TABLE OF CONTENTS Real fluids have viscosities that are functions of temperature and composition. This means that the viscosity will vary across the radius of a tubular reactor and that the velocity profile will be something other than parabolic. If the viscosity is lower near the wall, as in heating, the velocity profile will be flattened compared with the parabolic distribution. If the viscosity is higher near the wall, as in cooling, the velocity profile will be elongated. These phenomena can be important in laminar flow reactors, affecting performance and even operability. Generally speaking, a flattened velocity profile will improve performance by more closely approaching piston flow. Conversely, an elongated profile will usually hurt performance. This section gives a method for including the effects of variable viscosity in a reactor design problem. It is restricted to low Reynolds numbers, Re < 100, and is used mainly for reactions involving compounds with high molecular weights, such as greases, waxes, heavy oils, and synthetic polymers. It is usually possible to achieve turbulence with lower molecular weight compounds, and turbulence eliminates most of the problems associated with viscosity changes.
Variable viscosity in laminar tube flows is an example of the coupling of mass, energy, and momentum transport in a reactor design problem of practical significance. Elaborate computer codes are being devised that recognize this coupling in complex flow geometries. These codes are being verified and are becoming design tools for the reaction engineer. The present example is representative...
