Three-phase monolith reactors

In this chapter, first, the existing correlations for three-phase monolith reactors will be reviewed. It should be emphasized that most of these correlations were derived from a limited number of experiments, and care must be taken in applying them outside the ranges studied. Furthermore, most of the theoretical work concerns Taylor flow in cylindrical channels (see Chapter 9). However, for other geometries and flow patterns we have to rely on empirical or semiempirical correlations. Next, the modeling of the monolith reactors will be presented. On this basis, comparisons will be made between three basic types of continuous three-phase reactor monolith reactor (MR), trickle-bed reactor (TBR), and slurry reactor (SR). Finally, for MRs, factors important in the reactor design will be discussed. 

Industrial experience with the three-phase MR is limited, since only few large-scale industrial plants are running in the world today. Very little has been published about these industrial reactors and reactor scale-up. Also, most modeling has been done on cylindrical channel geometry, while most industrial reactors use sinusoidal or square geometry. Hence this chapter mainly summarizes our own experience with three-phase monolith reactors, with limited reference to the literature. 

Scaling up three-phase monolith reactors from pilot plant to industrial size is easy in some areas and more difficult in others. Since there is no interaction between the channels, the behavior within the monolith channels is independent of scale. Adding more parallel channels will not affect the flow, the mass and heat transfer, or reactions in each channel, as long as the flow distribution is uniform. Also, both the pilot plant reactors and the industrial reactors are adiabatic due to the absence of radial mixing. 

R.H. Patrick, Jr., T Klindera, L.L. Crynes, R.L. Cerro, and M.A. Abraham, Residence Time Distribution in Three-Phase Monolith Reactor, AIChE J. 4I(3) (A9 (1995). 

An alternative to the trickle-bed reactor, developed only recently, is the three-phase monolith reactor [3,4].The monolith catalyst has the shape of a block with straight narrow channels with the catalytic species deposited on the walls of these channels. Advantages of the monolith reactor compared with the trickle-bed reactor are its low pressure drop and the much smaller diffusion distance, because of the thin catalyst layer. They also are claimed to be intrinsically safe. 

Kreutzer, M.T., Du, R, Heiszwolf, J.J., Kapteijn, F., and Moulijn, J.A. (2001) Mass transfer characteristics of three-phase monolith reactors. Chem. Eng. 

M. T. Kreutzer, P. Du, J. J. Heiszwolf F. Kapteijn and J. A. Moulijn, Mass transfer characteristics of three-phase monolith reactors. Chemical Engineering Science, 2001, 56 (21-22), 6015-6023. 

Hydrogen peroxide is mostly produced on a large scale using the anthraquinone (AQ) autoxidation process. The key step is the selective liquid-phase hydrogenation of the AQs to their corresponding hydroquinones (Scheme 9.8). An industrial process has been designed at Chalmers University and developed and used by Akzo-Nobel on the pilot scale. It involves a three-phase monolith reactor but very few details... 

The approach is illustrated by a case study, a three-phase monolith reactor coupled to a recycling device, the Screw Impeller Stirred Reactor (SISR) developed at TU Delft (Kaptejn et al. 2001),. Cylindrical monoliths are placed in a stator, and a foam of gas and liquid is forced through the monolith channels with the aid of a screw (Fig. 1). Monolith reactors combine the advantages of slurry reactors and fixed beds minimized internal diffusion, low pressure drop and continuous operation (Nijhuis et al. 2001). 

In the design of optimal catalytic gas-Hquid reactors, hydrodynamics deserves special attention. Different flow regimes have been observed in co- and countercurrent operation. Segmented flow (often referred to as Taylor flow) with the gas bubbles having a diameter close to the tube diameter appeared to be the most advantageous as far as mass transfer and residence time distribution (RTD) is concerned. Many reviews on three-phase monolithic processes have been pubhshed [37-40]. 

Figure 25 Aspects controlling the performance of a three-phase catalytic reactor, indicating the flexibility of the use of monolithic catalysts.

Figure 3 shows two examples of reactors with a fixed catalyst for gas-liquid reactions, viz. the trickle-bed reactor and the three-phase monolith reaetor. In these reactors the flow of liquid phase usually approaches plug flow. The figure also shows an example of a batch reactor system for a liquid-liquid reaction consisting of a mixing tank and a fixed-bed reactor with upward flow. This set-up is applied in aromatic acylation. 

Irandoust, S., Cybulski, A., and Moulijn, J.A. (1998) The use of monolithic reactors for three-phase reactions, in Structured Catalysts and Reactors,... 

