The wet feed solid material to a continuous dryer contains 40 wt % water on a wet basis (see hint) and is dried to 25 wt % (wet basis) by a counter-current air flow. The dried solid product leaves at the rate of 800 kg/h (dry solid+moisture). Fresh air to the system is at 26C and has a humidity of 0.004 kg water/kg dry air. The moist air leaves the dryer at 35C and humidity of 0.02 kg water/kg dry air. Part of this air is recirculated and mixed with the fresh air before entering the heater (See drawing below). The heated mixed air enter the dryer at 65C and humidity of 0.010 kg water/kg dry air. The wet solid enters at 25C and leaves at the same temperature (25C). Calculate the mass flow rate of air recycled.

Hint: Wet basis weight percent of water = [ kg of water / (kg of water + kg of dry solid) ] x 100% Dry basis weight percent of water = [ kg of water / kg of dry solid ] x 100%)

FIGURE 9.10-4. Process flow for air recirculation in drying