Woven wire screens

Sustained commercial application, however, did not occur until 1997, when Zheng et al. (7) described the successful stripping of oxygen from water used in secondary oil field recovery. In 1999 Trent et al. (8,9) introduced the first commercial application involving simultaneous absorption, reaction, and stripping. Both of these involve gas/liquid contact using a woven wire screen for the rotor internals. 

The broad classes of packings for vapor-liquid contacting are either random or structured. The former are small, hollow structures with large surface per unit volumes that are usually randomly dumped into the tower. Structured packings are normally layers of elements fabricated form expanded metal, woven wire screen, or sheet metal, and are stacked into the tower very carefully. 

For highly exothermic and fast reactions the catalyst is often deposited on the outer surface of the support which is usually of very low porosity (e.g. FjOj on SiC for o-Xylene oxidation) (Ellis, 1972). In other applications (e.g. ammonia oxidation converters) the catalyst is in the form of a woven wire screen (or gauze) which is usually supported on a non-catalytic pad to prevent premature ignition (Gillespie, 1970). 

SCREEN ANALYSIS STANDARD SCREEN SERIES. Standard screens are used to measure the size (and size distribution) of particles in the size range between about 3 and 0.0015 in. (76 ram and 38 pm). Testing sieves are made of woven wire screens, the mesh and dimensions of which are carefully standardized. The openings are square. Each screen is identified in meshes per inch. The actual openings are smaller than those corresponding to the mesh numbers, however, because of the thickness of the wires. The characteristics of one common series, the Tyler standard... 

Sieving is a standard operation for separating powders according to particle size using woven wire screens. The Tyler mesh size represents the number of openings per inch or the number of parallel wires that form the opening. 

A wide variety of screens and meshes are available, ranging from fine photoetched or electroformed perforated screens to the heavy duty wedge wire screens used in centrifuge and high pressure screw press construction. Simple sieves and coarse screens were used as early as the sixteenth century for processing metal ores. Modern woven wire screens are precision made cloths with aperture sizes as small as 20 pm (smaller aperture sizes are supplied by some manufacturers) for industrial separations in filtration, clarification and extraction. Plastic meshes and plastic coated metal meshes are finding an increasing number of applications in separation processes. 

Three HiGee units were installed by Dow Chemicals in the US for stripping hypochlorous acid, used as the reactive chemical in a process. The rotating packed-bed used for stripping was selected after other alternatives such as a spray distillation tower were rejected for capital cost reasons. The packing selected was a woven wire screen, with the gas flowing counter-current to the liquid. Various packing surface areas were tested, with 2000-3000 m /m area densities, but these exhibited no performance differences so the lower value was used. 

They consist of an oscillating bar contacting a woven wire screen, and the material is forced through the screen by the oscillating-rotary motion of the bar (Figs. lA and B). 

The ability to wick liquid along the screen makes woven screen superior to perforated plate. Pore sizes much smaller than 10 pm are achievable using advanced laser drilling or machining techniques on a solid piece of metal. However, for flexible liquid acquisition systems, both the size and the number of holes affect performance. The number of pores in a woven wire screen is proportional to the product of the number of the warp and shute wires. Perforated plates are structurally more stable than woven screens at the cost of higher flow resistances due to fewer holes. However, since perforated plates cannot wick liquid to areas that dry out due to evaporation, they are not recommended as a primary PMD in future cryogenic propulsion systems. 

Problem 9-14 (Level 2) Satterfield and Cortez studied the mass-transfer characteristics of woven-wire screen catalysts. This kind of catalyst is used for the oxidation of NH3 to NO in the process for making rutric acid, and for the reaction of ammotua, oxygen, and methane to make hydrogen cyanide. A Pt/Rh wire usually is used for these reactions. 

Satterfield, C. N. and Cortez, D. H., Mass transfer characteristics of woven-wire screen catalysts, Ind, Eng. Chenu Fmdanu, 9 (4), 613-620 (1970). 

Probably the simplest technique for particle size measurement is sieving. Conventional woven-wire screens can be used to analyze powders, most of whose particles fall into the range of 50 jum to 1 cm. The range can be extended downward to 10 fim by using electroformed screens, which are more accurate than woven wire screens up to about 200 m. It is advisable to... 

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