1. Technical Field
This invention relates, generally, to systems, methods, and apparatus for coating particulate material and the coated particulate material so produced. More particularly, the present invention provides improved draft tube spout fluid bed (DTSFB) having the capability to coat particles with fluids, for example, for coating aerogel beads to provide a useful engineering material, for instance, an improved insulating material.
2. Description of Related Art
Coating particulate material can often enhance the physical and chemical properties of the material, for example, by reducing absorption of liquids and gases or simply protecting the particulate material from environmental degradation. For instance, the coating of particulate material can provide the following enhancements to particulate material: improved insulation properties, both thermal and electrical; improved abrasion resistance; and improved strength.
Aerogel beads were first developed in the 1930s and are the lightest solids known. Aerogels are low-density materials that have proven to be effective insulators. They typically have a thermal conductivity on the order of 0.01 Watts/meter-Kelvin, that is, less than one-third the thermal conductivity of polyurethane foam. However, only recently has the commercial scale production of aerogels been economically feasible. Aerogels are typically referred to by the nicknames “frozen smoke,” “solid smoke,” or “blue smoke.”
Due to their insulating properties and their low density, approximately, 140 kilograms per cubic meter, aerogels have been proposed for insulation in a broad range of applications from insulating liquid natural gas (LNG) supertankers, to insulating superconductor power cables, to insulating spacecraft, such as, insulating the external fuel tanks or crew return vehicle of the space shuttle.
However, typically, untreated aerogels are porous and tend to absorb gases and liquids. For example, about 95% of the surface of aerogel beads contain pores having an average pore size of about 20 nanometers (nm). The absorption of fluids by aerogels may typically destroy the material or, at the very least, interfere with the insulating properties of the beads. The present inventors sought to overcome the porous nature of aerogels by coating aerogels to minimize or prevent the absorption or infiltration of fluids, for example, by applying a polymer coating. However, the very light, low-density nature of aerogels makes them difficult to handle, especially when attempting to coat these minute particles, typically, less than 5 millimeter (mm) in diameter. Aspects of the present invention overcome these technical difficulties, as well as overcoming the disadvantages and limitations of the prior art methods and apparatus.
One device that has been used to handle particulate material is a draft-tube, spout-fluid-bed (DTSFB) mixer. The design and operation of the DTSFB mixer were investigated by Littman, et al. and are disclosed in U.S. Pat. Nos. 5,248,222 and 5,254,168, both of Littman (one of the co-inventors of the present invention), et al. (the disclosures of which are included by reference herein). Recent developments of the DTSFB mixer were reported by Plawsky, et al. (2003), the disclosure of which is also incorporated by reference herein, in which a “first generation” mixer was disclosed. Further improvements in the DTSFB mixer were reported by Park, et al, (2006) in which a “second generation” DTSFB mixer was designed and tested. (The disclosure of Park, et al. (2006) is also included by reference herein.) Aspects of the present invention provide advantages over both Park, et al, (2006) and Plawsky, et al. (2003), and other prior art methods and apparatus.