MIXING takes a spin

Spin cycle: with a vertical `ring' of water and higher than atmospheric pressure Centrisol avoids clumping of particles on a liquid's surfaceConventional batch mixing involves charging a vessel with the appropriate volume of liquid, then adding the required amount of solids. Some materials, such as the thickeners or sweeteners for foods and pharmaceuticals, pose clumping and insolubility problems. Dissolving aspartame in water, for example, can require mixing for several hours because in a batch vessel the powder tends to float.

Such problems could be obviated with the introduction of a novel mixing technology from process systems company Bran+Luebbe, Hamburg. Its engineers have developed an integrated system which converts the traditional batch process into continuous and in-line liquids and solids processing.

The Centrisol mixer system offers process companies the benefits of just-in-time production, continuous powder dispersion and mixing, solids dilution up to 60 per cent, and accurate dosing. Centrisol's centrifugal mixing technique allows the continuous and controlled feeding of granular solids into a liquid and immediate mixing of the components in a single step, claims project engineer Dorothee Klopping.

The process combines established liquid feed technology, a multi-screw feeding process controlled by loss-in-weight procedures, and the Centrisol injector, which is the heart of the system. `The Centrisol injector system allows operators to produce suspensions continuously, with highly accurate concentrations,' Klopping told PE.

The Centrisol injector consists of a cylindrical casing containing a rotor mechanism which is driven externally from underneath. All parts are made of stainless steel. The opening for solids is located on the flat top of the cylinder, with liquid entering and product mixture exiting through nozzles located in the cylinder wall.

WATERWALL

The first stage of the mixing process involves pumping precise quantities of liquid into the cylinder. The centrifugal forces acting on the liquid force it against the cylinder wall, leaving the rotor free from liquid. At a sufficiently high rotational speed, the pressure at the cylinder wall reaches several bar, forming a stable liquid phase. The forces maintain a `ring' of liquid with air in the centre, into which the solid is fed.

From start-up, air is contained within the water, which will clear after about 2min as the rotor reaches its optimum speed. Removing the air is an important factor in the process. Generally, the Centrisol will run at about 150-200l per hour at its slowest speed, so potential loss of product is only about 5l. At its top speed the largest of the four models of Centrisol mixer can process at a rate of 12 000l/hour.

Solid particles from the screw feeder fall freely and reach the liquid-free centre. Frictional forces then accelerate the particles away from the centre towards the liquid at the cylinder wall and once they strike the liquid, they are rapidly absorbed with a uniform distribution in the liquid phase.

The mixed liquid is withdrawn by an extraction pump, operating at a power in proportion to the pressure at the cylinder wall. Keeping the cylinder wall pressure constant is essential, as this determines dispersion characteristics. The Centrisol device may be regarded as a continuously stirred tank, which minimises air contact with the product.

Bran+Luebbe has identified several areas of application for the Centrisol Mixer System; two of the £100 000-plus systems have been purchased by German companies and examples of the four sizes, ranging from 500-12 000l/h, are installed in various test sites.

In food processing, the system has been used successfully in the production of syrup and the addition of stabilisers in egg and milk powders and thickeners to water or liquid foods. It has also been employed for adding pigments to cosmetics, mixing scouring powder into detergents and for thickening dilutions.

Per Johansson, systems projects engineer, at Bran+Luebbe's Brixworth site, told PE: `Typical liquids that the system suits are water and sorbitol; it can handle liquids up to 1000 centipoise. We have also performed some experiments with heating the input liquid to reduce the viscosity of the mixed solution. No thermal problems were experienced. And after use, the Centrisol's cleaning in place function is achieved by pumping water at 85iC through the system.

`In tests, several of our clients have have made products to food standards, such as toothpaste. They have tested these for any problems of bacterial growth and we have not had any comeback, so far. It's a fairly expensive piece of kit so it wouldn't really be suitable for water treatment, but we believe Centrisol will possibly create a brand new market for itself.'