Heat transfer fouling is the accumulation and formation of unwanted materials on heat transfer surfaces which leads to a decrease in the overall heat transfer coefficient. Fouling of heat transfer equipment increases energy consumption and maintenance costs and thus decreases processing efficiency. In the fuel ethanol industry, evaporator fouling occurs when thin stillage is concentrated. Fouling affects the efficiency and environmental footprint of more than 200 biorefineries in the US. Thin stillage is the liquid fraction of unfermented materials from fermentation and is composed of carbohydrate, protein, fat and ash.
Research on thin stillage fouling has focused on effects of corn oil, pH, Reynolds number, solids concentration and carbohydrates (Singh et al 1999, Wilkins et al 2006ab, Arora et al 2010, Challa et al 2014). However, temperature effects on fouling rates have not been studied. The objectives were to investigate the influence of bulk fluid temperature, initial probe temperature and their temperature difference on thin stillage fouling characteristics. Experiments were conducted using model thin stillage (1% starch solution) and commercial thin stillage with varying temperature conditions. Bulk temperatures varied from 60 to 80°C and initial probe temperatures varied from 100 to 120°C; thus temperature differences varied from 20 to 60°C. Fouling resistances were measured using an annular probe with a 7 L batch system. Mean fouling rate, maximum fouling resistance and induction period were used to characterize fouling behavior. For commercial thin stillage, fouling rate and maximum fouling resistance increased as the initial probe temperature increased from 100 to 120°C. At 120°C initial probe temperature, fouling rate increased with bulk temperature. For the model thin stillage, similar results were observed. Induction periods were longer than 5 hr for an initial probe temperature of 100°C. At an initial probe temperature of 120°C, longer induction periods, lower fouling rate and maximum fouling resistance were observed at lower bulk temperature. Bulk temperature and initial probe temperature had an effect on thin stillage fouling characteristics. Comparing with commercial thin stillage, bulk temperature had larger effects on model thin stillage. Using higher initial probe temperature, such as 120°C, and fluid bulk temperature of 80°C would provide repeatable and more rapid (within 5 hr) characterization of fouling behavior.