Impact of heat exchanger fouling in bioethanol plants

Bioethanol plays a major role in improving the sustainability of a carbon-intensive sector such as transportation. With a pressing need in the industry to reduce carbon intensity of the ethanol production itself, it is of paramount importance ensuring that thermal systems are as efficient as possible. One of the biggest challenges that leads to significant operational issues, extra energy and water consumption, unplanned plant shutdowns and unnecessary CO2 emissions is fouling, the progressive deposition of low thermal conductivity material on thermal surfaces. This unwanted deposition reduces the thermal efficiency of key equipment, including heat exchangers and evaporators and increases pressure drops. In this paper, a mathematical model for the most energy-intensive section of a typical 50 MGY dry-grind ethanol plant is developed to assess the economic and environmental impact of fouling. The model includes double effect evaporators integrated with the beer column and two reboilers and allows quantifying the Key Performance Indicators under different combinations of fouling rates, operating conditions and cleaning effectiveness. Results show that even if regular cleaning cycles are performed, following current industry standards, the impact of fouling is still significant, with emissions of 15,435 ton CO2/ year (81.67 g CO2/l), additional energy of 70,809 MWh/year (0.38 kWh/l), additional water use of 106,738 m3/ year (0.56 l/l), and additional costs of 3.53 MM$ per year (0.02 $/l). Moreover, by optimising the selection of cleaning agents, costs can be reduced up to 65-70 % while water consumption, energy use and emission release can be curbed by 70-80 %. Other strategies to mitigate fouling (e.g. increasing steam pressure) were also identified and discussed.