Study of CO₂ heat pump cycle layout for full electrification of milk powder spray dryer

The utility energy demand of spray dryers is the highest among the process operations of milk powder plants. This is mostly due to heating of air up to ~240 °C from ambient temperature, which results in significant use of thermal energy, conventionally supplied by combustion of fossil fuels. The exhaust air of the spray dryer can be utilized for heat recovery. The energy efficiency improvement potential is limited when only employing direct heat recovery, due to a large need for heat at high temperatures. High-temperature heat pump (HTHP) integration provides a full electrification possibility to substitute steam and natural gas consumption. However, considerable exergy losses due to the heat transfer temperature difference between refrigerants and process air hinder the integration of HTHP for providing heating in the full temperature span. In this study, a baseline CO₂ transcritical HTHP layout, which includes discretized gas coolers and internal heat exchanger is investigated for such a large temperature glide and multiple heating temperature application. In addition to this, the interaction between heat pump configuration, heat exchanger network layout, spray drying process and production, and energy-saving potential is analyzed in this study. The gas coolers in the layout are divided into subcomponents to achieve a good temperature profile match between heating air and refrigerants. A maximum total COP of 2.68, a minimum levelized cost of heat of 27.8 €/MWh, and a payback time of 8.6 years were obtained at a discharge pressure of 23 MPa.