Energy targeting and process integration of spray dryer with heat recovery systems

Abstract

Energy recovery is essential as it reduces fuel cost and pollution due to emission of flue gases. The energy recovery potential can be easily determined by pinch analysis, where individual fluid streams obey mass balance. However, in this paper, pinch analysis based methodology is proposed for energy targeting of evaporative systems such as dryers where the mass balance of individual streams is not possible due to loss of identity of one of the streams (feed) because of evaporation of water content and separation of the solid part. Initially, a spray dryer with a counterflow arrangement is selected to demonstrate the applicability of pinch analysis for targeting of energy requirement considered as the base case. Additionally, three different heat recovery configurations integrated with spray dryers are explored by pinch analysis for energy targeting as: (i) direct heat recovery, (ii) indirect heat recovery, and (iii) hybrid heat recovery systems. Further heat recovery potentials and their effect on the performance of the drying process are evaluated appropriately for different cases based on exhaust gas temperatures. The pinch analysis shows that with decreasing a mass flow rate of supply gases/air, the exhaust losses are also reducing for spray dryer integrated with all configuration of the heat recovery system. For spray dryer with indirect heat recovery configuration, energy-saving potential with and without exhaust gas/air temperature constraint is achieved as 57 and 68%, respectively, for a given illustrative example. Among all configurations, a hybrid heat recovery system with 80% recirculation is having the highest thermal energy saving potential (74%) compared to the base case. Moreover, the reduction in the driving potential of spray dryers for direct and hybrid heat recovery configurations is appropriately analyzed for new as well as retrofitted drying systems. It is observed that the effect of driving potential is insignificant for a lower recirculation ratio. Further, techno-economic analysis reveals that there are substantial energy savings even after considering the driving potential suitably in new and retrofitted spray dryers with heat recovery systems.