Modified Manufacturing Process Yields Remarkably Uniform Silica Spheres

Shell

Tiny silica pellets, or spheres, are found in a wide variety of applications -- everything from the little bags of absorbents used to absorb moisture in consumer electronics packaging to critical catalyst carriers in complex chemical reactions.

While some of the more basic uses for silica particles don´t require uniform, consistent sphere sizing, particle size and pore volume distribution are important for catalyst supports and many other petroleum, chemical, and process industry applications. For these large-scale processes, silica particles of uniform size with a high crushing strength are greatly desired. To achieve these qualities, silica spheres are produced using something called the sol-gel method. This requires a silica hydrosol to be prepared by mixing an aqueous solution of an alkali metal silicate with an acid aqueous solution. The resulting hydrosol is converted into a droplet form and the droplets are gelled to produce substantially spherical hydrogel particles. At this point, the alkali metal content is reduced, usually by washing the hydrogel particles in water, which are then dried and calcined, or powderized, to remove volatile matter. What´s left are silica spheres with a high crushing strength.

The problem, however, is that the particles are not very uniform in size or porosity. Although the droplets have a uniform diameter, the dried hydrogel particles themselves do not, a result of uncontrolled shrinkage during drying. Researchers at Shell discovered that the key to narrow particle size distribution and narrow pore volume distribution (another way of saying controlled uniform size and porosity) was the drying process. In response, they developed a patented multi-stage drying process using standard sol-gel chemistry and drying equipment. By carefully controlling the initial, partial drying phase, silica particles could be created having very narrow distribution ranges of both particle size and pore volume.

The first step requires the hydrogel particles to be partially dried relatively quickly, using humid air and temperatures generally above 100°C. Any standard drying equipment can be used, including tray dryers, fluid bed dryers, or rotary dryers. Shell researchers determined that the amount of water left in the partially dried hydrogel particles controls the pore volume. To control pore diameter, the partially-dried hydrogel particles are subjected to a hydrothermal treatment, bathing them in either hot water or steam. This procedure enables controlled growth of pore diameter while leaving pore volume substantially unchanged. Because the drying occurs in two steps, moisture is removed gradually, preventing cracks and creating high crush strength.

The spheres are washed with acid to remove sodium, which would otherwise make the spheres receptible to hydrothermal treatment, changing pore structure in aqueous, high temperature conditions. At this point a final drying occurs, typically for several hours at elevated temperatures of up to 200°C. However, the conditions under which this final drying occurs are not critical. In fact, it does not appear that the properties of the resulting silica spheres are much affected by the drying conditions in the final phase, which is used to calcine the silica. The end result is remarkably uniform alkaline or neutral silica spheres, ideal for use as catalytic carriers with active compounds in such processes as hydrometallization of heavy hydrocarbon oils.

This technology is currently available for sale or license, with full documentation and technical support.