Packed-bed discharges

Figure 18.12 Geometries for packed-bed discharges, (a) Planar configuration, (b) Coaxial configuration. (Reprinted with permission from Ref. [62]. Copyright 2012, John Wiley Sons, Inc.)...

There are three types of Hquid content in a packed bed (/) in a submerged bed, there is Hquid filling the larger channels, pores, and interstitial spaces (2) in a drained bed, there is Hquid held by capillary action and surface tension at points of particle contact, or near-contact, as weU as a zone saturated with Hquid corresponding to a capillary height in the bed at the Hquid discharge face of the cake and (3) essentially undrainable Hquid exists within the body of each particle or in fine, deep pores without free access to the surface except perhaps by diffusion or compaction. 

For larger diameter columns, and for low liquid rates, the distributor must be almost exactly level (e.g., within 6 mm for a 3-m diameter) or all pour points will not function. On the other hand, the rises must be high enough to accommodate the backup caused by high liquid rates. The needed head can be estimated from the orifice equation, with a discharge coefficient of 0.5. In some cases the orinces discharge directly into tubes that extend to the packed bed (the Tubed drip-pan distributor ). 

The benefits of the use of micromembranes for the selective removal of one or more products during reaction have been demonstrated for equdibrium-limited reactions [289]. For example, the performance of hydrophilic ZSM-5 and NaA membranes over multichannel microreactors prepared from electro-discharge micromachining of commercial porous stainless steel plates was studied by Yeung et al. in the Knoevenagel condensation [290,291] and andine oxidation to azoxybenzene [292]. For such kind of reactions, the zeolite micromembrane role consists of the selective removal of water, which indeed yields higher conversions, better product purity, and a reduction in catalyst deactivation in comparison to the traditional packed bed reactor. 

We present results on the catalytic hydrogenation of carbon dioxide to methanol in a conventional tubular packed-bed reactor filled with a copper based catalyst. In addition, results of an alternative approach using a dielectric-barrier discharge (DBD) reactor with and without the aid of a catalyst are presented. 

The presence of a catalyst has a positive effect (factor 10) on the methanol formation at low temperatures. Compared to the performance of the same catalyst in the packed-bed reactor, the simultaneous presence of discharge and catalyst led to a substantial lowering of the optimum temperature range from 220 to about 100 °C. 

J. N. Chung, O. A. Plumb, and W. C. Lee, Condensation in a Porous Region Bounded by a Cold Vertical Surface, Heat and Mass Transfer in Frost, Ice, Packed Beds, and Environmental Discharge, ASME HTD, (139) 43-50,1990. 

Special Modifications of DBD Surface, Packed-Bed, and Ferroelectric Discharges... 

Figure 4-98. General schematie of a packed-bed eorona discharge.

---