Dry powder curtains

In cold climates, especially when temperatures are below freezing, a water curtain may suffer operating problems when ice forms at the spray nozzles or the water drops produce snow. Either of these occurrences could result in a significant reduction in the effectiveness of a water curtain. 

It is known that dry powders, such as sodium bicarbonate, calcium chloride, magnesium oxide, and calcium hydroxide, readily react with hydrogen fluoride. Use of similar materials for the control of liquid spills was discussed in Chapter 3 of this guideline. 

A recent small-scale study (Schatz, 1993) evaluated several powders— either metal carbonates, oxides, or hydroxides—to determine reaction rate constants and applicability for mitigating an HF cloud. Several powders were found to have sufficiently high reaction rates and HF removal efficiencies to make them feasible for HF mitigation. Powder weight ratios as low as twice the stoichiometric ratios achieved the same HF removal efficiency (90%) as the highest water-to-HF ratios (40 1) used in the spray tests described in Section 4.3. Upon reaction with HF, the dry powders usually form a wet, nontoxic mud that is easy to collect and remove. 

The technology for applying dry powders for firefighting purposes is well developed for small-scale situations, such as restaurant kitchen firefighting systems. (A scale-up to large industrial systems has not been done as yet, but no inherent technical problems would preclude such a scale-up.) However, to prevent lump formation, powder must be carefully stored and kept dry. The system should also receive a complete performance test periodically, to ensure that it will function when needed without having to discharge the powder inventory. 

Using dry powders for the mitigation of airborne or liquid HF and other toxic materials is possible. The technique would work well where the maximum spill is known in advance so that stationary dry powder systems of finite, yet sufficient, capacity could be designed. 

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Vinyl chloride/vinylidene chloride monomers can be polymerized by emulsion pol3rmerization. Poly(vinyl chloride) (E-PVC) product is mostly applied as the dried form. It is spray-dried and milled to form fine powders ("crumbs") which is mixed with plasticizer to form a plastisol, i.e., dispersion of pol) vinyl chloride) particles in liquid organic media. The plastisol is poured into molds to make rubber doUs, shower curtains, embossed wall coverings and many of other common objects. In packing materials, especially for food packaging, the films of poly(vinylidene chloride) latexes are used, which are highly impermeable for both, oxygen and water vapor. 

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