In part 1 of this series on wastewater and water treatment mixers, we discussed how to select a mixer for chemical preparation. In this post, we’ll show you how to select a mixer for flash mixing and flocculation.
Done in a Flash?
Aggressive Agitation + Speed for Flash Mixing
Flash mixing is used to evenly distribute coagulating chemicals in water, allowing micro-flocs to form. As the precursor to flocculation, flash mixing increases the efficiency of flocculation and reduces chemical wastage. Because the components need to be quickly and evenly dispersed, this process is all about brute force. You’ll need a mixer that can generate a high shear mixing pattern (violent agitation) in the shortest time possible. An ideal flash mixer will have the following features:
- Impeller – For high speed, high shear mixing, your best bet are pitch-blade impellers. They create a turbulent fluid flow that allows solids to collide and shear. For our clients, we recommend the P4 Pitch impeller blade and customize the size and diameter to their process.
- High G Value – Because the residence time in flash mixing is short (less than 60 seconds), the mixer needs to be configured with a high G value / velocity gradient (400 – 1000 sec-1). This has to be calculated by your mixing team: at Dynamix, we typically add an additional 15% on top of the calculated G value to make sure everything gets evenly dispersed.
- Tank Configuration – While not part of the mixer per se, tweaking the tank configuration can minimize short circuiting. To maximize particle contact, we recommend flowing in from the bottom and the flowing out from the top.
What the Floc?
Gentle Mixing for Successful Floc Formation
After flash mixing is complete, the micro-floc undergoes flocculation, a process that separates solids suspended in water. To be successful, micro-flocs must first attach themselves to impurities in the water and then cluster together to form larger, visible agglomerates known as macro-flocs. The challenge is that floc particles are very sensitive: they must be mixed in a way that allows them collide and bind with each other. The high shear, high speed mixer that you use for flash mixing would destroy floc particles. Instead, you need a mixer that creates a gentle, axial flow pattern that generates low shear over a large area of influence. An ideal flocculation mixer will have the following features:
- Impeller Shape & Size - For low speed, low shear mixing, your best bet are large airfoil impellers. They create laminar fluid flow over large areas of influence that protect the integrity of the solids. For our clients, we recommend Dynaflow™ blades with an impeller diameter-to-tank ratio of 0.3 – 0.45.
- Impeller Tip Speed – Another way to minimize shear is to manage the impeller tip speed. As with G values, determining the appropriate tip speed is dependent on your specific process. In general, we recommend a maximum tip speed of 480 feet/min for our Dynaflow™ impellers.
- Low G Value – Because the residence time for flocculation is considerable longer than flash mixing (>30mins), the mixer needs to be configured with a low G value (20 – 100 sec-1). Despite the low G values needed for flocculation, it’s still advisable to add an additional 15% on top of the calculated G value to ensure sufficient power.
Summary
As you can see, flash mixing and flocculation require very different mixing solutions. If you’re interested in learning more about mixing solutions for your process, post a comment or email us at info@dynamixinc.com.
Mixing is what we do, so it’s no surprise that we love the array of mixed drinks that come with the holiday season.
One favorite is eggnog, a drink that has several mixing steps. Making it can be tricky, so we thought we’d share our process for mastering a festive holiday eggnog.
Ingredients
- 6 eggs
- 3/4 cup sugar
- 1 quart milk
- 1 pint cream
- 6 ounces bourbon (optional)
- 6 ounces spiced rum (optional)
- 1 whole nutmeg, for grating
Instructions
- Separate egg whites from the yolks into separate bowls
- Vigorously mix egg yolks until they turn light in colour, adding 1/2 cup of sugar as you mix
- Mix in milk, cream, and (if desired) liquor
- Next, vigorously mix egg whites with the remaining sugar until they peak
- Fold the whites into the egg yolk mixture
- Finish by topping with freshly grated nutmeg
About Consuming Raw Eggs: To review health and safety guidelines related to the consumption of raw eggs, visit your government’s website: Health Canada and US FDA.
Disclaimer: The recipes found on this website are provided as suggestions. We assume no obligation or liability, and make no warranties, with respect to these recipes.
We first mentioned impellers in our blog discussing 4 factors for configuring your mixer. In this post, we’ll look more closely at the types of impellers and their applications.
The impeller is the component of a mixer that does the actual mixing and produces the results. That’s because when an impeller rotates, it creates fluid flow and shear patterns. While there are many types of impellers, here are the 4 main types we use when engineering mixers:
1. Airfoil
Generally most efficient because it produces the maximum pumping with the lowest shear.
Standard DynaflowTM
High Solidity DynaflowTM for Gas Dispersion
2. Pitch Blade
Ideal for viscous mixtures and for applications requiring a combination of pumping and shearing.
Standard Pitch P4
Modified High Efficiency Pitch
3. Radial Blade
Ideal for applications where shear is the primary requirement, or where agitation close to the bottom of the tank is desired.
Radial Blade
4. Marine-Type Propellers
Ideal for low-viscosity, high-speed direct drive mixers.
Propeller Blade
Summary
The mixer you need depends on your process and the chemicals you use in that process. If you’re interested in learning more about mixing solutions for your process, post a comment or email us at info@dynamixinc.com.
Chemical preparation in wastewater and water treatment directly impacts many downstream processes. Let’s take a look at several common chemicals used and some key factors that influence their preparation and mixing.
