What is mixing?
The simplest way to think of mixing is the process of fusing ingredients that are otherwise separate and independent through an external force. The ideal result of mixing is a uniform, completely homogeneous solution. This goal is readily achieved in thinner, waterlike substances, however, highly viscous materials do not easily achieve this type of uniformity, and requires detailed understanding of the rheologies and agitation forces involved.
In order to understand the fundamentals of high viscosity mixing (high viscosity or otherwise), it is important to understand the nature of flow as characterized by the Reynolds number.
How do we characterize flow?
There are three types of flow that can be achieved in a certain volume of fluid. Laminar Flow, Transitional Flow, and Turbulent flow. Turbulent flow is normally what one tries to achieve inside a standard turbine-type mixer. Under 5,000 centipoise, the flow that comes from mixing is considered chaotic. The movement of the faces of the blades in the mixer is significant and can range from 2 to 10 times the diameter of the blades. Low viscosity mixes have a higher turbulence and extended flow. The higher the viscosity, the less turbulent the flow.
Flow is described in terms of the intensity it creates. It is measured by comparing the fluctuating velocity against the mean velocity. Isotropic turbulence is the rare case in which the root, mean and square fluctuating velocity measurements are the same. The main symptom for this kind of turbulence is that it has no rotation (vorticity).
The Reynolds Number
The Reynolds number is a quantifier that is used to measure turbulence. It is a dimensionless measurement that is created from the viscosity, the density and the flow velocity of the substances in the turbine. One of the main uses of the Reynolds number is to differentiate between the stable laminar and turbulent flows.
Once you have a Reynolds number measurement for a flow, you can determine the character of the eddy currents that form. A typical range of turbulent flow as measured by the Reynolds number is from 20,000 and above. A traditional boundary between laminar and turbulent flow is usually around2,000. This boundary is highly debated, and has led to this area being characterized as the “transitional” zone. Many of the predictive empirically derived equations break down in this zone. Below 2,000, flow is more streamlined (hence this area being call Laminar),
Turbulent flow is usually defined by the relationship between the fluid velocity and a point in a related eddy, a sizing that can fluctuate according to the turbulence type. These flows will eventually move into each other and form new eddies; this is where chaotic turbulence comes from. These conditions lead to a better mix and a more uniform structure, however the challenge of high viscosity mixing is creating this chaotic turbulence by relying on the pushing structures of impellers rather than then left over momentum from the last push of the impeller.
Successful Mixing in Viscous Flow
When it comes to viscous flow, how does one bring the same uniformity and homogeneity you would find in a turbulent system to the laminar system? The insights you discover here will help you choose the right type of high viscosity mixer depending on the application that you need it for.
In high viscosity mixing, you must have a mixer that moves your substances at a high intensity with a high degree of interlacing as well. Interlacing creates angles and pushing movement of the impeller to force two streams of laminar motion to collide into one another. If you are using a viscous system, the laminar impeller is the better choice when compared to the turbulent impeller, because the laminar impeller has significantly more surface area. Eddy currents are less likely to form when substances are being mixed in the viscous range, and the mix moves in a streamlined way and actually stops flowing before it reaches the blade. Mixes here are not promoted through any turbulent flow. The energy from the blade itself also tends to dissipate, leading to heat buildup around the blade and within the substance mix, but no exchange of heat. This kind of reaction leads to a low Reynolds number and differential of velocities between streamlines.
Knowing this, here are vetted tips that will help you to choose the best mixer for the viscous process.
Consider the Purpose of Your Mix
In most cases, a mix is conducted to blend two or more ingredients in a uniform way. The success or failure of that uniformity of the product that results affects many parameters in the resulting mix. Before anything, consider the characteristics of the mix that you are trying to prioritize - durability, reaction, stability or performance. You may also need to consider the factors that you will need to achieve your mark, including mix, load and cleaning technique along with hardware downtime, mixer design and power consumption. Consider the temperature and pressure controls as well as the vacuum on the mixer itself, and review the entire system holistically rather than just its performance.
