Liquid blending is defined as the mixing of two or more miscible liquids to a measurable level of uniformity. It is often quantitatively characterized by a “blend time.” This is the time required from the addition or contact of the ingredients to the point at which any sample taken from the vessel can be expected to show a ratio of mixed components within five percent of a sample taken from any other point. “Crude” blending is achieved when the composition is within ten percent. “Precise” blending is achieved when the composition is within two percent. As a rule of thumb, crude blending takes about half the time of normal blending, precise blending takes about twice as long.
Vessel Design Considerations
When the viscosities are water-like we expect the flow to be turbulent. Almost any vessel configuration is allowed. It can be short and squatty or tall and skinny. It can be round, square, rectangular, horizontal cylindrical or even irregularly shaped. However, the vessel geometry will restrict where the mixer(s) must be placed for effective blending. For example, you can’t expect to be able to place one mixer close to the end of a long horizontal cylindrical tank. Odd shaped vessels often require more than one mixer as do very shallow pits and sumps.
When the liquid exhibits significant viscosity, the flow will be laminar. In this flow regime, your choice of vessel geometry is restricted. Relatively big impellers will be required and these will often run close to the sides of the vessel. Round vessels are the best choice, but square vessels with rounded corners and filleted bottoms may sometimes be used. Tall skinny vessels can be mixed, but as the aspect ratio increases the blend time becomes much longer at the equivalent level of power and cost of the agitator.