PLASTIC PROCESSING
SIDD
This Special Introduction and Distribution Device was initially developed and patented for the introduction and mixing of colorants in a melt stream. This is one of the most difficult of all mixing tasks because of the high-resolution capability of the human eye/brain complex plus its remarkable color discrimination power.
The structure consists of six cylindrical pipes of equal size, which are naturally nested about the axis of a pipe. Each contains a single mixing element. Additive is introduced at the apex of an inlet cone and flows over this cone before entering the six cavities. The additive exits the device in the form of twelve spokes of a wheel.
The A/t value generated may be shown to be 288/p 2F. For a 1% additive flow rate this gives us an A/t value of 2921. In addition we have been able to obtain a significant improvement in the distribution and dispersion of the additive. The SIDD can be applied to situations where the additive viscosity value is much lower or the same as the main product.
These three devices all possess an intriguing property. As the additive fraction size goes down, the A/t value goes up. This is the reverse of experience with most conventional designs of static mixer systems.
Viscosity and Pressure Drop Considerations
Most polymeric materials are shear sensitive. That is to say, their viscosity is dependent on the shear rate to which they are exposed. Usually an increased shear rate produces a lowering of viscosity and is referred to as shear thinning. Viscosity versus shear rate data is usually provided by the polymer manufacturer in the form of a graph on log/log paper. We list below the shear rate and pressure drop formula for each of the devices we have described and others together with the units involved.
Main pipe inside diameter = D inches
Main product flow rate = Q gpm
Shear rate = g sec-1
Viscosity = m cP
Pressure drop = dP psi
Axial length = L inches
Additive fraction = F
Number of mixing elements = nments = n
| Device | A/T Value | Shear Rate | Pressure Drop | Comments | ||
|---|---|---|---|---|---|---|
| Open Pipe | --- | 39.2Q/D3 | 22.73x10-6Wm L/D4 | Hagen-Poiseuille Equation | ||
| Helix | np | 96.0Q/D3 | 205×10-6Qm n/D3 | Helix L/D ratio = 1.5:1 | ||
| L/H unit | 8/p2 F | 215Q/D3 | 5.9×10-4Qm /D3 | Each hole dia. = 0.45D | ||
| H/L unit | 3.6/F | 81.7Q/Dt2 | 22.1Qm L/Dt3 | Where annulus dia.= 2D/3 | ||
| SIDD | 288/p2 F | 593Q/D3 | 12.6×10-4 Qm/D3 | Each hole dia. = 0.3D | ||
| PARAFLO | 8pN/F | Contact factory for shear rate and pressure drop calculations. | ||||