Your expert source for

                      mixing technology

Ratio Effects

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), viscosity ratios

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

that follow provide a method of selecting the element number based on the process viscosity and flowrate.

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 (N_Fr) is very important. If N_Fr is below 15, mixing is retarded by the density difference, if above, this effect is practically negligible.The relationship between N_Fr 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 N_Fr = 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 be practical.

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.