Mixing Tank Design Criteria

Mixing is an essential operation in many fields of engineering. It has central significance in food processing, pharmaceutical manufacturing, chemical engineering, biotechnology, agrochemicals, dye production, and water purification among countless other applications. Many mixing schemes using mixing tanks have been developed to meet various production and processing objectives. In agricultural systems, a mixing system is used in a number of processes, such as the preparation of chemical concentrations, balancing the amounts of nutrients in fertilizer tanks, blending various materials, and processing farm products.

Mixing Tank Design Criteria​

How does a mixing tank work?

A mixing tank works by releasing bubbles in sequence at regular intervals and that’s why mixing tank design is so important. This creates immediate vertical circulation in the container and mixes 100% of its contents. The sudden release of air shocks the fluids, propelling the fluid molecules into motion.
Agitator manufacturers are frequently required to provide the customer with equipment designed according to specific process specifications. This work demands a good understanding of mixing and the consequence of the proposed equipment in the process. The designer must define in detail the mechanical specifications of the stirrer and vessel. In summary, the customer expectation is a mechanical specification of the vessel and a good understanding of the mixing influence on the process.
A good practice during the equipment design stage is to maintain continuous contact with the customer regarding to the process and the equipment they expect to buy. With VisiMix, the agitator manufacturer has an opportunity to demonstrate to his customer the influence of the equipment in the process (turbulent and laminar regimes) through VisiMix reports, including calculations of:
 Blending
 Suspension
 Dissolution
 Emulsification
 Gas dispersion
 Heat transfer
 Chemical reactions
 Mechanical stability of shafts

Agitator Manufacturer Advantages

Once the mixing characteristics of the process are analyzed by the customer and after his agreement, the agitator manufacturer can proceed to calculate the mechanical parameters.
After understanding the mixing parameters well, it is possible to determine the simple mixing parameter used in calculating mechanical parameters in software like Agitation Intensity. If one assumes agitation intensity to be proportional to the rate at which a specific tank contents are turned, then intensity would vary linearly with the fluid velocities in the tank. Such a criterion suggests that the range from mild to violent intensity be defined by average bulk fluid velocities, which are designated on a scale from 1 to 10. With VisiMix, you calculate this velocity and feed this into the corresponding Agitation Intensity mechanical parameter calculation.

Mixing tank design calculations

The mixing tank design criteria is a very amorphous question because tank mixing, and all of those parameters, are service parameters. So, before we define what is the criteria to design a tank, we need to ask what is the purpose of this tank and what is the purpose of this mixing system? VisiMix provides data for checking the application of your equipment and giving production processes, comparing different designs of tanks, baffles and impellers.

Consequently, it can help in the selection of the best ways to improve mixing regimes, according to your technical possibilities, by selecting or designing new mixing units. But all of this is completely different if we have a process that we need to manufacture a formulation or cream, or we have a process that we have a typical reaction, a process or a fermentation process.

Baffle and Flocculation Tank Design

We are able to provide for every process what is required, but we need to know what is the process we are dealing with. Several practical considerations of general characters which should be considered can be the following: equal values of mixing power -suspension is usually more efficient in unbuffered equipment- and embedding with a larger type diameter is advisable.

Or when talking about mixing of liquid, liquid system is usually performed in baffle tanks, so smaller drops are formed in the tank in small impellers, due to the force of the diameter of the tank at high velocities, as you need to generate enough shear rate to break up the drops to small ones. And when we talk about mixing of multiphase systems with a light dispersed phase, in most cases of these kind of systems, if they are not with baffle, do not have enough shear force to generate stable drops that will be representative, and the time of the drop breaking can be reduced or increased according to the shape of the impeller.

We need to ask about what is the drop size that we are interested to generate in order to have good results. And for the gas liquid, for instance, we need to consider the capability to transfer the gas to the liquid media by sparger or in the heat space. We need to consider the capability to transfer this gas that is into the gas phase to dissolve in the liquid phase.

Flocculation Tank DesignSo, we need the mass transfer coefficient for it and the capability to transfer, and for this we need high shear rate. But if we increase the high shear rate, maybe we will have high circulation flow and it will take out the gas very fast to the top of the tank, and now we have not enough residence time. All of these are there and we are able to calculate all of them.

And it is really not a problem. So, as general criteria, as I explain always, if someone will ask you, what is a good mixing, you must answer that you don’t know. Because a mixing is for a process. We need to understand what is the required mixing for some process that we are managing. In general ways, but continue to have some others ways to guide us in order to provide the condition that is required for the process.

You cannot ask everyone, “please come and run 5 km”. This is because, immediately, if this person is a normal person, he or she will ask you “okay, well, what is the direction to run?” or if you ask them to please come and apply 5 kilonewton force them might ask you: “okay, to where?” And all of this is required. So, it is very, very difficult to define what is the tank criteria if we don’t know the process.

And once we know the process, and any other relevant considerations, like some mechanical considerations that are very simple to know; vibration of the shaft and capability of the shaft to support the bending, we calculate with the solver. Nonetheless, we are entering more in the designing of the equipment by itself which is more related to the mechanical engineering field and not to chemical engineering.

Once we know well our process, and once we know the quantities and we want to manufacture, we will be able to discuss with mechanical engineers and to define the best ways to generate our equipment.

VisiMix Video

The Influence of Mixing in the Process
New methodology using VisiMix software for the purpose of checking the influence of mixing in the processes.

VisiMix Demo Operation
Learning how to input data into the VisiMix software and get results the will help us understand the influence of mixing in our processes.

Lab Experiments
Learning how to set up the relevant experiments at the lab scale, to develop the processes from an engineering point of view.