Lime Slurry Preparation
When preparing lime slurries for control pH, uniformity is critical. Why? Because lime solids must reach/stay in suspension if they are to serve their purpose in downstream processes. For this reason, it’s essential that wastewater and water treatment facilities have lime slurry mixers that can efficiently and effectively make up/maintain uniform slurries.
In order to successfully suspend/re-suspend solids, a mixer needs to create a flow pattern that involves the entire tank. Axial flow patterns have large areas of influence, so they’re very effective at pulling settled solids back into suspension. This slow, high torque mixing is especially important in high viscosity mixtures, like uniform slurries that contain up to 30% solids. Because the range of solids can vary, mixers should be engineered to meet the unique needs of the slurry. Here are a few things to consider when selecting a mixing solution for lime slurry preparation:
Impeller Shape & Size
- Lime slurry mixers should have impellers with airfoil blades because they promote a long, laminar flow that maximizes the area of influence. The diameter of the impeller should also be customized to the tank: 0.3 to 0.5 impeller to tank diameter ratio would be optimal. While we have a full range of impeller types at Dynamix, we typically recommend our Dynaflow™ airfoil blade for lime slurry mixing.
Shaft Diameter & Design
- In any mixing solution, the mixer shaft should be engineered to prevent the destructive forces caused when critical speeds are reached. At Dynamix we design the shafts to operate 65% below the first critical speed: this is well below industry best practice, which is to stay at least 20% below the first critical speed. This minimizes shaft vibration and deflection.
Baffle, Tank & Ratio Design
- Optimized mixing doesn’t stop at the mixer. That’s why we recommend configuring the non-mixer components of your process, including baffle sizing and ideal tank selection.
Activated Carbon Slurry
Activated carbon slurries, which is used in wastewater and water treatment to remove organics for taste, odour, and colour control, have different requirements than other treatment chemicals. An activated carbon slurry mixer must be designed to perform 3 functions:
Wetting Out
- Dry carbon particles tend to float: if they’re to be incorporated into the mixture, proper wetting during makeup is critical. As with lime slurry mixing, a mixer that generates an axial flow pattern will be most effective for mixing dry carbon. Carbon slurry particles tend to floate, so they need to be “sucked” into the liquid from the surface. The axial flow impeller placed close to the liquid interface will provide the best mixing for wetting out the particles.
Suspension
- Once the carbon particles have been wetted out, they need to be brought into suspension. We recommend using a radial flow impeller. The impeller should also be positioned close to the tank bottom.
Re-suspension
- To keep activated carbon particles suspended, the process used when bringing them to suspension should be repeated continuously during storage until they are ready for use.
Polymer Mixing
Polymers are used in wastewater and water treatment to enhance flocculation by optimizing the strength, size and weight of flocs. Before they can be added to the flocculation process, the powder or neat forms of polymers need to be activated and aged.
Activation & Aging
Polymer activation is a complex process involving high speed, high shear mixing in a pump for liquid polymers and in an edductor for dry polymers. Top-entry mixers are used for polymer aging. Activated polymers can easily be damaged during the aging process. To protect them, we recommend using a low shear, high torque mixer and a low speed. A laminar flow pattern, like the one created from the Dynaflow™ airfoil impeller, protects the integrity of solids and maintains product quality. The pattern also creates less drag than a pitch blade, so it’s more efficient.
Facilities that receive their polymers in totes have to prevent stratification. The solution for this job is a portable mixer that can be easily inserted into plastic totes. For our clients, we recommend the ITM. The collapsible impellers allow insertion of blades much larger than the small tote opening in order to generate the desired axial flow pattern.
Summary
The mixer you need depends on your process and the chemicals you use in that process. If you’re interested in learning more about mixing solutions for your process, post a comment or email us at info@dynamixinc.com.
Learn about how engineering mixing systems for your water treatment process can reduce costs and increase efficiencies.
Whether your operation is municipal or industrial, understanding how to configure mixers to your specific environment, process, and tank are critical in the optimization of your plant. (more…)
If you’re in an industry that includes fluid mixing in its processes, you probably know that finding the right mixer for your needs is critical to maintaining quality control, reducing costs, and optimizing efficiency.
The key here is to engineer a mixer that addresses the specific needs of your process. When configuring a mixer for your process, your mixing engineer should consider the following 4 factors.
Factor #1 – Tank Type & Volume
This determines the amount of fluid your tank can hold. This is important because it will determine the size and position of the mixer and its mounting.
Factor #2 – Viscosity
This is “thickness” or “internal friction” of your fluid. It’s important because it will determine the impeller and horse power configurations.
Factor #3 – Specific Gravity
This is the density of the solid in comparison to water. It’s important because it will determine the type of pumping action that is required to adequately mix your fluid.
Factor #4 – Environment & Process
This is the setting where your mixing will take place. It’s important because it can determine the type of power source and finishes.
Engineering the Solution
With this information in hand, your mixing engineer should configure a mixer with the following qualities:
- Motor with suitable horse power and energy source
- Gear drive optimized for torque capacity
- Mounting fitted according to your tank needs
- Impeller sized to maximize efficiency
Summary
The mixer you need depends on your process and the chemicals you use in that process. If you’re interested in learning more about mixing solutions for your process, post a comment or email us at info@dynamixinc.com.
