I will introduce the knowledge about the flow pattern of the agitator.
Mechanical and electrical engineers of chemical plants are familiar with stirrers and mixing tanks.
But have you ever seriously considered mixing?
We tend to be interested in the mechanical part of the stirrer and neglect the chemical engineering part.
Or, since it is a technology specific to reactions, it is often decided at the research stage or by the process engineer in the previous process.
Even if we ask the manufacturer to do the work, there are many cases where the researcher and the manufacturer have to meet directly, and the mechanical engineer does not get involved.
Even so, a mechanical engineer who does not know agitators is questionable as a chemical plant.
So let’s understand the basic part of the agitator.
The agitator is too deep, leave it to the experts
- Stirring capacity performance
- flow pattern
- Stirring conditions and baffle plates
- suspension of particles
- Batch is liquid-liquid stirring
Stirring capacity performance
What is the performance that indicates stirring ability?
Thinking from a zero base is surprisingly difficult.
There are some indicators, so let’s take a look.
The greatest indicator of stirring capacity performance is stirring power.
Especially in batch chemical plants, it is a good level to consider that “stirring performance = stirring power”.
This is related to scaling up.
Is it possible to reproduce the phenomenon that occurred in the flask-level experiment using a real device?
This is a problem between chemical plant research and the field.
This is because when scaling up, it is not possible to make all factors similar, and something must be sacrificed.
When considering chemical reactions, there is a certain relationship between various elements and dimensions.
- Handling amount is related to volume
- Thermal control is related to area
- Reaction rate is (may be) related to specific surface area
- Gas-liquid reactions are (may be) related to interfaces
There are various differences as to whether it works on the first power or the second power, but I think you can intuitively understand that it is physically related.
The important thing here is that when you scale up, the area/volume ratio changes.
It’s easy to understand if you think of a ball.
The sphere has a surface area of 4πr2 and a volume of 4/3πr3.
If r is doubled, the surface area is quadrupled and the volume is eight times.
Surface area and volume cannot be scaled up in the same relationship at this moment.
If you try to match the heat transfer performance when scaling up, other elements such as reactions will not meet.
It is the stirring power that tries to match these multiple elements somehow evenly.
There is a convention that if you keep the stirring power/volume constant, most of the elements will be the same even if you scale up.
Discharge flow/circulation flow
As long as it is a stirrer, the liquid flow is one indicator.
In the world of agitators, there are two types of flow: discharge flow and circulation flow.
Outlet flow is the flow of fluid directly expelled from the impeller.
A circulating flow is a flow that occursaround the discharge flow along with the flow of the discharge flow.
Stirring turns → liquid moves in radial direction (discharge) → liquid disperses (circulation)
I think that’s a good image.
I think you can get a rough idea of this by stirring hot water with your hands in the bathroom.
This shows the qualitative relationship between discharge flow and circulation flow.
The shape of the blade affects the discharge flow.The faster
the discharge flow , the faster the circulation flow.
The more the discharge flow circulates around the entire stirring container, the larger the circulating flow becomes, allowing for more uniform stirring.
From the perspective of flow velocity, the faster the discharge flow or circulation flow, the better it seems, but the problem is that this is not necessarily the case.
Shear rate is also one of the stirring performance.
The shear velocity is the velocity itself at the tip of the blade. It is closely related to the discharge flow.
Shear rate is a concept originally learned in fluid mechanics and materials mechanics. The idea is the same.
The shear rate in stirring is almost determined by the impeller diameter and rotation speed.
Fast shear rate → Fast discharge flow → Fast circulation flow
I think you can understand this relationship intuitively.
The larger the blade diameter, the faster the shearing speed, so the idea is to sacrifice the rotational speed when scaling up.
Let’s take a look at the flow pattern of liquid due to stirring.
This greatly affects the shape of the stirring blade.
There are various shapes of stirring blades, such as paddles, disc turbines, and propellers, which are commonly used.
The direction of the discharge flow is divided into radial and vertical directions depending on the shape of the vane.
