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Ind. Eng. Chem. Res. 2007, 46, 8574-8583
Enhancement of Catalyst Performance Using Pressure Pulses on Macroporous Structured Catalysts Jasper J. W. Bakker,* Willem J. Groendijk, Karen M. de Lathouder, Freek Kapteijn, Jacob A. Moulijn, Michiel T. Kreutzer, and Sten A. Wallin Catalysis Engineering, DelftChemTech, Delft UniVersity of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands, and Core R&D, The Dow Chemical Company, 1776 Building, Midland, Michigan 48674
This paper explores the enhancement of catalyst performance by inducing convection inside a macroporous catalyst carrier. The catalyst carrier used was an acicular mullite monolith with highly permeable microstructured walls. The monolith walls were coated with colloidal silica and impregnated with Pd. After performing the proper preparation steps, the high permeability of the walls was preserved and the catalyst was uniformly distributed with a low effective diffusion length. The catalyst performance of the selective hydrogenation of 3-methyl-1-pentyn-3-ol was investigated in a monolithic stirrer reactor, under internal diffusion-limited conditions, using different silica-supported palladium monoliths. At 2.3 bar H2, 20 mol/m3 alkyne, and 25 °C, using equal catalyst loadings, the activity per unit monolith volume was 100% greater and the maximum yield of the alkene was 9% greater for the mullite monolith than for the eggshell-coated monolith with impermeable walls. This confirms that internal diffusional limitations were reduced. Pressure pulses of passing gas bubbles inside the monolith channels induced a convective enhancement, thereby making mass transfer limitations inside the highly permeable walls absent. Introduction Heterogeneous catalysis in liquids is severely hindered by slow diffusion inside particles, with detrimental effects on activity, selectivity, and stability of the catalyst. There are three ways to overcome diffusional problems: (1) one can make a less active catalyst, tilting the balance of diffusion and reaction in favor of the latter; (2) one can make a smaller catalyst, which is very effective because diffusional time scales with the square of length; (3) one can force the liquid to flow through the porous catalytic material. The subject of this paper is the design of structured catalysts that exploit this third option: convective enhancement of catalyst effectiveness. It is illustrative to consider some characteristic numbers: assume a spherical catalyst particle with dp ) 0.6 mm, with a first-order rate constant of kV ) 0.1 s-1 and an intraparticle diffusion coefficient Deff ) 10-10 m2/s. Then, the characteristic time for reaction 1/kV amounts to 10 s, and the characteristic time for diffusion is (dp/6)2/ Deff ) 100 s. We can balance these times by either reducing the catalyst’s activity tenfold or by making a 0.6 mm eggshell catalyst with a 70 µm active layer. Both these measures increase the required catalyst volume tenfold. Alternatively, if we are able to transport matter through the catalyst with an intraparticle velocity of only dp×kV ) 60 µm/s, the diffusional problems vanish and the per-volume activity can be maintained. For a more comprehensive theoretical analysis of convective enhancement, the reader is referred to Nir and Pismen.1 Several reports of intraparticle convective enhancement of reactions rates have been reported in the open literature. The rich dynamics of multiphase flow is usually used to create such convection, and more often than not, a gas phase is involved. Wilhite and co-workers2 showed that both selectivity and yield could be improved in trickle beds by operating in the pulsing regime. It is well-known in electrochemistry that evolution of * To whom correspondence should be addressed. Tel.: 0031152786733. Fax: 0031152785006. E-mail:
[email protected].
gas disrupts concentration boundary layers. Intraparticle production of CO2 by immobilized yeast cells and the subsequent convection caused by this gas is known to enhance the effectiveness of this reaction.3 Van den Heuvel and co-workers4,5 have designed convective enhancement on purpose by entrapping a gas bubble inside a catalyst particle and subsequently subjecting that particle to variation in static pressure by recycling the particle in a vertical-loop reactor. As the particle traverses this loop, the variation in pressure causes the bubble to shrink and expand, squeezing liquid in and out of the particle. In this study, we used monoliths as structured support materials.6-9 A monolith consists of thousands of small (∼1 mm) parallel straight channels, separated by thin walls in which external mass transfer can be decoupled from internal mass transfer by independently varying the wall thickness and the channel diameter.10 The channel walls of a monolith are usually coated with large surface area catalyst carriers to create an eggshell catalyst.11 A disadvantage, in particular for the eggshellwashcoated monoliths, is the lower catalyst inventory when compared to packed-bed reactors. Convective enhancement would be a welcome method to increase the activity per unit monolith volume. When local or temporal pressure variations are used to induce flow inside the particle, it is imperative that the permeability is high. In recent years, various groups have reported the synthesis of high-porosity support materials for catalysts that exhibit such a high permeability.12-17 In this work, we use high-porosity honeycomb monoliths, where the walls consist of fused, interlocked, microstructured acicular mullite grains with a continually interconnected open-pore system (parts a and b of Figure 1).18-20 By controlling nucleation and crystal growth, different mullite grain sizes and, consequently, different pore sizes can be produced. For a comparison, a less porous cordierite monolith wall is shown in Figure 1c. One can conceive many different hydrodynamic mechanisms that create pressure pulses. In this paper, we will explore the use of Laplace pressures in capillary channels as the forcing mechanism. For a gas bubble traversing a capillary channel, a
10.1021/ie070033o CCC: $37.00 © 2007 American Chemical Society Published on Web 06/07/2007
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8575 Table 1. Properties of the Different Monoliths Used in This Study
property channel size, mm wall thickness, µm specific surface area, m2/g wall porosity, % average pore size, µm pore volume up to pore size of 10 µm, % intrinsic permeability, 10-12 m2
Figure 1. (a) SEM micrograph of the microstructured walls of the acicular mullite monolith; (b) backscattered electron image of a polished cross section of a wall intersection showing the highly permeable walls made of interlocked elongated mullite grains with an interconnected pore system; (c) backscattered electron image of a polished cross section of a cordierite monolith wall intersection.
acicular mullite large pores
acicular mullite small pores
cordierite
1 240 0.22 71 45 10
1 240 0.54 62 5 80
1.1 180 0.21 36 8 60
1
