CFD Stirs Up Mixing

Progress in simulation provides improved mixer performance

By Seán Ottewell, Editor at Large

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The era of mixing simulations requiring experts in computational fluid dynamics (CFD) and sometimes taking weeks to run is long gone. Driven by huge leaps in computing and related technologies, companies such as Ansys, Comsol and Flow Science are bringing easy-to-use mixing simulation to the engineer’s desktop.

“Advances in parallelization and high-performance computing, as well as templatization, have brought accurate CFD simulation into the reach of non-expert chemical engineers,” says Bill Kulp, lead product marketing manager, fluids, Ansys Inc., Pittsburgh, Pa.

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Exemplifying this, the company has just begun a project with Nalco Champion, Houston, that will give chemical engineers there — none of whom are experts in simulation — access to Ansys Fluent and a simulation app based on an analysis control technology (ACT) template to quickly and efficiently scale up processes for new chemicals.

“The company has been quite frustrated with failures, especially with new reaction designs that won’t work with their library of tank models, and are looking to CFD simulation to reduce costs and increase the success rate of their scale-ups,” explains Ansys senior account manager Erik Shank.

Up to now, Nalco Champion’s workflow typically involved completing hundreds of new reactions, 10–15% of which wind up moving into production test phases. Of these, only a handful eventually go forward, based on reaction yield, availability/cost of feedstock chemicals and other relevant data.

With each scale-up costing around $250,000, Nalco intends to use simulation to improve process costs and efficiency by eliminating at a very early stage the ones that are less likely to succeed.

“Chemical engineers can now focus on reactions and not complex CFD. The overall goal is to automate the process with a user defined field (UDF)/ACT extension to allow them to submit reactions into the simulation for on the fly decision-making,” adds Shank.

Success In India

Kulp points to a 2016 project carried out with Aditya Birla Science & Technology, Navi Mumbai, India, which resulted in an improved impeller for the mixing tank used in viscose staple fiber (VSF) manufacturing.

VSF is produced by dissolving a wood pulp slurry in caustic soda and then forcing the solution through tiny holes in a metal cap. Mixing the slurry and caustic soda solution is both time consuming and expensive in terms of electricity use.

The Ansys team started with a steady-state multiphase simulation of the existing mixer to better understand the turbulent nature of the VSF mixing process. They employed a number of models to do this, including the frozen rotor mixing model for impeller motion and the Euler-Euler inhomogenous multiphase model to simulate the liquid/solid mixing in the system.

This led to an initial impeller design, which then went through six iterations to optimize the tradeoff between mixing performance and power consumption. The final design (Figure 1) uses a curved-blade impeller placed near the bottom of the tank and provides a five-fold improvement in mixing of the solid suspensions together with a 12% cut in electricity consumption.

Faster Decision-Making

CFD technology itself continues to advance, driven by improved computing resources and robust, accurate and scalable numerical methods. Important, too, is the ability to successfully perform multivariate analyses.

“These enable our customers to carry out hundreds of ‘what if?’ types of analyses early in the design phase and quickly assess product performance for strength, power, thermal, pressure, flow rate, electrical or a number of other performance requirements. Through this digital exploration, designers and product engineers can identify optimal combinations while eliminating outlying designs — saving time and money. This multivariate design-of-experiment approach helps them to identify the ‘best’ operating condition rather than one that is simply ‘good enough,’” says Kulp.

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