Gypsum calcination in a fluidised bed reactor
Gypsum (calcium sulphate dihydrate) is of great industrial importance with over 95,000 ktonnes being used in the world per annum. The greatest use of gypsum is in the production of plaster (calcium sulphate hemihydrate) for use as an interior finisher. Plaster is produced by the calcination (thermal decomposition) of gypsum. The most popular design is a continuous calciner where gypsum is fed continuously and is directly heated by hot air. There are a number of different phenomena occurring within a calciner, including heat transfer, mass transfer, particle and gas mixing, elutriation and the dehydration reaction itself. All these processes interact with each other. Although a lot of research has been carried out in these areas already, the literature has been found to contain significant discrepancies. This study contains experimental work which has been carried out in order to better understand the physical processes occurring within a gypsum calciner. The rate of dehydration of gypsum (35-67μm in diameter) has been studied in a fluidised bed reactor. Experiments were carried out at bed temperatures of 100 to 170°C. The fluidising gas was air with water vapour pressures of 0.001 to 0.30 atm. The dehydrations were under differential conditions. The results show that the dehydration under these conditions can be successfully modelled using the two dimensional Avrami-Er'ovev expression. A study of the fluidisation and elutriation properties of gypsum in batch vessels (cylindrical and conical) has been carried out. The mechanics of elutriation has been investigated and modelled for various freeboards, superficial gas velocities and air humidities. Tracer tests have also been carried out on a laboratory scale continuous conical kettle. Sodium carbonate was used as the inert tracer material. Runs were carried out at different air and gas flowrates and different bed temperatures. Residence time distributions were elucidated. Finally, the above experimental data and component models have been investigated for their applicability to producing a model of the laboratory scale gypsum calciner.