Chapter 1 Introduction




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Chapter 1

Introduction



1.1 Introduction

Multiphase reactions involving reactions between gas and liquid phases over a solid catalyst surface are used extensively in chemical, petroleum, petrochemical, biochemical, material, and environmental industrial processes for a wide variety of products. Examples include the catalytic hydrogenation of petroleum fractions to remove sulfur impurities (Irandoust et. al. 1990) and the catalytic oxidation of liquid hydrocarbons with air or oxygen (Levenspiel, 1996, 1998).


For such multiphase reactions, different types of reactors have been used over the years, such as stirred tank slurry reactors, slurry bubble column reactors, packed bed reactors and others (Kapteijn et al. 2001; Nijhuis et al. 2001; Krishna et al. 1994). The reactor choice has been governed by the reaction chemistry, the reactor hydrodynamics, the simplicity of use and manufacture, and the need to enhance mass transfer rate for mass transfer limited reactions. Many investigations have been performed to enhance the understanding of such reactors. An excellent recent review of multiphase reactors can be found in Dudukovic’ et al. (2002).
However, conventional reactors have several drawbacks. For example, in a packed bed reactor, a large pressure drop is encountered even at moderate gas and liquid flow rates. This limits the operation of the packed bed at high gas and liquid velocities, and hence limits the reactor productivity. High pressure drop also translates into high energy input to the system. Moreover, lack of full utilization of catalyst in the bed and poor mass transfer rates have always been issues in such reactors. On the other hand, in continuous stirred tank reactors, a considerable amount of mechanical energy needs to be invested to achieve the desired degree of mixing. Additionally, after the process is completed, the catalyst needs to be separated from the product and recycled, thus increasing the operating cost.

1.2 An Overview of Monolith Reactor

Although conventional reactors will continue to play a major role in various process industries in coming years, researchers have started looking for alternatives to mitigate the above mentioned shortcomings.


One such alternative is the use of a monolith reactor, a tubular reactor stacked with catalyst coated (or impregnated) monoliths instead of random packing. Figure 1 shows a monolith in which parallel channels are separated by walls made of cordierite (magnesium aluminosilicate) or other ceramic materials. Hence, no radial mixing occurs. Monoliths can carry active catalyst in two ways: the surface can have a washcoat of the active catalyst, or the structure can be impregnated with active catalyst. Monoliths are industrially produced by extrusion of a paste containing catalyst particles or by extrusion of a support on which the catalyst can be coated (washcoating). In monoliths the channel cross-sections are usually rectangular, but circles, triangles, hexagons or more complex geometries also exist. To increase the surface area, internal fins can also be provided. These fins have a stabilizing effect on the gas-liquid flow and allow operation in counter-current mode without flooding (Heibel et al. 2004; Lebens et al. 1999a, b, c, 1997). A general overview of characteristics, fabrication, and typical applications can be found in Williams (2001), Garcia-Bordeje et al. (2002), and Gulati (1998). An earlier work by Lachman and Williams (1992) also provides a good introduction to monolith production, raw materials, and its application in catalytic two-phase processes.

Figure 1.1: Monolith catalyst carrier: a proposed substitute for random packed bed.

Some geometrical features of monolith are worth mentioning here. The number of channels per unit cross sectional area, the “cell density”, typically ranges from 25 to 1200 channels per square inch (cpsi). The void fraction varies between 0.5 and 0.9 and is frequently expressed as the open frontal area (OFA). Typical values for the wall thickness range between 0.05 and 0.5 mm. A monolith structure is characterized by the wall thickness and cell density, which are independent of each other. Various important geometrical parameters are related to each other as follows:

cell density



1-1

open frontal area



1-2

geometric surface area



1-3

hydraulic diameter



1-4

The parameters in equations 1-1 to 1-4 are defined as: tw-wall thickness, L- length from one channel wall center to the other, R – fillet radius (R is normally not specified, since it varies with the die wear; however its effect on geometric properties is included for the sake of completeness - from Cybulski et al. (1998). The relevant dimensions are shown in Figure 1-2

Figure 1-2: Cross section of a single cell (not to scale)

Monolith reactors can be operated in two distinct flow regimes, Taylor flow regime (or slug flow regime) and annular flow regime (Coleman et al., 1999). Taylor flow regime is characterized by the movement of a train of alternate gas bubbles and liquid slugs through the monolith capillary channels. On the other hand, in annular flow regime, the liquid falls on the sides of the capillary walls and the gas flow through the core. The details of all the flow regimes in a monolith have been described in more detail in Chapter Two. The monolith reactor (as a multiphase reactor) operating in both flow regimes is claimed to have several advantages: very low pressure drop, excellent mass transfer properties, high surface/volume ratio, short diffusion distance, low axial dispersion, and ease of reactor scale-up, among others (Boger et al. 2004; Nijhuis et al. 2001; Edvinsson et al. 1998). However, there are still a few drawbacks associated with monolith reactors. These include the high cost of manufacturing the structure and poor heat transfer. However, metallic monoliths have been studied in some recent works for application in exothermic reactions (Boger et al. 2005)
A monolith, as a structure, is not new to the scientific and industrial community. Monoliths have been successfully used in solid catalyzed gaseous reactions (gas-solid reactions) such as in automobile catalytic converters and in abatement of NOx and CO emissions from power plants (Cybulski et al. 1999). Akzo Nobel has started production of hydrogen peroxide using an industrial scale three-phase monolith reactor by the hydrogenation of anthraquinone to the corresponding hydroquinones (Albers et. al 2001). Recent works on the use of monolith as a bioreactor (Ebrahimi et al. 2004) and photobioreactor (Son, 2001) have also been reported.
Although research involving multiphase monolith has progressed in the last decade, many important aspects of this research are still to be explored. Monolith reactors will gain industrial acceptance only after extensive experimental verifications and further insight into their operation.

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