High viscosity, no worries
How to homogenize, mix, heat and cool highly viscous liquids with no moving parts

The SMX static mixer and its cousin the SMR mixer heat exchanger use geometry to provide thorough mixing and excellent heat transfer in viscous fluids, yet with low energy consumption and low pressure drop.

Over the last 50 years the polymer industry has developed hundreds of polymer types and grades in response to customer demand. But regardless of the type of polymer, anyone manufacturing or using polymers faces the problem of laminar flow. Turbulent flow at Reynolds numbers of 2300 or above is dominated by kinematic forces. In laminar flow, by contrast, viscous forces dominate and the flow profile across a pipe becomes a parabola.
High frictional forces at the inside surface of the pipe and in the outer layers of the fluid mean that near the pipe wall the velocity tends to zero. Heat transfer is difficult because any external heating or cooling of the pipe wall will have little effect on the fluid except in the thin layer next to the wall, which is therefore likely to burn or freeze. Since mixing is poor, heat can be transferred to the bulk of the fluid only by conduction—which is very slow. The
result is high temperature differences across the fluid, and great difficulty in predicting the amount of heat transferred because of the complex relationship between temperature and polymer viscosity.
When viscous products are heated, a laminar velocity profile can result in poor
heat transfer efficiency due to stagnation inside the heat exchanger, hot spots and poor product quality.

Cooling suffers from the same problems, with the extra danger that the pipe or heat exchanger may block completely with product that solidifies after over-cooling. Products such as silicones are especially tricky in this respect.
Poor heat transfer is by no means the only problem affecting high-viscosity materials in laminar flow. Other operations that are difficult to perform are :
-Homogenization, which is needed to reduce differences in temperature and viscosity within a single fluid. Failure to do this in injection molding and extrusion lines, for example, yields poor product quality.
-Mixing, which is needed to overcome differences in viscosity between different
fluids, such as a polymer and a solvent. Ordinary pipes or mixing vessels give very poor results, and also introduce extra residence time distribution that is harmful to product quality.
The way to better mixing…
In the early 1970s, engineers at Bayer AG in Germany began developing a new type of static mixer designed to solve these problems. Their solution, which was developed under the name BKM, is now available as the SMX static mixing element. An SMX mixing element consists of a number of intermeshing bars placed in the flow channel. High-viscosity product flowing through the mixer is continuously split, stretched and recombined. Each SMX element is positioned at 90° from the next, so that inhomogeneity is resolved two-dimensionally in each subsequent element.
The fluid stream is subdivided up to 32,000 times, and the principles of geometry ensure that the product has a flow profile that is close to ideal (plug flow), with minimal residence time.
The small picture above demonstrates the impressive performance of the SMX mixer using colored epoxy resin. In a real application, of course, the colors could represent different temperatures (viscosities) or different materials.
The advantages of mixing with the SMX are:
-High mixing efficiency even under variable process conditions. The SMX is the only static mixer that can mix fluids with viscosity differences of up to 1,000,000:1, such as polymer and solvent.
-The fixed geometry ensures reliable scale-up at ratios up to 100:1.
-Excellent temperature uniformity, with temperature differences reduced by a factor of 50 or more.
-Easily retrofitted to existing pipes and extruder barrels.
-High mechanical strength, as a result of internal notches on each mixing element.
-No dead zones.
-No moving parts, so no maintenance is needed.
…and better heat transfer
A further development of the SMX principle came in the mid-1970s after engineers at Hoechst AG, Frankfurt, experimented with heat transfer. They started by installing SMX elements in a jacketed pipe, which produced the temperature profile shown in the left figure on this page.
Radial mixing introduced by the SMX elements ensures that heat is transferred by convection as well as conduction, with heat being “pumped” from the walls to the center and back again. The result is a huge increase in heat transfer (right figure on this page).
The jacketed-pipe SMX heat exchanger works for pipe diameters up to 75 mm, but at larger sizes the ratio of heat transfer area to throughput becomes too small. The Hoechst engineers therefore took the next logical step, which was to replace the solid bars of the SMX unit by hollow tubes carrying the heat transfer fluid. The result was named the SMR mixer heat exchanger. Its main advantages are:
-High heat transfer capacity in a single flow channel, with no maldistribution.
-Tubes act as both mixing elements and active heat transfer surface, so heat transfer coefficients are high.
-The flow profile approaches plug flow, ensuring uniform time, temperature and shear history.
-Very low pressure drop.
-Easy and reliable scale-up at ratios of up to 100:1).
Furthermore the heat exchangers are compact, because heat transfer areas of 90–150 m2 area/m3 heat exchanger are possible. They are ideal for cooling, heating and precise reactor temperature control of sensitive, viscous liquids.