kg/m3

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48. Kolodziej, A.; Lojewska, J., Optimization of structured catalyst carriers for VOC combustion. Catal. Today 2005, 105 (3-4), 378-384.


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