Cost reduction and energy efficiency in EPP processing

The processing of EPP (expanded polypropylene) is an energy-intensive process in plastics processing. Due to the process-specific use of steam, compressed air and cooling water, which are often not further utilised for energy purposes after the end of the process, i.e. are simply lost unused, the energy costs significantly influence the total costs for the end product. The steam required for production is often generated from treated fresh water. Thus, the water must be heated from room temperature to approx. 180-190 °C steam temperature via the steam generator. At the same time, the generation of compressed air generates a lot of waste heat potential that is only partially used or not used at all. By using a compressed air heating system (CAHS) with e.g. 300 kW shaft power, 2,900 m³/h of compressed air can be generated at 8.5 bar and at the same time 11 m³ of water can be heated from 30 °C to 95 °C. This requires 776 kWh of natural gas. In comparison, 325 kWh of electricity must be used for the same amount of compressed air. To heat the 11 m³ of feed water by 65 °C, 973 kWh of natural gas must be used at a boiler efficiency of 90 %. Table 1 shows an example of the energy costs incurred for the provision of the media described. The conventional generation of steam and compressed air without heat recovery from compressors, the conventional generation with heat recovery from compressors and the use of a CAHS are considered.

Table 1: Comparison of energy costs for the provision of compressed air and heat

Figure 1: Energy costs for the provision of compressed air and feedwater heating

The bar charts in Figure 1 illustrate the possibilities that the technology generates in terms of cost savings. Compared to conventional provision, there is an energy cost reduction of 345,000 €/a; if conventional compressors are equipped with heat recovery, the saving is still 300,000 €. Thermal efficiencies of over 100% can be achieved through the heat pump effect that occurs during the compression and subsequent expansion of air. The prerequisite for this is that the condensation heat from the condensing heat exchanger as well as that from the compressed air cooling can be used. Due to the low temperatures (< 30 °C) at which the fresh water is available, this process represents one of the ideal applications.