Sludge convection drying process: Numerical modeling of a heat pump assisted continuous dryer

In a previous stage of the current study, a numerical model of sewage sludge drying was developed. Air stream with a given temperature and speed passes over the mud to dry the product. A heated floor is also added to the process to perform a proper drying of the product. This model is coupled to a heat pump model exchanging with the air stream by way of intermediate exchangers. The present paper aims at investigating the performance of this continuous dryer. The specific moisture extraction rate (SMER) is calculated for a given configuration with fixed air temperature, humidity and velocity as well as fixed dryer dimensions and mud thickness. Furthermore, a reversal product operation was performed to evaluate this technique. In this work, two cases are studied. The first one, with the weakest relative humidity of the drying air (11%), has given a SMER result of 1.44 kgw.(kW h)(-1). The second one, with higher relative humidity (16%), has permitted to obtain a SMER indicator of 1.99 kgw.(kW h)(-1). This comparison shows the importance of maintaining relatively high humidity conditions for the air stream entering the evaporator, notably due to a lower compressor work. Moreover, these results highlight the strong interest of a heat pump system coupled to a dryer, in comparison to a conventional dryer for which the SMER is equal to 1 kgw.(kW h)(-1).