Unit Operations of Chemical Engineering
The VisiMix tool allows you to analyze and study most of mixing processes. It is always good to confirm some of these parameters by experimentation; by using VisiMix you can minimize the number of experiments needed.
Let us take a look at some of these different phenomena that occur in various mixing equipment in stirred vessels.
First is the phenomenon of blending. Normally, we make the assumption that blending occurs instantaneously; thus, if there are multiple competing reactions that are faster than the mixing action, then the outcome may be different from what is desired. For example, when dye is dispersed in a vessel filled with water, the dye will eventually become uniformly distributed, whereas the reaction rates will vary throughout the vessel due to difference in concentration. The blending profile can be altered by many factors such as location of addition point, addition rate, fluid properties of the additive and bulk (density, viscosity, miscibility, speed of stirrer, its size, design and location in vessel), and also vessel internals such as baffles and heating/cooling coils.
Solid–Liquid Mixing Process
In solid–liquid mixing, the goal is to suspend particles; in other words, if they are denser than the liquid, then they want to settle and your goal is to lift them up from the base of the vessel and separate them. If you have very light solids, the challenge will be to wet the solids; meaning, to spread liquid over the entire surface of the solid or to draw down the particles from the liquid surface into the fluid.
When you suspend solids, you want to determine the degree or level of suspension: partial suspension (complete of bottom suspension) and uniform suspension (we never really achieve complete uniformity of suspensions but we approach it); depending on your process, either of them may be suitable and acceptable. For example, if partial suspension is acceptable for dissolution of very soluble solids, then you do not need to lift them all up; if they are very soluble and you keep swelling them at the bottom of the vessel, then that would be sufficient.
Liquid–Liquid Dispersion Process
In liquid dispersion, droplets are broken up but when they meet each other they coalesce according to the force of collision and the viscosity of the fluid between them. Droplet break-up and dispersion requires threshold energy; meaning, you need to put in a certain minimum amount of energy before you can actually break droplets.
However, the coalescence and aggregation of droplets continue when while breaking the droplets and are also influenced by the energy input and the properties of your fluid. This is a challenging area for processes. Droplet size and droplet distribution can be calculated using VisiMix.
For liquid–liquid dispersion, the process objectives will vary according to the desired product but will include all of these:
– Nature of the dispersion: O/W, W/O, O/W/O, W/O/W
– Droplet size and distribution of resulting dispersion
– Kinetics of the dispersion: the time scale to reach equilibrium or required DSD
– Stability of the dispersion and the tendency to resist an increasing coalescence rate
All the above are very challenging and it is hard to find a tool capable of addressing all aspects of liquid dispersion.
In gas–liquid mixing, we break up gas bubbles into small bubbles to create interfacial area and then distribute them uniformly throughout the vessel to affect the process we are carrying out.
Effect of Mixing on Reactions
There are few effects of mixing on reactions because mixing affects how reactants are brought together and its effect is determined by the local concentration and temperature reaction rate, and other properties. That affects the course of reaction rates and selectivity. For single reactions, blending can affect the concentration of reactants and, thus, the reaction rate; for multiple reactions, the blending can also affect the selectivity and for multiple systems, mixing affects droplet and bubble sizes and the differential area for mass transfer. It is important to vary mixing in your studies to determine the effects of mixing and to determine the critical effects for the desired purpose.
Mixing challenges require close attention. If all you are doing is blending two miscible liquids to homogeneity, this is the easiest mixing objective, but when scale-up is involved, then it is important that you pay attention to these challenges. If you have multi-phase systems, then you have to address many skills of mixing frequently mass transfer aspects and always incomplete understood physics. It is important to be able to do experiments or to use a tool that gives you insights as to what is occurring during the mixing process.
Mixing Challenges Requiring Closer Attention
In conclusion, the most challenging mixing operations are:
- Higher density, higher viscosity miscible liquid into thin bulk
- Blending/reacting as viscosity increases and rheology changes
- Incorporation, dispersion, dissolution of powders into viscous bulk
- Incorporating/dispersing poor-wetting solids into a liquid
- Agglomeration of polymer pellets during dissolution
- Achieving suspension of fragile needle-shaped crystals with minimal attrition
- Settling and packing of solids on vessel bottom
- Encrustation on wetted surfaces during crystallization or precipitation
- Droplet size prediction for concentrated and surfactant-laden dispersions
- Catastrophic phase inversion during formulation
- “Fouling” or gel formation in emulsion polymerization
Finally, some important lessons – Because mixing is very complex, process failure can happen if you do not pay adequate attention and process failure is a costly thing to encounter. It can cause lower reaction yields, longer reaction times, unacceptable product properties, or generate impurities that require higher cost for purification or handing the waste. In addition, it can cause higher production cost for product rework.
Early and careful assessment is important. If you are able to do an early assessment, then you can affect your process in very positive ways; for example, because you can determine how to identify and make changes quickly and avoid costly mistakes, the cost to make changes is much lower. The opportunity to influence process decisions is very good and avoids or prevents process scale-up problems. Hence, you can achieve your process goals and save money, and that is what we are after. VisiMix allows you very quickly assess mixing issues in your process vessel. In VisiMix, you can calculate hydrodynamics and determine the turbulence energy and dissipation inside the vessel by looking at the fluid, and make heat and mass transfer calculations. There is also a set of tools to assess mechanical reliability, alkylation stress, and vibration. Once the hydrodynamic properties are determined, you can use solid–liquid mixing, liquid–liquid mixing, multi-phase systems, and gas–liquid and liquid dispersions by applying the hydrodynamic properties in the turbulence calculations, which also address chemical reactions. This is what makes VisiMix a high-productivity tool.
This article is based on Dr. Victor Atiemo-Obeng’s lecture on VisiMix Forum.
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.
Learning how to set up the relevant experiments at the lab scale, to develop the processes from an engineering point of view.