Bonded Phases - The Fluidized Bed Method for Bonded Phase Synthesis
The Fluidized Bed Method for Bonded Phase Synthesis
The idea of synthesizing bonded phases by means of a fluidized bed reactor was first suggested by Unger (14). The underlying principle behind this alternative process was to be able to maintain more constant reaction conditions and, consequently synthesize a more reproducible product. The fluidization can best be described as a contact process in which particulate matter is transformed into a fluid-like state by a fluid stream that may be a gas or liquid. The fluidization is accomplished by the upward passage of a fluid through the bed of particles until a velocity is attained where the upward drag force acting on the surface of the particles is equal to the gravitational force downwards. Under these conditions, the particles move apart and become suspended by the fluid flow and the bed is said to be fluidized.
The properties of fluidized beds have been extensively examined in combustion research and one of the most arresting properties of the fluidized bed system is the extremely high rate of mass or heat transfer that can take place within it. For example, the transfer of heat to a metal body such as an iron bar when inserted into a fluidized bed is nearly ten times faster, than if inserted into a stream of hot gas at the same temperature. An iron bar, one inch in diameter, plunged into a fluidized bed where combustion is taking place, will be heated to red heat in a few seconds. This extremely rapid heat transfer is partly due to the turbulence of the gas in the system (consequently the normal laws of diffusion or heat transfer do not apply) and partly, but to a lesser extent, due to the transfer of heat directly by the solid particles of the fluidized bed to the iron bar. In fact, the idea of using the fluidized bed in bonded phase synthesis was probably induced by the rapid transfer properties of the system. It was expected, that bonded phase synthesis in a fluidized bed would guarantee more complete reaction and, as a consequence, the product would contain significantly less unreacted silanol groups. However, while a slightly greater bonding yield was indeed realized, the main advantage of the fluidized bed system was found to be, the reproducibility of the product, its ease of operation and automation and, above all, its greater flexibility.
The fluidized bed apparatus could be operated in such a manner that it allowed far more complex multi-stage syntheses to be carried out rapidly and with relative ease (e.g. the synthesis of oligomeric phases). The technique was developed by Khong and Simpson (15,16,17) and a diagram of their apparatus is shown in figure 4.
Figure 4. The Fluidized Bed Apparatus for the Synthesis of Bonded Phases