- Radial direction for vertical paddle and flat blade disc turbines
- Propeller is vertical
Radial discharge is also intuitive.
The blades rotate → The liquid is pushed out radially by centrifugal force
The direction of the blade can be set in various directions such as vertical direction, horizontal direction, and diagonal direction.
Orienting the blades vertically maximizes radial discharge.
The vertical ejection is weaker by this amount.
Contrary to the paddle, the discharge flow flows in the same vertical direction as the stirring shaft with propeller blades.
A strong vertical flow is a flow that tries to spit out the liquid at the bottom of the tank.
It is not often used in batch-type chemical plants because it dislikes slurry build-up.
position of blade
The flow pattern and vane position are closely related.
If the impeller is close to the bottom of the tank, the vertical flow balance will change (both discharge flow and circulation flow).
This is because the closer the impeller is to the bottom of the tank, the sooner the lower flow hits the bottom of the tank.
In some cases, an attempt is made to design uniform flow by arranging the impellers in several stages, such as at the top, middle, and bottom of the tank.
Solid vortex and quasi-free vortex
The flow pattern is divided into parts with and without stirring blades.
Let’s take a look at the horizontal cross-section of the tank.
It’s called a whirlpool.
We distinguish between solid vortex flow and quasi-free vortex flow.
A solid swirl is a stream that flows at the same speed as the impeller motion.
The part through which the impeller passes can be considered as a solid swirl.
It is easy to understand if you think of the speed of eddy flow as a linear relationship proportional to the blade diameter and rotation speed.
Semi-free eddy flow is the flow in the area surrounded by the stirring blades and tank.
The discharge flow from the tip of the stirring blade is the driving force, and the tank wall is fixed.
Like solid eddy flow, it is strongly influenced not only by factors such as the blade diameter and rotation speed, but also by the density and viscosity of the liquid.
The higher the viscosity, the weaker the discharge flow, and the quasi-free eddy flow attenuates quickly.
A similar idea can be used when a tank is cut vertically , and the rise of liquid due to stirring may be discussed. It’s called a vortex.
Stirring conditions and baffle plates
The stirrer comes with a baffle plate.
Let’s check the effect of the baffle board again.
- Reduction of quasi-eddy flow region
- Changes in circulation flow
It has the effect of intentionally changing the simple flow of liquid caused by stirring, such as solid swirl flow or discharge flow.
By complicating the flow pattern, we hope to have the effect of making the flow of liquid uniform and stirring it quickly.
The stirring power will increase accordingly.
As a familiar example, consider the following.
- There is no baffle board when you just add milk to coffee and stir it with a stirrer.
- When the stirrer is stirring and the liquid is flowing, if you stop the stirrer, the stirrer will be treated as a baffle.
Stirring conditions with no baffle plate
The dimensions of the stirring blades are a design condition for the stirring machine.
It is generally said that the following conditions are appropriate for the dimensions of the stirring blade without a baffle plate.
|Width of stirring blade
|Diameter of stirring tank x 1/3
|Height of stirring blade
|Diameter of stirring tank x 0.05 to 0.1
|Inclination of stirring blade
|Stirring blade width x 1/4
In practice, the user rarely designs the design, but rather relies on the agitator manufacturer, so they probably don’t think about it much.
Stirring conditions with full baffle plates
The conditions for the stirring blades with baffle plates naturally differ from those without baffle plates.
This optimal condition is called the perfect baffle condition.
Even if you delve too deeply into mathematical and chemical engineering knowledge, it is difficult to use it in practice.
Full baffle conditions are for disk turbines and paddles.
- Stirring speed is about 2/3 of that without baffle plate.
- The width of the paddle blade is the diameter of the stirring tank x 1/2
There is a lot of specialized data, and it is likely that each company is accumulating data.
At the user level, it tends to be left up to the manufacturer, and the most appropriate stirring blades and baffle plates are selected from the manufacturer’s lineup.
suspension of particles
Slurry systems are often handled in batch chemical plants, so particle suspension is a subject of investigation.
Two states are considered separately when a liquid containing solids is stirred in a bath.
