Digestate : Sludge evaporation is better when reciprocated

The HRS Unicus Series uses a patented stainless-steel scraping mechanism which moves hydraulically back and forth within each interior tube. This movement performs two key functions: it minimises potential fouling by keeping the tube wall clean and creates turbulence within the product. Taken together, these actions increase heat transfer in the material, creating a highly efficient process which is ideal for extremely viscous and high fouling products.

Because the speed of the scrapers can be controlled separately and a number of different scraper designs have been developed for different products, the action of the Unicus Series and the speed of the scrapers can be optimised for a particular material. The Unicus Series has proven particularly useful in treating the digestate from anaerobic digestion processes where the texture and consistency are particularly challenging.

Each Unicus Series consists of three elements: a hydraulic cylinder and power pack, a separation chamber to ensure hygiene and preserve product separation from the motor, and the heat exchanger itself. The heat exchanger consists of a number of tubes, each of which contains a stainless-steel rod to which the appropriate scraping elements are fitted. Using a range of materials including Teflon and PEEK (polyether ether ketone), these provide different internal geometry setups according to the application, such as 120° scrapers for large particulates and 360° scrapers for viscous fluids without particulates. 

A standard range of Unicus models for environmental uses provide heat transfer areas from 0.7 to 10 m², while larger models up to 120 m² can be made for specific uses. The Unicus Series is fully scalable by increasing the shell diameter and adding interior tubes (as many as 80 in a single shell). Specially designed seals separate the inner tubes from the separation chamber, and like the scrapers, these are tailored to the product application. These seals prevent leakage of the sludge or digestate. 

The Unicus Series is particularly suitable for evaporation applications, where preventing fouling is particularly important. Because of this, HRS has developed a specific version of the Unicus Series for use in evaporation units where volume reduction of the product is required. The scraping action keeps the heat transfer surfaces clean so that Unicus evaporators can concentrate materials to a level that is unattainable using traditional technologies. Unicus evaporators can be used in multi-effect setups or in combination with mechanical vapour recompression, where suitable.