Ch. Vial

Static Mixers in the Process Industries—A Review

This paper summarizes the field of static mixers including recent improvements and applications to industrial processes. The most commonly used static mixers are described and compared. Their respective advantages and limitations are emphasized. Efficiencies of static mixers are compared based both on theory and experimental results from the literature. The operations, which can benefit from the use of static mixers, are explored, namely, mixing of miscible fluids, liquid–liquid and gas–liquid interface generation, liquid–solid dispersion and heat transfer. Design parameters governing the performance of the various mixers in these applications are reported. The key parameters needed for the selection of a suitable mixer are highlighted.

Influence of operating conditions and impeller design on the continuous manufacturing of food foams

The influence of impeller design and operating conditions on the continuous manufacture of food foams was studied in a narrow gap mechanically stirred unit using a model food. Quality and texture of foams were characterised on the basis of density, bubble size and rheological measurements. Mixing mechanisms were investigated using residence time distribution (RTD) experiments. The results show that mean bubble size in foams depends only on the intensity of viscous forces generated by the impeller. For a similar intensity of shear forces, impellers with a small internal volume are to be preferred for foaming because they improve bubble dispersion and mixing of surface-active agents. These allow the manufacture of foams with a lower density, which has been shown to improve their texture and increase their mechanical strength. RTD analysis is shown to be a simple diagnosis tool for estimating the mixing efficiency of new impellers which may be helpful in improving choice of adequate impeller design and operating conditions to produce foams with desired properties.

Effect of Matrix Elasticity on the Continuous Foaming of Food Models

The aim is to understand the effect of matrix elasticity on continuous foaming using food models based on glucose syrup. This was modified by adding polyacrylamide (PAA) with 2% whey protein isolate (WPI) or Tween 80 as foaming agents. Foaming was conducted in a stirred column. Rotation speed N and gas-to-liquid flow ratio (G/L) were varied. Overrun, average bubble size d32, texture and stability were measured using densimetry, image analysis, and rheometry, respectively. Experimental results showed that 0.01% PAA did not modify the viscosity of 2% WPI models, but conferred low elastic behavior. PAA (0.05%) doubled matrix viscosity and drastically increased elasticity. The increase of elasticity became slower for further PAA addition. Foaming experiments demonstrated that theoretical overrun could not be achieved for inelastic WPI models in two cases: for high viscosity and low N, as dispersion effectiveness was reduced; for high G/L and N because of enhanced coalescence. Matrix elasticity was shown to increase overrun at constant viscosity for high G/L by enhancing interface stabilization. However, in elastic models, gas dispersion was more difficult and d32 was higher than in inelastic fluids of similar viscosity. Finally, when the limiting step was dispersion, foaming was shown to be negatively affected by matrix elasticity.