Heat pump integration in the plastic granule drying industrial process

Drying of plastic granules is an essential process for molding industry, it is energy intensive but fundamental for the quality of final products. Grundfos is a company working in the water treatment sector and owns multiple plants for granules drying in their production lines. This study focuses on one of its plants whose equipment is produced by Labotek. It consists of 7 drying hoppers and one desiccant flexible dryer unit treating up to 450 m3/h of air. The average elaborated mass of plastic is 80 kg/h and it has an initial moisture content of 1% on mass basis. The continuous drying process is carried out in a closed loop circuit where the air is dried down to a dew point of -40 °C by means of a molecular sieve placed in a desiccant bed. The thermal energy stream requirements involve both heating and cooling and a heat pump would be capable of cooling the waste stream by extracting and promoting the heat. The plant currently relies on electrical heaters and chilled water cooling coils. The hoppers air temperature requirements go from 80 °C to 140 °C. Several models using EES (engineering equation solver) have been developed to study the possibility for the integration of a CO2 heat pump (HP) in the system to fulfill the heating and cooling needs of the drying phase. A fist simplified design approach aimed to decouple the heat exchangers capacities to face the energy balance challenges and deeply analyze the system. The study then, passing through a third design model, addresses to an hybrid layout with the heat pump supported by an additional cooler and electrical heaters. The final heat pump proposed solution was investigated with the components model where the system relies on Bitzer 4PTE-7k compressor. This solution shows a heat pump configuration working with an evaporating temperature of 15 °C and a gas cooler optimized pressure of 122.1 bar. The air temperature are 20 °C for the bed inlet and 80.77 °C for the main supply air. The gas cooler capacity is 6.88 kW and the HP has a COP of 4.151. The calculated potential reduction of electrical energy consumption is 17.60 % and the SMER is 0.05181 kWh/kg resulting in an increase of 21.36 % compared to the existing configuration. This system would cover 59.8 % of the cooling load and 66.6 % of the heating load for drying operations. The study analyzes also how different mass flow rates of plastic and different volume flow rates of air can affect the performance of the integrated system showing that a high plastic load carries to poor overall performance and that high air flow rates produced small increases of the COP.