Methanol is useful in the transesterification step during production of biodiesel. When the methanol reacts with vegetable or animal fats it produces a usable fuel. Methanol can become saturated with water in this process. To keep costs down and lower waste, it is important to dehydrate the methanol in order to produce reusable methanol. After frequently passing the mixture through a methanol dehydration step, it can be reused.
Dehydration of methanol is performed by molecular sieve dehydration units (MSDUs). The wet methanol passes through a vessel filled with molecular sieve beads. There are small pores on the surface of each bead. The pore sizes (usually 3A) are small enough to allow water (2.6A) to pass through but not methanol (~4.5A). The sieves absorb the water and allow the methanol to pass through the bed.