Multiphase CFD for the process industry: status and applications
Today, Computational Fluid Dynamics (CFD) is known by many process industry professionals. Less known is the fact that today's CFD is much more than some 'flow simulation of water or air'. The simulations can simultaneously include gas and solids as a 'separate phase', and there are many proven ways to do that. Translated to process applications, this means gas bubbles, liquids, flocs, crystals and particles of all kinds. This what we call 'multiphase CFD'. The three phases can be combined in a single simulation.This article gives you some basic explanation and three very different practical examples in which multiphase CFD was used to solve practical problems. After reading this small article, you'll be much more aware about the possibilities so you can harvest them.
What are the 'three phases'?
First, let us quickly summarize what these phases can be. Many unit processes contain more than just water, solids or air. Figure 1 shows the plug and play of CFD 'gears'. Although you can 'play' with viscosity, mass transfer, kinetics, ... we highlighted the 'phases' for you: liquid, gas and solids, and some examples of each.
And which phases do we have to use when? It depends on the objective and the nature of the process. For example, if your question is 'will solids wash out, and if so, which fraction? Where do solids accumulate?' You might use a 2-phase model containing water and the solids with a given density distribution. In the same process, your question might be: 'will flow distribution be optimal with this inlet structure?' In the latter case, you might even use a 1-phase model (water only). On the other hand, a 'heavy' fluidized bed should probably always modelled at least in 2-phase, as the solids impact the liquid flow too much. So this is what we mean with OBJECTIVE and NATURE OF THE PROCESS.
Figure 1: The plug and play of gears makes CFD extremely powerful - not many practitioners are aware of today's possibilities
Example 1: Aeration modelling (2-phase)
The scale at which aeration modelling can be applied varies enormously. Figure 2 shows pure oxygen modelling in water (2 coarse bubble aerators) in a bioreactor in the chemical industry. We typically use such models to answer the following questions:
- Can we save pure oxygen?
- How can we optimise mixing?
Figure 2: small changes in positioning of the pure oxygen units leads to very different mixing performance and gas/liquid transfer efficiency
Example 2: 'Granular systems' modelling (2-phase)
This is an example of 2-phase modelling of water and solids, whereby the solids behave more like individual granules and are heavy and concentrated enough to impact the water flow. Further, we added the real size/density distribution (different sizes in the same simulation),as granules do not have 1 single size. Figure 3 shows the example of pellet softening, a well known drinking water treatment process to remove calcium. 'Similar species' in the process industry are fluidized bed systems and crystallisers. We used the model to answer the following questions:
- Can we reduce the size of the reactors without the risk of washout?
- Can we optimise the reactor shape to get the optimal pellet distribution? How does this distribution look like as function of height?
- How to design the inlet structure as such that feed is distributed evenly, and shortcircuiting avoided?
So these questions typically relate to saving OpEx and CapEx (smaller reactor),and process reliability
Again, we started testing different changes virtually with the model. A 1-phase model in this case would have made no sense, because of the mass of the solids and their impact on the water flow...
Figure 3: distribution of granules with different sizes in a pellet softening reactor (same simulation, but we visualised the classes separately)
Often, process industry practitioners have experience with 1-phase CFD modelling. This article shows only a few out of many examples where multiphase CFD modelling was applied to solve practical questions in the process industry. Which phases to include (1, 2 or 3-phase) depends on two factors:
- The specific project objective (e.g. 'do you want to quantify solids wash out' vs 'do you want to know how flow distributes')
- The specific technology (e.g. a fluidized granular bed cannot be modelled using 1-phase only given its impact on the water flow)
Hence, we want to pick the simplest approach, but not an oversimplified one. Today's computational power has removed barriers with regard to multiphase that still existed 5 years ago.