(335y) Microfiltration of Thin Stillage: Process Design and Economic Analysis

The corn based dry grind process is the most widely used method in the US for fuel ethanol production. Within the dry grind corn process, approximately 6 L thin stillage (TS) is produced concomitantly with 1 L ethanol. TS contain 5 to 10% solids and they are concentrated using evaporators. TS concentration requires evaporation of large amounts of water as well as evaporator maintenance. Evaporator maintenance requires excess evaporator capacity at the facility, increasing capital expenses, requiring plant slow downs and/or shut downs. Membrane technology may provide an efficient means for dewatering and reducing fouling of evaporators by solids removal. In plant scale operations, multistage membrane systems have been adopted for cost minimization. We considered application of dynamic optimization to the optimal design and operation of a continuous microfiltration (MF) system for corn dry grind process. The objectives of our study were to (1) develop a process model to design a multistage microfiltration system, (2) optimize area requirement and number of stages for multistage system and (3) economic analysis of multistage MF system for a 40 million gal/yr ethanol plant. Total area requirement decreased with number of stages but there was tradeoff between higher capital costs involved at higher number of stages. For target CF=15, 5 stage membrane system was found to be optimum with area requirement of 655 m2 for minimum cost. Increase in the input stream flow rate from 405,000 to 760,000 gal/day significantly increased the total capital cost of the system by 47%. Comparison of single stage with optimal system showed 50% reduction in operating cost. Optimal system also showed potential to process more than twice the amount of input flow rate compared to 4 effect evaporator system for given conditions.