Obviously, the least experience has been accumulated with monoliths, particularly in three-phase applications. They are also more expensive than the other reactors. Therefore, the use of monoliths can only be economically ju.stified for three-phase processes in which it offers a distinct advantage, like higher yield, improved. selectivity, increased throughput of a plant, or lower overall investment or operating costs. Of particular interest are situations in which a MR substantially simplifies the design or operation of a unit. 

Nijhuis TA, Kreutzer MT, Romijn ACJ, Kapteijn F, Moulijn JA. Monolithic catalysts as more efficient three-phase reactors. Catal Today 2001 66 157-165. 

Andersson B, Irandoust S, Cybulski A. Modeling of monolith reactors in three-phase processes. In Cybulski A, Moulijn JA, eds. Structured Catalysts and Reactors. Chemical Industries, Vol. 71. New York Marcel Dekker, 1998 267-304. 

Smits HA, Stankiewicz A, Glasz WC, Fogl THA, Moulijn JA. Selective three-phase hydrogenation of unsaturated hydrocarbons in a monolithic reactor. Chem Eng Sci 1996 51 3019-3025. 

An interesting monolithic configuration has recently been disclosed that can be suitable for three-phase processes carried out in countercurrent mode [10]. This can be particularly important for processes in which both thermodynamic and kinetic factors favor countercurrent operation, such as catalytic hydrodesulfurization. The flooding of a reactor is a considerable limitation for the countercurrent process run in conventional fixed-bed reactors. Flooding will not occur to that extent in the new monolith. A configuration of channels of the new monolith is such that subchannels open to the eentcrline are formed at the walls. The liquid flows downward, being confined in these subchannels and kept there by surface tension forces. The gas flows upward in the center of the channel. The results of studies on the new monolith concept are presented in Chapter 11 of this book. 

Lie et al. [35] modeled the CO oxidation by O2 on a Pt/Al203 catalyst with an isothermal monolithic converter in order to assess the effect of cyclic feeding on the performance of the reactor. The kinetic model of Herz and Marin [59] was used, which consists of a closed sequence of elementary steps. The reactor model is essentially as described by Eqs. (25)-(27), but now includes accumulation terms for all three phases the gas phase, the pores of the washcoat, and the catalyst surface. 

In the last 15 years, the use of monoliths has been extended to include applications for performing multiphase reactions. Particular interest has been focused on the application of monolith reactors in three-phase catalytic reactions, such as hydrogenations, oxidations, and bioreactions. There is also growing interest in the chemical industries to find new applications for monoliths as catalyst support in three-phase catalytic reactions. 

Of primary interest for the industrial application of monolith reactors is to compare them with other conventional three-phase reactors. Two main categories of three-phase reactors are slurry reactors, in which the solid catalyst is suspended, and packed-bed reactors, where the solid catalyst is fixed. Generally, the overall rate of reactions is often limited by mass transfer steps. Hence, these steps are usually considered in the choice of reactor type. Furthermore, the heat transfer characteristics of chemical reactors are of essential importance, not only due to energy costs but also due to the control mode of the reactor. In addition, the ease of handling and maintenance of the reactor have a major role in the choice of the reactor type. More extensive treatment of conventional reactors can be found in the works by Gianetto and Silveston [11], Ramachandran and Chaudhari [12], Shah [13,14], Shah and Sharma [15], and Trambouze et al. [16], among others. 

V. APPLICATION OF MONOLITH REACTORS IN THREE-PHASE PROCESSES... 

Cybulski et al. [39] have studied the performance of a commercial-scale monolith reactor for liquid-phase methanol synthesis by computer simulations. The authors developed a mathematical model of the monolith reactor and investigated the influence of several design parameters for the actual process. Optimal process conditions were derived for the three-phase methanol synthesis. The optimum catalyst thickness for the monolith was found to be of the same order as the particle size for negligible intraparticle diffusion (50-75 p.m). Recirculation of the solvent with decompression was shown to result in higher CO conversion. It was concluded that the performance of a monolith reactor is fully commensurable with slurry columns, autoclaves, and trickle-bed reactors. 

R.K. Edvinsson and A. Cybulski, A comparative analysis of the trickle-bed and the monolithic reactor for three-phase hydrogenations, Chem. Eng. Sci. 49(248) 5653 (1994). 

R. K. Edvinsson, Monolith reactors in three-phase processes. PhD dissertation, Chalmers University of Technology, Gdteborg, 1994. 

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