Producing energy by transforming crops into bio-ethanol at a time when famine and food shortages affects large areas of the world’s population is clearly not a sustainable option. But new technology which enables bio-waste to be turned into bio-ethanol means that energy can now be harvested without damaging the food chain.

Ethanol, a popular type of bio-fuel, is traditionally produced by fermenting sugars present in plants such as corn and sugarcane with yeast. The process requires the use of the most edible parts of the plants, which are rich in starch and certain types of sugars called hexoses, resulting in the loss of a valuable source of nutrition.

Following a project at Delft University of Technology in the Netherlands, researchers have succeeded in genetically modifying a strain of yeast that can convert bio-waste materials such as straw, woodchips and corn husks, into ethanol. And a technical process using the yeast has been developed at the Danish technical university. The technique opens up a previously untapped source of plant-based energy without compromising world food supplies.

Food for thought
One company utilising this process is GEA Niro, a spray-drying specialist. GEA Niro specialises in the development, design and engineering of powder technology. And the company’s expertise in spray-drying proteins plays a major role in the process by drying the by-product of ethanol – yeast cream – which is decanted from the resulting slurry, into protein powder, which can be used for animal feed.

The drying process is achieved in three steps. The yeast cream is concentrated to a point where the slurry at 60˚C has a viscosity of around 150 centipoise. This concentration is achieved in an energy-efficient way by using a falling film evaporator to a level of about 20% total solids.

The concentration is then dried in a spray-drying plant equipped with either rotary or nozzle atomisation depending on the required properties of the final powder. The resulting powder is transported to silos for immediate use or packed into bags for storage or distribution.

In any industrial process, operational cost must be kept under control and removing water from a substance can be a costly affair. For example, heating 1kg of water by 1˚C requires 4.18kJ and when heated to 100˚C it requires further a 2,400kJ to evaporate. Therefore the specific water evaporation rate is the energy used to remove a specific amount of water in the evaporation plant.

Niro says its falling-film evaporation plant operating at low pressure and using thermal or mechanical recompression of vapour has a low specific energy consumption.

Where there are no particular property demands for the dried yeast cream powders, Niro recommends spray drying using rotary atomisation. The powder produced using this method will have a bulk density of about 500kg/m3 and an average powder size of 100 microns. This makes it suitable for mixing with animal feedstock.

For more complex powder properties, Niro recommends using a spray dryer with nozzle atomisation for drying the concentrated slurry. To keep the initial investment as low as possible in acquiring a spray dryer, the company suggests using the highest possible drying air temperature. This can be achieved using a direct gas heater to increase the temperature of the main air.

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