Effect of Viscosity and Volumetric Ratio
Viscosity ratios below 100:1 (0.01<m 1/m 2<100) have little effect on the degree of blending achieved.Provided the volumetric ratios of the process streams are similar (V1/V2<100),
up to 1000:1 can be readily blended with a minimal number of
elements. Viscosity ratios above 1000:1 or
volumetric ratios above 100:1 require
special attention in the Mixer design. The design recommendations
provide a method of selecting the element number based on the process viscosity
However note that they are limited to the following: -
CONTACT ZAIN if your situation
falls outside the range above.
Effect of Density Ratio
Extreme density ratios are uncommon with liquid-liquid blending, however they
can be a problem when blending two gases.The mechanism of gas mixing is predominantly eddy turbulence and molecular
diffusion. It is generally excepted that the mixing of two gas streams can be
achieved in a long section of empty pipe under very turbulent conditions.
However if the density difference between the two gases is large, and the pipe
diameter is large, mixing can be difficult.For gases of different densities the effect of the densiometric Froude number (NFr) is very important. If NFr is below 15, mixing is
retarded by the density difference, if above, this effect is practically
negligible.The relationship between NFr and Re can be expressed as.
The high exponent of D explains why it is more difficult to mix in a large
pipe even when Reynolds numbers are quite high.
As an example oxygen and air are to be mixed (D r /r = 0.1) in a pipe of D=24", at a Reynolds
number of 35,000. From this we can obtain NFr = 1, indicating a
strong segregation by density. As a rough indication the length required for a
proper mix would be 100-150 pipe diameters, or 91 m which can become too long to
A static mixer can accomplish the same degree of mixing with 2 to 4 element
sections (1 to 2 pipe diameters). The major mixing mechanisms are radial mixing
and shear from the elements.