Degassing a liquid stream using an ultrasonic whistle

The removal of unwanted dissolved gas from a liquid is an important unit operation in many processes; gas chromatography has a degassing stage for example; photoresist substrates are degassed before being applied to silicon chips. The degassing technologies used in these cases include membrane, vacuum and ultrasound. Another important application is the production of carbonated beverages. With beer and lager production, excess carbon dioxide in solution is commonly displaced by nitrogen in a batch process. This is time consuming and expensive. The work described here relates to the latter application and investigates the suitability of an ultrasonic whistle (jet-edge type) for degassing liquids. The attractions of a whistle are that no external power supply is required and that it is also a flow-through device. The mechanism of operation is as follows-A flow of liquid (the 'jet') through a nozzle impinges on the 'edge', causing it to vibrate ultrasonically. Ultrasound of a particular frequency and above a threshold intensity causes stable cavitation in a liquid, and consequent degassing through rectified diffusion. The design of the nozzle and 'edge' are carefully matched to achieve resonance at the required frequency. Initial experiments with carbonated water have been promising-up to 30% of the carbon dioxide was removed from a flow of 110 litres/min. Increased gas removal is expected following further optimisation.