Effect of pH on Fouling Characteristics and Deposit Compositions in Dry-Grind Thin Stillage

Cereal Chem. 83(3):311–314 Dry-grind corn processing facilities produce ethanol, carbon dioxide, and distillers dried grains with solubles (DDGS). To produce DDGS, drygrind corn processors concentrate thin stillage in multieffect evaporators. Concentration of thin stillage uses large amounts of energy, and efficient operation is important for long-term economic stability of the industry. Little data are available on fouling of evaporators during thin stillage concentration. We evaluated how thin stillage pH and acid type used during pH adjustment affected fouling as measured by induction period, fouling rate, and deposit composition. Using an annular fouling apparatus, fouling tests were conducted at pH 3.5, 4.0, and 4.5. In a second experiment, we used two types of acid, HCl or H2SO4, to adjust thin stillage to pH 3.5. Induction periods were shorter at pH 3.5 than at pH 4.0 or 4.5. As pH increased, fouling deposit protein decreased and ash increased. Concentrations of most elements, including P, Ca, Mg, Mn, and K, increased with an increase in pH. Phosphorus was the most abundant mineral element in fouling deposits. Induction periods were similar for the two acids. Thin stillage pH has an influence on deposit concentration, fouling rate, and induction period. In the United States, fuel ethanol is produced primarily from corn by wet-milling and dry-grind (DG) processes. DG facilities require lower capital investment than wet-milling facilities, but they produce only one coproduct, distillers dried grains with solubles (DDGS). DDGS is sold as an animal food, primarily for ruminants. To meet growing demand for ethanol, farmer-owned cooperatives and others have built DG facilities throughout the corn-producing states of the United States. Decreased production costs are needed to make DG processing more competitive and stable. Several strategies can be pursued to compensate for declining DDGS prices. One of these is to increase DG process efficiency. Concentration of thin stillage is one of the more energy-intensive unit operations in DG processing. Evaporation and drying operations account for 40–45% of thermal energy and 30–40% of electrical energy used in a DG facility (Meredith 2003). Thin stillage is the liquid fraction that results from centrifuging material left over after ethanol distillation (whole stillage) and is composed of soluble proteins, ash, lipids, and carbohydrates that were not converted into ethanol during fermentation. Thin stillage is concentrated from 4–6% (wb) solids to 25–30% (wb) solids using multieffect evaporators and combined with the insoluble fraction from whole stillage centrifugation (wet grains) to form DDGS (Singh et al 1999). Evaporators accumulate deposits on their surfaces that reduce heat transfer and increase pressure loss in a process known as fouling. Fouling decreases energy efficiency and increases operating costs through higher heating fluid temperatures and increased cleaning of evaporators. The fouling of heated surfaces in fluid dairy processing has been reported in numerous studies. In dairy processing situations, pH and its relationship to the isoelectric point of dairy proteins have been shown to play key roles in fouling mechanisms (Belmar-Beiny and Fryer 1993; Visser and Jeurnink 1997). When solution pH is at or near a protein’s isoelectric point, the protein has neutral charge and protein molecules aggregate due to a lack of electrostatic repulsion. Protein aggregates can then attach to a heated surface (Visser and Jeurnink 1997). A similar effect has been observed with other proteins such as bovine serum albumin and gelatin (Fukuzaki et al 1995). The typical pH range for thin stillage is 3.7–4.7 (Jones and Ingledew 1994). Slurry pH is adjusted by H2SO4 and HCl addition before saccharification and fermentation for optimal yeast growth and glucoamylase enzyme activity (Kelsall and Lyons 2003; Russell 2003). There are few publications regarding fouling of thin stillage evaporators. Singh et al (1999) found that thin stillage from corn wet milling fouled at a rate 67% less than DG thin stillage. There are few published data on fouling rate and deposit composition. This study was designed to determine effect of pH on thin stillage fouling behavior and fouling deposit composition. MATERIALS AND METHODS Fouling Test Apparatus The apparatus for measuring fouling characteristics was based on previous work (Fischer et al 1975; Singh et al 1999; Agbisit et al 2003; Wilkins et al 2006) and consisted of a stainless steel rod inserted into a cylindrical housing, creating an annulus (0.00042 m cross-sectional area). Thin stillage flowed through the annular space between the concentric rod and cylindrical housing. The rod interior was equipped with an electrical resistance heater and four thermocouples arranged to heat a portion of the rod and to measure the interior surface temperature. Thin stillage was pumped through the testing loop at a linear velocity of 5.2 m/sec and bulk temperature (Tb) was maintained at 40 ± 2°C throughout each test. When bulk temperature of thin stillage reached 40°C, power was supplied to the resistance heater within the rod; power was adjusted to an average initial temperature of 100°C within the rod, after which power was maintained constant (± 15 W). As material was deposited on the heated portion of the rod, the rate of heat transfer decreased due to the low thermal conductivity of the fouling deposit, and interior temperature (Ttc) increased. Each test was terminated when the interior rod temperature reached 200°C or when test time reached 8 hr, whichever occurred sooner. Fouling resistance, Rft (m·K/kW), was calculated from Ttc and heater power (Q) as testing progressed. Q was kept constant throughout each test. Rod surface temperature (Ts) was determined by: