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The Role of Engineering Technologies: Virtual Prototyping

By: Brian Bell, Peter Spicka and Aniruddha Mukhopadhyay
Posted: October 14, 2008, from the May 2006 issue of GCI Magazine.

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CFD has been used to help to understand fluid flow phenomena in many types of equipment where these processes take place (mixers, emulsifiers and crystallizers for example) and, in addition, that phenomena in the packaging materials that eventually will contain product.

Analysis of Solids Suspension in a Liquid Medium

Mechanical agitation is widely used in cosmetic industry operations involving both liquid-liquid and solid-liquid flows. In cases of solid suspensions in liquid media, the typical process requirement is for the solid phase to be suspended such that the suspension either is self-sustained or requires minimal shaking by the end-user. The issues are dissolution and any plausible reaction rates, as well as feed uniformity. The challenge is in understanding the fluid dynamics in the vessel and relating this knowledge to design through adjustable process parameters.

The motivation of this study is to elucidate the criteria of minimum suspension speed using CFD to establish scale-up criteria for stirred tanks suspending solids. One critical parameter for such solids-in-liquid system is the “just-suspended” speed (NJS). However, designing based on only NJS or on power draw does not necessarily lead to an optimum design. The impeller system has to be designed to provide optimum flow pattern for the suspension duty. As such, numerous correlations exist for NJS. Some of the correlations were developed with a narrow range of impeller blade styles, sizes and position in the lab scale tank. Due to the large variability involved, the prediction of NJS by means of correlations is often questionable. Thus the objective of CFD modeling is to predict NJS, solids distribution, cloud height, amount of solids unsuspended and the scale-up criteria.

Analyzing In-line Static Mixer Performance

Compared to the stirred tanks, static mixers operate in continuous mode. Several varieties and generations of static mixers are used in the cosmetics and toiletries industry.2–5 Static mixers are used predominantly for homogenization with relatively gentle shear forces such that the fluids are sheared locally to mix through intersecting flow channels by continuous splitting, extension and transport. Variations in concentration, temperature and velocity are equalized over the flow cross-sections in the mixer elements. Mixing is not random but follows the geometric pattern of these elements.

The engineering objective of static mixer flow analysis is to quantify the state of mixing by a parameter called the “coefficient of variation,” which typically is used to estimate the macromixing and determine the minimum number of elements required for a given unit operation. Gathering clear experimental pictures at various sections of a static mixer is expensive and not always feasible, whereas parametric variations in geometry, number of elements, flow conditions and fluid properties are easily studied using CFD.