A model-based analysis of a near-adiabatic system with two high-temperature heat pumps for sustainable paper drying
Climate change poses a significant threat to the environment, with industrial processes being among the largest contributors to the global carbon footprint. The paper industry contributes significantly to carbon emissions due to its high energy demand, with its energy mix relying on roughly one-third fossil fuels, one-third nonfossil- based sources, and one-third electricity. This highlights the urgent need for the paper industry to shift towards green and sustainable practices. The drying section is the most energy-intensive stage of the papermaking process, accounting for around 60% of total energy consumption. Therefore, there is an opportunity to reduce energy consumption and carbon emissions. This paper proposes and models the concept of a novel technology carrier, which consists of two high-temperature heat pumps operating simultaneously to create a near-adiabatic process for paper drying. The first heat pump is an open-cycle system that captures exhaust air from the drying hood and compresses it to increase its temperature. This hot, compressed air is then used for the following purposes: to act as the heat source for a second heat pump, to heat process water, to preheat incoming fresh air, and to generate electricity. The second heat pump absorbs heat from the first one and delivers steam at above 200 °C and 6 bar(a), using water as its refrigerant. The Python model is also used to investigate the following: how multistage compression with intercooling manages compressor discharge temperature in a high-temperature heat pump using water as a refrigerant, the effects of variations in moisture content and exhaust air temperature on heat recovery system performance, and potential methods for improving the heat recovery system's coefficient of performance to enhance overall efficiency. The insights presented here lay the groundwork for future research and collaboration with the paper industry, supporting the transition towards sustainability.