- solid particles simply floating
- Solid particles are evenly distributed
If all you want is for the solid particles to float, simply continuing to stir will likely solve the problem.
Adding the condition of uniform distribution to this, it is necessary to design it as an agitator.
Relationship between stirring speed and dissolution speed
If the slurry liquid is allowed to stand without stirring, the solid content will settle.
This sedimentation speed varies greatly depending on the performance of the slurry.
Now, what will happen if you move the stirring blade in this state?
- When the stirring speed is low , it will partially float.
- When the stirring speed is high , all particles float.
I think you can sort of understand.
When the stirring speed is low, both the discharge flow and the circulation flow are weak, and a flow pattern is formed only near the stirring blades.
Other parts are in the same state as a stationary liquid.
Even if the solid content floats due to the movement of the liquid, the stationary part remains as it is.
As you increase the stirring speed, all the solids in the tank will float at some point.
Floating limit stirring speed
The stirring speed at which all the solids in the tank float is called the floating limit stirring speed.
It seems to be the definition of the condition where there is no baffle plate, but you don’t need to be too conscious of it.
From the perspective of whether solids float or not, it depends on various factors such as the shape of the stirrer, solid particle size, solid density, solid bulk density, liquid density, and liquid viscosity, but from a direct operational perspective, it is the discharge speed . .
The following factors can be considered to increase the ejection speed.
- Diameter of stirring blade
- Stirring blade shape
- Rotation speed of stirring blade
This is an important point in terms of design philosophy when choosing the optimal product from a manufacturer’s lineup.
Batch is liquid-liquid stirring
What does a stirrer stir? is first.
Batch chemical plants fall into one of the following four types.
- uniform liquid (liquid)
- Heterogeneous liquid (liquid-liquid)
- Liquid and gas (gas-liquid)
- Liquid and solid (solid-liquid)
Homogeneous liquids are highly unlikely in reaction processes.
Devices that store uniform liquid include raw material tanks and recovery tanks.
These are necessary before and after the reaction.
Raw materials are required before the reaction, and recovery is required after the reaction.
Basically, a stirrer is not attached to such a homogeneous liquid.
Inhomogeneous liquids require agitation.
This applies to most batch chemical processes.
When a reaction occurs, two layers are usually formed: a water layer and an oil layer.
In many cases, one of them is the target substance and the other is the impurity.
A stirrer is required for the reaction itself, but also for the subsequent cleaning process.
For example, suppose that a water layer and an oil layer are formed after the reaction, the target is the water layer, and the oil layer is discharged outside after the reaction.
If the liquid separation is completed and only the aqueous layer remains, would you proceed with the next reaction?
I almost always do a cleaning process.
Add fresh oil to the remaining water layer and mix with a stirrer.
Then separate the oil again.
Repeat this several times to reach the desired composition, and then perform the next reaction.
For this reason, a stirrer is almost essential in the reaction process.
Most use standard agitators.
Stirring of liquid and gas
Some reactions involve liquids and gases.
In batch-type chemical reactions, gas is generated during the reaction, so gas emissions are a hot topic.
Stirring of liquids and solids
Stirring of liquids and solids is also very common in batch chemical processes.
Used in crystallization.
The performance of the stirrer is also important here.
However, standard agitators are often used.
Solid-liquid stirring crystallization is used more frequently than gas-liquid stirring special reactions.
It is risky to use a special stirrer for its crystallization.
Batch chemical plants require versatility.
Many low viscosity liquids
Low-viscosity liquids of 100 mPa·s or less are often used in batch chemical processes.
In particular, it is often less than 10mPa・s.
Because it is a low-viscosity liquid, the performance required of the stirrer is naturally fixed.
I explained the flow pattern of the stirrer.
Outlet flow/circulation flow, radial and vertical flow patterns, solid vortex/quasi-free vortex, baffle condition, solid suspension.
Since it is a difficult field, we tend to leave it up to the agitator manufacturer, but let’s keep the basics of the design concept in mind.
Please feel free to post any concerns, questions, or concerns you may have regarding the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)
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