Catalytic Material with Enhanced Contacting Efficiency for Volatile

Article pubs.acs.org/IECR

Catalytic Material with Enhanced Contacting Efficiency for Volatile Organic Compound Removal at Ultrashort Contact Time Sabrina Wahid and Bruce J. Tatarchuk* Center for Microfibrous Materials Manufacturing (CM3), Department of Chemical Engineering, Auburn University, 212 Ross Hall, Auburn, Alabama 36849, United States ABSTRACT: A unique catalyst structure, microfibrous entrapped catalyst (MFEC), was prepared by entrapping support particles (γ-Al2O3, 150−250 μm diam) into metal (Ni-200) microfibers and forming thin flexible sheets. Pleated structures of MFECs containing Pd/γ-Al2O3, Pd−Mn/γ-Al2O3, and Pd−Ce/γ-Al2O3 were investigated systematically for volatile organic compound (VOC) removal at various face velocities (ca. 3−30 m/s) and at low temperatures ( n-hexane. In general, the ease for decomposing VOC by precious metal catalysts follows the general order of alcohols > aldehydes > aromatics > ketones > alkenes > alkanes.26,35,36 An important goal of this work was to investigate the performance of pleated MFEC at high face velocities for catalytic applications. Many performance aspects such as pressure drop, chemical conversion, amount of catalyst utilization, cost of construction, catalyst life, etc., would be involved in evaluating the overall performance of a reactor system.37 Since it is difficult to define a single term that can judge all these aspects, only a few selected parameters were examined here: chemical conversion and pressure drop per unit length of the reactor bed. A similar approach was employed by Kołodziej and Łokewska38 for evaluating the performance of structured reactors with short channels. As the characteristic length of a given system decreases, the mass transport (i.e., chemical conversion) and momentum transport (i.e., pressure drop) increases. While the former is desirable, the latter is not since the system will need to be operated at higher pressures. For a given material, there should be an optimum characteristic length which gives the best performance. Hence, heterogeneous contacting ef f iciency (ηHCE), a dimensionless number, is defined as the product of mass efficiency (i.e., conversion achieved) to flow efficiency (i.e., inverse of pressure drop), per unit surface area of the catalyst (a measure of catalyst utilization). The mass efficiency (ηmass) is defined as the log reduction of VOC concentration achieved per unit surface area of the catalyst, as shown in eq 2.

( )

log ηmass =

C in Cout

n p(πd p2)

(2)

The flow efficiency (η flow ) is defined as the inverse dimensionless pressure drop across the material. ηflow =

1 ρυ 2n (πdp 2) 2 o p

(3)

ΔP

So, the overall ηHCE is defined as the product of ηmass and ηflow which is independent of the amount of catalyst, according to eq 4. The higher the ηHCE value, the better the performance of the structured reactor, as it indicates higher conversion with lower pressure drop. ηHCE = ηmassηflow

1 = 2

( ) ρυ

log

C in Cout

ΔP

o

2

(4)

Figure 12 shows the comparisons of ηHCE versus superficial velocity for various pleated MFEC and different VOCs at the same inlet conditions. In each experiment, system temperature, catalyst loading, and media thickness were kept the same for different pleated configurations. It is evident from the plot that an increase in pleat number increased the ηHCE value. With an increase in the PF, the effective velocities in MFEC were cut down by an equivalent factor and the pressure drop decreased accordingly. For the same reason, the residence times as well as the associated conversions increased with the increase in PF. This clearly demonstrates that the high PF value of MFEC designs can provide higher conversions with lower pressure

Figure 11. n-Hexane conversion at different system velocities using Wshaped structure: (a) Pd/Al2O3; (b) Pd−Ce/Al2O3; and (c) Pd−Mn/ Al2O3.

metal. However, when PdO catalyst is modified by reducible oxides (e.g., CeO2, MnOx), catalytic activity depends on both the oxygen activation on the active metal and the capacity of the oxide for providing active lattice oxygen. MnOx and CeO2 provide high oxygen storage capacity by taking up oxygen under oxidizing conditions and releasing it under the reducing conditions.29−34 Therefore, PdO modified by reducible oxides G

dx.doi.org/10.1021/ie400282q | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 12. ηHCE versus system velocity for different pleated MFEC structures (flat and W-shaped structure): (a) ethanol; (b) toluene; and (c) nhexane (T = 473 K). H

dx.doi.org/10.1021/ie400282q | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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df = diameter of the fibers (μm) dp = catalyst particle diameter (mm) af = external surface area per unit vol of fibers (1/m) ap = external surface area per unit vol of particles (1/m) np = number of catalyst particles T = temperature (K) PF = pleat factor Re = Reynolds number

drop. Furthermore, for higher reactive compound e.g. ethanol, the value of ηHCE is comparatively higher than less reactive compounds. In these experiments, MFEC reactor operation was not in the interphase mass transfer controlled regime, and hence, there is further scope for improving the reactant conversion in MFEC by increasing the catalytic activity. The bed composition and properties (PF, voidage, catalyst loading, etc.) of MFEC structure also requires further optimization in order to derive their full benefit.

Greek Symbols

4. CONCLUSION A novel catalytic structure with enhanced heterogeneous contacting efficiency was developed for VOCs removal where the process involved high gas velocities. These high gas velocities indicated to the low intralayer residence times. The results demonstrated that pleated MFEC had caused a decrease in the effective velocity (inside MFEC media). This decreased effective velocity resulted in lower pressure drop and also higher residence time or, in other words, higher conversions. This dual advantage achieved with pleating MFEC media leads to higher ηHCE values. Furthermore, the low intralayer residence time inside the MFEC reduced the VOC removal and caused the surface reaction rate to be the limiting factor for overall reaction rate. This VOC removal can be explained by the Mars−Van Krevelen mechanism which is an oxidation− reduction process. The results also verified high oxygen storage capacities of reducible oxides while noble metal oxide modified by these oxides showed better performance. Pleated MFEC showed promising performance in this study as it provided a large surface/volume ratio for the entrapped catalyst. This attribute facilitates the weight and volume demand of the air handling system in a limiting space. As the bed properties of microfibrous materials can easily be tailored to the requirement of specific applications, our future work will focus on the comparison of pleated MFEC with other conventional heterogeneous contacting systems, e.g., wash-coated monoliths and packed bed catalyst particulates.

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ρ = gas density (kg/m3) ρc = catalyst support density (kg/m3) μ = viscosity (kg/m·s) ε = bed void fraction φf = sphericity of the metal fibers, 6/(dfaf) φp = sphericity of particles, 6/(dpap) ηmass = mass efficiency ηflow = flow efficiency

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AUTHOR INFORMATION

Corresponding Author

*Mailing address: 314 Ross Hall, Department of Chemical Engineering, Auburn University, Auburn, AL 36857, USA. Tel.: +1 334 844 2023. Fax: +1 334 844 2065. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by U.S. Army TankAutomotive Research, Development and Engineering Center (TARDEC) under contract no. W56HZV-05-C-0686. The authors want to thank Mr. Troy Barron and Ronald Putt for their technical assistance.

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NOMENCLATURE ΔP = pressure drop across the reactor (kPa) L = length/thickness of reactor bed (mm) νo = face velocity (m/s) Cin = reactant concentration at the inlet (ppmv) Cout = reactant concentration at the outlet (ppmv) dc = characteristic length of reactor (m) I

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