(46a) Mitigation of Heat Transfer Fouling in Thin Stillage Evaporators

During fuel ethanol production, fermentation of corn starch leads to fractionation of constituents, one of which is termed thin stillage. When this fraction is subsequently concentrated using evaporators, deposits routinely form on evaporator surfaces through a process termed fouling. Heat transfer fouling is a major factor adversely affecting evaporator efficiency and increases water, chemical and energy use at bioethanol production facilities. Due to fouling deposits, evaporators must be cleaned at intervals of 1 to 2 weeks or even more frequently, thereby negatively impacting profitability and capacity of the facility and ethanol production of at several levels. First, evaporator operations must be over-designed in order to compensate for decreased process throughput during cleaning, which leads to increased capital costs. As deposits form, evaporator efficiency is reduced, increasing energy requirements for evaporation. Further, cleaning of fouling deposits increase water used during ethanol production; following cleaning, water and cleaning chemicals must be either reused or treated. Since more than 200 U.S. fuel ethanol plants use evaporators and are adversely affected by heat transfer fouling, which increases the overall environmental footprint and costs of biofuel production, there is a critical need to more fully understand the causes of fouling and to develop strategies to mitigate fouling in thin stillage evaporators. An annular probe was used to quantify fouling tendencies of fluids passing over heated surfaces. Three streams derived from corn based ethanol fermentation were evaluated: original thin stillage, diluted thin stillage (DTS) and permeate from microfiltration of thin stillage (MFP). In additional experimentation, we examined the relative fouling tendencies of individual components of thin stillage (e.g., ash, sugar contents) to determine which components caused increased fouling. Reductions in solids concentrations and changes in composition resulted in reduced fouling rates and fouling resistances. At 10 h of fouling, 50 and 90% reductions in fouling resistance were observed in DTS and MFP streams, respectively, although both streams had similar solids level decreases from 7.2 to 3.5%. In DTS and MFP, induction periods were prolonged by factors of 4.3 and 9.5, respectively, compared to the induction period for thin stillage fouling. Mean fouling rates were decreased by factors of 2.3 and 23.4 for DTS and MFP, respectively. Fouling of MFP took twice the time to reach a probe temperature of 200°C than did thin stillage (22 vs 10 h, respectively). Reduced solids content alone did not explain decreased fouling tendencies in microfiltered stillage. These results are indicative that selective separation of components from the thin stillage may improve evaporation efficiency and mitigate evaporator fouling.