How Your New Mix Should Behave
If you are dealing with a Newtonian fluid, the viscosity will be low and constant. If a viscosity measurement is above 500 centipoise, there is a good chance it is a non-Newtonian fluid. There are 3 main types of non-Newtonian fluids with different characteristics and behaviors:
- Thixotropic mixes will have a lower viscosity while rheopectic products will rise in viscosity in a time dependent system with a consistent shear rate.
- Time independent systems with increasing shear will cause psuedoplastic products to move down in viscosity while dilatant mixes will increase. Bingham plastic will not flow until there is a certain minimum of shear stress.
- Viscoelastic fluids will be able to recover elastically if it is deformed by the mixer blade flow. The most common type of fluid sub-class in this profile are polymeric fluids. Polymeric fluids also exhibit the Weissenberg effect, an effect that causes the mix to move up the shaft.
Depending on which profile your product fits and the condition that your process creates, you can choose the right mixer for the job.
Decide Whether You Want Shear
Processes with low Reynolds numbers usually create shear - a phenomenon that is created through the separation of velocities between streamlines. The stress that occurs between the differences leaves a byproduct of heat, a stress that tears apart the particles. This is a condition that is usually brought about within the viscous flow range. If you are looking to maintain the size of the particles during the mixing phase, you may not want shear, although it is helpful in the process of deagglomeration.
Keeping Temperatures in Range
If a process must be kept between a certain range of temperatures, extra equipment may be required for cooling or heating purposes. If the case is especially demanding, you may need an automated, command control system for heat exchange. Every device that you add here will require a higher investment of time and capital, including operation and maintenance costs.
Staying Within a Budget
Although you will always be able to add features to add to the functionality of your system, your budget must obviously be considered. Match your profit projections with the projected length of the project against your operating costs and other capital required. Be sure to include the downtime during startup and installation, which will increase with the complexity of the system. Finally, consider the resale value and the other uses of the equipment outside of the primary function of the hardware.
Points to Consider
As you consider the parameters of the various mixers that may fulfill your project needs, there are a few points that you may want to take into account as well.
- Rotational type high viscosity mixers work at low blade tip speeds with a range between 200 to 800 ft/min.
- You can produce shear through the motion of the blade through the mix. The shear that you produce is affected by a number of factors: blade width, speed, design, number, product viscosity and the proximity of the blades to the wall and to each other.
- Shear is directly related to speed in a Newtonian system.
- Shear increases viscosity in dilatant substances and decreases viscosity in psuedoplastics in non-Newtonian systems. As shear is held constant in a non-Newtonian system, the viscosity of rheopectic and thixotropic materials is directly related to time.
- As the space between the blade mixers and the shell of the tank increases, shear and power demand increase alongside viscosity.
- Horizontal mixers can be especially good if the geometry of the blades are in order alongside having the correct viscosity and rheology of the product. If you get no tumble from the product, you can compensate by using Z type blades with dual arms that force extrusion action. These blades cost substantially more, however.
- You can obtain both vertical and horizontal mixers with a saw tooth or high speed chopper blade type; you do not have to use the high torque, slow speed blades that most mass production efforts use. High speed blades with a tip speed between 2,000 and 5,000 ft/min generate more shear. However, high-speed blades will only generate flow within a short distance when it comes to viscous materials. In most cases, a low-speed blade is used as a feeder into the high-speed blade to improve homogeneity and heat transfer. This is especially important if the overall batch viscosity at the beginning of the process is low.
- If you need to improve the axial batch movements in a vertical mixer, use an auger. They should be operated alongside a tubular shell in order to maintain their vertical efficiency. Budget extra time for cleaning.
- You may want to use open helical blades if you are working in a horizontal plane that has already created vertical tumbling through energy discharge from the rotating blade tips. This process will help the axial mixing process, which is especially important when you are trying to mix viscoelastic fluids.
- If you are looking to maximize the performance of your open helical blades, use them alongside supplementary agitation. You may also lower or raise the blade. This extra energy will reduce the incidence of stationary toroidal vortices.