Rectifying or Refining Column
The rectifying column, sometimes called the refining column, is used in the second stage of separation, which also has a stack of sieve trays to boost the separation of ethanol and water. Alcohol from the top of the beer column, now in vapor form, is fed into the bottom of the rectifier column, providing the heat source for the rectifier column. The alcohol continues to rise and more water is removed, increasing to approximately 90% alcohol out of the top of the rectifier column. After the alcohol vapors leave the top of the rectifier column they enter the condenser, where the vapors are condensed. Two-thirds of the 90% alcohol produced here is returned to the top of the rectifier column as 90% reflux. The remaining third of the alcohol is sent to the azeotropic ethanol-water mixture (89.4 mol% ethanol and 10.6 mol% water) storage tank. Temperature at the top of the rectifier column is around 68-71 °C, whereas in the bottom of the column it is about 85-88°C. The azeotropic mixture is then dried to make nearly anhydrous fuel grade ethanol (99-99.8 wt%) in the next stage, and this process is discussed in Chapter 15.