Annular packed beds

FIGURE 3.2 Annular packed-bed reactor used for adiabatic reactions favored by low pressure. 

Example 4.5 Entropy production in a flow through an annular packed bed The introduction of suitable packing into a fluid flow passage considerably enhances wall-to-fluid heat transfer, and hence reduces the entropy production due to heat transfer but increases the entropy production due to fluid-flow friction. Heat transfer to a fluid flowing in an annulus has a technical importance because we can heat or cool either or both of the surfaces independently. Entropy production provides a new criterion in analyzing such processes. In terms of the velocity and temperature profiles, the local rate of entropy production per unit volume of an incompressible Newtonian fluid for a two-dimensional annular flow is... 

Assuming fully developed velocity and temperature profiles for the control volume of an annular packed bed, the energy equation is... 

St and h are the Stanton number and the heat transfer coefficient, respectively. By adding the fundamental solutionsand/2, we can obtain the temperature profile for the annular packed bed with asymmetric heating... 

Relations for the Nusselt number and the effective thermal conductivity kc for the annular packed bed are... 

The heat transfer parameters have been derived for an annular packed bed in the range 200 < Re < 800 and... 

A two-flux radiation field model for an annular packed bed photocatalytic oxidation reactor was presented by Raupp et al. (1997). Similar to other annular flow reactors, the UV source was located at the center of the cylindrical reactor. Yet, the photocatalyst was not introduced in the form of a thin film but rather as spherules filling the annular space between the lamp and the housing. The principal assumptions made in this model included a steady state, isothermal operation, cylindrical symmetry. 

Different photocatalytic reactor configurations are reported in the literature including annular reactor with a small thin film of catalyst coated on the iimer surface of the outer annulus, flat-plate fluidized beds, annular packed beds, and catalyst coated on honeycombed monoliths, porous fibrous mesh, and optical fiber bundles (31,32). 

Continuous annular chromatography (CAC) has been the subject of several recent experimental studies (120, 121), models (122, 123) and a brief review (124). The equipment is very similar to the CRAE (Figure 7). Feed, eluent, and regeneration solutions (if necessary) are fed to fixed points or arcs at the top of an annular packed bed which rotates slowly about its axis. As the chromatogram develops, the components separate... 

C. Void fraction near the walls of annular packed beds... 

Annular packed beds (APBs) involving the flow of fluids are used in... 

FIGURE 2.6. Annular packed-bed reactor (AIChE Jour., 43(3), G.B. Raupp, J.A. Nico and S. Annangi, Two-flux ladiation-fleld model for an annular packed photocatalytic oxidation reactor, 792-801. Reproduced with... 

The debate still remains regarding which configurations are the most adequate for the photocatalytic reactors to photoconvert air borne pollutants (Ollis and Al-Ekabi, 1993). Several options have been described (Chapter 11) fluidized bed (Brucato et al., 1992 Dibble and Raupp, 1992 Yue et al., 1983 ), annular packed bed (Raupp et al., 1997), coated honeycomb (Sauer and Ollis, 1994 Suzuki et al., 1991 Suzuki, 1993), fixed powder layer (Formenti et al., 1971 Peral and Ollis, 1992) and fiber optic reactor (Peill and Hoffmann, 1995). The Photo-CREC-Air reactor (Ibrahim and de Lasa, 2002) optimizes TiOa-mesh iiradiation and air contacting the supported TiOa. As reported in the upcoming section, this configuration yields model pollutant conversions with high apparent quantum efficiencies. 

G. E. Mueller, Radial void fraction correlation for annular packed beds, AICHE Journal 45(1 l)pp. 2458-2460,1999. 

---