Getting the mix right

Mixtures of solids in liquids, dispersions and emulsions or pastes are as variable as the components that can be brought together: and then there is the added complication of the particle size distribution that might be desirable for any second or solid phase. Different techniques are being developed for mixing, particularly alongside new developments using nano-particles and catalysed reactions.

Most traditional mixing systems expend a lot of energy to disperse the components in the mixture, but a growing emphasis on energy cost reduction means other novel mixing techniques are proving effective.

Inducing vortices to mix a fluid flow can eliminate any stratification, or radial gradients of temperature and material composition, and such an “unpowered” or static mixer can, typically, be applied to low viscosity liquid-liquid blending, as well as mixing of process gas streams. The energy to create the vortices comes from the introduction of head loss and recent designs are developments of the original helical elements used for this duty.

The Kenics High Efficiency Vortex (HEV) mixer, from Chemineer, offers low head loss and an open, non-blocking structure, with triangular, side-wall mounted vortex-inducing elements producing the mixing action. Kenics HEV mixers have found wide use in low viscosity liquid mixing in large lines, particularly for water distribution and treatment in up to 2,200mm line diameters, and for gaseous use.

Chemineer recently supplied such a static mixer for a natural gas processing plant in Abu Dhabi. Specified by Fluor for mixing of the gas flows from four desert plants to a sulphur recovery unit in the Habshan complex, the 56-inch diameter Kenics HEV unit was chosen to help produce complete stream uniformity from a combination of the vortex structures and the mixing elements. The hydrogen sulphide in these gas streams is then converted to sulphur using the Claus Sulphur Recovery process by passing them through a furnace and over catalysts.

Further Chemineer product development has introduced an injector design optimised for use with the HEV-style mixer. This has recently been introduced as the Water Vortex Mixer (WVM). Its operational performance allows the unit to be offered for the injection and mixing of liquids even for low speed flows in pipelines, down to 0.1m/sec. Performance has been validated by the BHR group.

In direct contrast to the relatively gentle mixing action of flow-induced vortices, an ultrasonic mixing system has been developed that uses high power transducers to generate ultrasound waves that produce cavitation within a liquid. Imploding cavitation bubbles cause intensive shock waves in the liquid, and this disintegrates and disperses any particles or globules of a second phase.

The dispersion effects continue with time, disintegrating solid particles, such that the technique has been used in the laboratory to create dispersions, emulsions or pastes with a very fine particle size, down to much less than a micron. Industrial size processing units are available, with stainless steel Sonotrode active elements from Hielscher capable of producing up to 16kW of ultrasonic power, for example emulsifying up to 32m3/hr of oil and water, or dispersing/de-agglomerating up to 6m3/hr of powder in suspension. A smaller 1kW unit can, typically, process 4litres/min of a mixture in a flow cell, using a recirculation mode to establish the correct processing parameters.

Another advantage of this “ultrasonication” is that it can accelerate mixing or reaction processes and can turn a batch reaction into an in-line process, as has been achieved in a recent biodiesel production project.

This is a major application area for the technology and a three million gallons per year biodiesel plant commissioned by Pleasant Valley Biofuels in Idaho has a production process designed by Next Energy Systems of British Columbia using Hielscher ultrasonic mixing technology, with three Hielscher 1kW ultrasonic reactors. Initial test runs were conducted on a high level of free fatty acid (FFA) waste vegetable oil and after the transesterification into biodiesel occurred, separation of the by-product glycerin, by gravity alone, occurred within 15 minutes. This is far faster than conventional bio-diesel processing, where usually the 5-hour separation is accelerated with a centrifuge stage.

Hielscher has reported many industrial applications for ultrasonication: it is one of the most effective techniques for particle size reduction of inkjet ink pigments, which can be reduced in size from 500 microns to around 10 nanometres. The cell disruption effects of ultrasound have also been used for microbial and enzyme inactivation to aid the preservation of fruit juices and honey. The technique is also being used in the production of nano-particles and in the promotion of catalyst functions by activation of their surface layers.

A combination of vortex mixing and cavitation is used in the Sonolator process, from the Sonic Corp., in which two liquids can be mixed together using metered flows direct from their storage tanks. The liquid components to be added are delivered from a high pressure, positive displacement pump into a special orifice within the Sonolator, a mixing section in the pipeline that replaces any need for a mixing vessel or tank.

The orifice causes the liquid to be forced to strike a knife edge-type blade at high speed, creating vortices and extreme cavitation that results in very effective mixing and homogenisation. It is claimed to be applicable to paint/pigment dispersions, emulsions, lubricants, greases, resins and adhesives.

High shear in-line and in-tank mixing systems have been developed for more difficult mixing applications, where a high shear or milling-type action is needed to create a fine dispersion or emulsion. One of the pioneers of this style of mixer is Silverson. It recognises the difficulties of assessing the capabilities of a mixer in a specific application and has developed a comprehensive demonstration facility to enable customers test new products or formulations using their own materials on any of the Silverson range of high-shear mixers.

This range has recently been extended with the UHS line of hygienic mixers. It follows increasing demands from customers for ultra-hygienic mixing equipment and the UHS has been developed to meet EHEDG certification and conform to relevant cGMP and FDA guidelines.