This is the third paper on modelling and simulation of crossflow grain dryers. The first paper focused on the development of a Distributed Parameter Model, (DPM). The second reported on the effects of design modifications on the performance of the dryer. This paper presents an optimization technique to choose the best design and operating conditions to minimize capital and operating costs. Crossflow dryer design modifications including airflow reversal, temperature staging, and expanded bed thickness in the lower portion of the drying section have been simulated simultaneously using a DPM based simulation program. Two-level factorial designs were used to evaluate design modifications based on outlet grain moisture differential and energy consumption for drying. The design modifications were studied over a wide range of inlet air temperatures (121-149-degrees-C), airflow rates (976-1953 kg/h m2) and initial grain moisture contents (0.32-0.4 db) to ensure that the results are valid over a wide range of operating conditions. Expanding the bed width in the lower portion of the drying section resulted in a large improvement in energy efficiency. Airflow reversal greatly decreased the grain moisture differential at the dryer outlet. The factorial designs indicate that using these two design modifications in conjunction with one another will result in an overall decrease in grain moisture differential and an increase in drying efficiency. The optimal design was a crossflow dryer with a 0.3 m bed thickness expanding to a 0.6 m bed halfway through the drying section with an airflow reversal halfway down the drying section, operating at an airflow rate of 976 kg/h m2 and an inlet air temperature in the range 121-149-degrees-C.