Packing performance, random pressure drop

Several structured packing elements have been compared with regard to catalyst holdup, heat transfer performance, and pressure drop [120]. The results indicate that using catalyst coating gave lower pressure drop than packed beds but had a much lower catalyst inventory per reactor volume. On the other hand, a particle-packed structure exploited the advantages of structured flow vdiile not sacrificing much in catalyst holdup compared to a randomly packed bed. This alternative retained the favorable pressure drop characteristics so that smaller particle-sized catalysts could be used. Despite their lower... 

Eckert [125] provides some basic guidelines to good packing selection for various system performance requirements. Kunesh [126] illustrates the often-determined pressure drop advantage of random packed towers over the usual valve tray. See Figure 9-19 [126],... 

Dixon and coworkers [25] have performed several CFD simulations of fixed beds with catalyst particles of different geometries (Figure 15.9). The vast number of surfaces and the problems with meshing the void fraction in a packed bed have made it necessary to limit the number of particles and use periodic boundary conditions to obtain a representative flow pattern. Hollow cylinders have a much higher contact area between the fluid and particles at the same pressure drop. However, with a random packing of the particles, there wiU be a large variation... 

Structured packings are produced by a number of manufacturers. The basic construction and performance of the various proprietary types available are similar. They are available in metal, plastics and stoneware. The advantage of structured packings over random packing is their low HETP (typically less than 0.5 m) and low pressure drop (around 100 Pa/m). They are being increasingly used in the following applications ... 

Experiments were also performed to compare the holdup and flow distribution in a bed, randomly packed with 3 mm spherical alumina particle, under the same flow conditions as was done for structured packing. However, it was evident that the successful operating conditions for structured packing were too severe for random packed bed, due to very high pressure drop. For very low liquid velocity ( 1 mm/s) and no gas flow, when the experiment was possible, the liquid distribution was poor as indicated by a low uniformity factor ( 40%). However, this information is insufficient to compare the distribution characteristics of structured and random packings. 

Pilot plant smdied have also been performed by Larsen et al. [37], who obtained stable operation and more than 95% SO2 removal from flue gas streams with a gas-side pressure drop of less than 1000 Pa. The importance of the membrane structure on the SO2 removal has been studied by Iversen et al. [6], who calculated the influence of the membrane resistance on the estimated membrane area required for 95% SO2 removal from a coal-fired power plant. Authors performed experiments on different hydrophobic membranes with sodium sulfite as absorbent to measure the SO2 flux and the overall mass-transfer coefficient. The gas mixture contained 1000 ppm of SO2 in N2. For the same thickness, porosity, and pore size, membranes with a structure similar to random spheres (typical of stretched membranes) showed a better performance than those with a closely packed spheres stmcture. 

Kunesh [126] presents an overview of the basis for selecting random packing for a column application. In first deciding between a trayed tower or a packed one, a comparative performance design and its mechanical interpretation should be completed, considering pressure drop, capacity limitations, performance efficiencies (HETP), material/heat balances for each alternate. For one example relating to differences in liquid distribution performance, see Reference 126. 

The drawbacks of randomly packed beds in microchannels are the high pressure drop and effects related to the nonuniform packing of the small catalyst particles, namely, channeling and maldistribution of the fluids. A large RTD results, which diminishes the reactor performance and, in the case of sequential reaction networks, the product selectivity. The reactor or the catalyst may be modified such that a structured bed is obtained. 

The catalytic filaments were introduced into the tubular reactor in the form of threads. A bundle of 100 filaments with a diameter of 7 pm each formedthreads of diameter of about 0.5 mm. The catalytic threads were placed in parallel into the tube to form a cylindrical catalytic bed of several centimeters length. This arrangement gives about 300 threads per cm within the tube cross section with a porosity of = 0.8. The specific surface per volume is in the order of 10 m m and, thus, about 50 times higher compared to washcoated tubes of the same inner diameter [8]. The performance comparison under identical experimental conditions with randomly packed beds with particles of silica and y-alumina of different shapes and sizes showed significantly broader residence time distribution compared to the structured filamentous packing with about five times lower pressure drop for the same hydraulic diameter and comparable gas flow rates. 

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