Sludge drying in CO_2 heat pump dryer

摘要

Thermal drying of sludge significantly enhances the number of possibilities of its further usage. The traditional methods of sludge drying are based on the process of evaporation of water in a temperature of minimum 100 ℃. Consequently a significant amount of energy is required and the process has a high environmental impact. The heat pump drying system is an alternative for the traditional method of sludge drying. It is energy efficient and avoids emissions from the drying installation. Sludge is dried by closed loop circulating air treated in the heat pump. A novelty is applying carbon dioxide (CO_2) as a refrigerant. CO_2 as a natural working fluid in heat pump systems is increasingly important in view of the HFC (Hydrofluorocarbons) alternatives. Due to the critical parameters of CO_2 (T_c= 304,13 K and p_c = 7,38 MPa) the process in the CO_2 heat pump is the transcritical cycle. It provides the possibility of additional hygienization of the dried product which becomes sanitary safe. The article introduces the idea of CO_2 heat pump dryer for sewage sludge drying and presents data from laboratory measurements of sludge drying in controlled temperature and humidity environment.