Catalytic Wall versus Fix Bed Reactors

The catalytic wall reactor (CWR), mainly used with a tubular geometry, also known as Tubular Wall Reactor (TWR), consists of a reacting stream flowing in a cylindrical tube, which has the wall covered with a thin catalyst layer. Such reactor design attracted attention since early ’50s due to the fact that it can handle highly exothermic reactions with good temperature control (Baron et al. 1952, Chambre et al. 1956, Katz 1959, Hudson 1965, Solomon and Hudson 1967). Smith and Carberry (1974) and later Huang and Varma (1980) highlighted that there are several advantages o f the CWRs compared to the conventional flxed-bed reactors (FBR). The latter are usually heat transfer limited due to large gas-solid thermal resistances, subject at the same time to significant intraphase mass resistances. Within CWRs, the heat is generated at the wall eliminating the large gas-solid thermal resistances, thus much higher heat generation rates are achievable without excessive temperature rise. Since the catalyst is used in a thin layer its efficiency is much higher and the amount o f catalyst required is significantly reduced. For complex reaction systems, the diffusion o f intermediate desired product from the catalytic wall to the bulk gas phase is faster than from a catalyst pellet, resulting in shorter residence time and potentially enhanced yield. In contrast to FBR where high gas velocities are required to minimise the gas film around the pellet, CWRs can make use o f lower velocities. Consequently, lower pressure drops are achieved. Finally, the parametric sensitivity o f CWRs is minimal providing a higher degree of stability than FBR (Smith and Carberry 1975), which in turn leads to better temperature control. Thus, catalyst destruction and deactivation due to hot spots is minimised and the catalyst life extended.

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2.7.2 A p p lic a tio n s o f C a ta ly tic W a ll R e a cto rs

The application of catalyst-coated wall reactors has been investigated for various exothermic reactions such as oxidation of sulphur dioxide (Baron et al. 1952), hydrogenation of carbon monoxide (Goyal et al. 1982, 1983; Dalai et al. 1992a, 1992b), oxidation of naphthalene over V2O5 catalysts (Smith and Carberry 1975 , 1976, Parent et al. 1983), conversion of synthesis gas to gaseous and liquid fuel via Fischer-Tropsch synthesis (Goyal et al. 1984, 1988), hydrogenation of nitrobenzene to aniline (Amon et al. 1999). Extensive attention has been dedicated to catalytic combustion in monoliths which are considered a specific catalytic wall reactor (Young and Finlayson 1976a, 1976b; Kolaczkowski 1995,1999; Groppi et al. 1995a, 1995b; Hayes et al. 1992, 1996; Cominos and Gavriilidis 2001).

R eactants

Bulk fluid H EAT T R A N SF E R

Products

M A SS T R A N SF E R R E A C T IO N

C onvection + C onduction C onvection + D iffusion H om ogen eou s

Inter-phase resitances Interphase

D iffusion S tagn ant film

Radiation

Intraphase resitances Intraphase

D iffusion

C atalyst layer C onduction H eterogeneous

Figure 2.3 Reaction and transport processes in catalytic wall reactors

2.7.3 G e n e r a l F o rm o f G o v e r n in g E q u a tio n s

The mathematical formulation of an adequate model for a chemical reactor consists of equations of momentum, mass, and energy balances at a degree of complexity that is most of the time a compromise between the accuracy of the model and the solving effort and time (Froment and Bischoff 1990). The complexity of the physical and chemical phenomena present in catalytic wall reactors is characteristic to heterogeneous chemical

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reactors. An illustrative summary is given in Figure 2.3. The movement o f the reacting stream in the reactor is usually coupled with heat and mass transfer phenomena, and chemical reactions. Heterogeneous reaction takes place in the catalyst layer and at the surface, at the same time homogeneous reactions are possible in the gas phase. Momentum, mass, and heat transfer by convection, diffusion, and conduction characterise the gas-phase. For the catalyst layer, the predominant transport phenomena are mass diffusion in the catalyst pores and heat transfer by conduction and radiation. The complexity o f the system arises due to strong interactions between these phenomena. The general form of the governing equations is presented below.

Momentum balance. This balance is obtained by application o f Newtons’s second law on a moving fluid element as presented in the classical work o f Bird et al. (1960), where usually only pressure drops and friction forces has to be considered (Froment and Bischoff 1990).

— pv = - [V -p w ] - Vp - [V t] (2.7-1)

5t

Momentum Rate Convection Pressure force Viscous force per unit volume contribution contribution contribution

Most of the time, the computational effort for solving a reactor model that includes the motion equation besides the equations o f mass and energy balance is quite demanding. Rather than solving the momentum balance, the velocity profiles are approximated according to the reactor geometry and flow regime (i.e. flat velocity profile for turbulent flow, parabolic velocity profile for laminar flow). Pressure drop can be estimated using empirical correlations (Geankoplis 1993).

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$ - = -[V -Piv] - VJ, - I V i/j (2.7-2)

CT j

Rate of increase of Convection Diffusion Chemical reaction

mass concentration contribution contribution contribution of species i per unit

volume

In general terms, the diffusion mass flux, Ji, consists of contribution o f concentration, pressure, forced, and thermal diffusion. When the flow is laminar or perfectly ordered the term V J., results from molecular diffusion only. It can be written more explicitly as an extension o f Pick’s law for diffusion in a binary system (Froment and Bischoff 1990):

Ji =P'Di,m^®i (2.7-3)

Di,m is the effective binary diffusivity for the diffusion o f the species i in the multicomponent mixture, and o)j the mass fraction. Using more specific multicomponent diffusion law, such as Stefan-Maxwell can increase the model accuracy, but the computational effort increases.

Energy Balance. The rigorous equation of energy balance for a multicomponent system given by Bird et al. (1960) as applied by Forment and Bischoff (1990) in chemical reactors is

SPiCp,iÇ = -IP iC p,v.V T + V .(X V T )-n(-A H j)rj - U -V H , +Q „, (2.7-4)

1 Ca i j i

Energy Rate per Convection Conduction Reaction beat Molecular diffusion Radiation

unit volume contribution contribution contribution (mixing) contribution

contribution

The radiative term is significant only at high temperature, and even then it can be lumped with the conductive term by using an effective conductivity for X. The contribution of the molecular diffusion term, where Hj are the partial mass

enthalpies, is negligible for gaseous mixtures at low pressures that have an almost ideal behaviour (Sandler 1989).

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Continuity equation. In a closed system the fundamental property o f mass conservation needs to be fulfilled.

^ + Vpv = 0 (2.7-5)

a