CFD in the mix

Many engineers in the chemical processing industries will be aware of computational fluid dynamics (CFD) as a software tool that offers the ability to simulate fluid flow, heat transfer and chemical reactions.

But how many of them ever turn to it as a method to solve their design issues? The answer is 'not as many as you might expect'. When discussing mixing this appears even more surprising - after all, mixing involves fluids and the whole raison d'etre of CFD is to model fluids.

It seems that many companies only consider CFD when they come across a design challenge. This can, for example, be where a mixing tank or reactor is proving increasingly inefficient; when capacity needs to be increased at low cost; or if erosion of impellers is proving too high a cost. Even then, it is only when standard methods prove incapable of giving a true insight into the problem that CFD simulation is most commonly employed - in a troubleshooting role.

However, even for companies with no perceived problem, there are real possibilities of achieving increased operating efficiency, higher product quality and an increased ability to prove regulatory compliance by applying CFD.

The above issues cannot be comprehensively addressed without an understanding of how a process works. Furthermore, once a process is understood, an ability to quickly test a variety of design changes to improve operation will speed the design process, reduce downtime and cut costs.

While some involved with mixing processes may be familiar with CFD, others will not. It appears that the 'expert' tag often attached to use of the software dissuades many people from considering its use.

However, recent developments in CFD codes have made the software more accessible and easy to use for the non-expert, while the more advanced general purpose codes have been developed to provide even greater accuracy of the physics at work.

The high-capability CFD codes from the larger CFD vendors have had multiphase models for a few years now, enabling a range of different phases to be modelled within the same simulation. This is important when a fluid, or fluids, and particulate matter and gaseous bubble behaviour suspended in fluid need all to be accounted for.

Driven by requirements for even greater accuracy, CFD models are now available that not only account for different phases, but also provide information on the different distributions of bubble or particle sizes at varying locations within a device.

For many in the chemical process industry, this level of detail is not needed to improve the performance of their mixing tanks. However, for some, the ability to predict particle/bubble size and distribution, and account for the effects this induces, could prove very significant.

For example, in the immediate vicinity of an impeller there will be a zone of high turbulence where bubbles will be broken down into smaller bubbles. Further away from the impeller, these smaller bubbles will begin to coalesce and form larger bubbles.

The size of bubbles is important as it determines the rate of gas-liquid mass transfer as smaller bubbles have a relatively large surface area. In addition, bubble size also determines the speed at which the bubbles rise.

A knowledge of the range of bubble sizes present and their distribution within a tank will therefore reveal how efficient a mixing tank is and inform and guide decisions on how the design should be amended to increase efficiency.

Today CFD remains an under-used resource in the design and engineering of many chemical process. However, the technology is now within reach of a far wider range of engineers as it becomes much more user-friendly and customisable for each industry's needs.

Indeed, the easier-to-use, more automated framework is the blueprint for the direction CFD is moving. This is with the purpose of allowing CFD to be used to evaluate design candidates ever earlier in the design process, enabling only those with potential to be invested in to be developed, saving time and money and ensuring optimum efficiency prior to manufacture and installation.

For engineers in the chemicals and related industries, the real value of CFD is in avoiding design issues in the first place, rather than troubleshooting existing ones. At the same time, for the most advanced physics, new models mean ever more detailed insight is available for those who require it.

Edward Wynn and Edward Throp are both senior CFD engineers in chemical process at Fluent Europe Ltd