Fractional distillation is the most common form of separation technology used in solvent recovery plants. In most cases, the distillation is operated in a batch state. New feed is charged to the distillation column in a batch and products are removed in batches. Unless the process is disturbed due to changes in feed, heat, ambient temperature, or condensing, the amount of feed being added and the amount of product being removed are normally equal. This is known as a batch fractional distillation system.
Industrial distillation is typically performed in large, vertical cylindrical columns known as “distillation or fractionation towers” or “distillation columns” .The distillation towers have liquid outlets at intervals up the column which allow for the withdrawal of different fractions or products having different boiling points or boiling ranges. By increasing the temperature of the product inside the columns, the different chemicals are separated. The “lightest” products (those with the lowest boiling point) exit from the top of the columns and the “heaviest” products (those with the highest boiling point) exit from the bottom of the column.
Large-scale industrial towers use reflux to achieve a more complete separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower of a typical, large-scale industrial distillation tower. Inside the tower, the reflux liquid flowing downwards provides the cooling needed to condense the vapors flowing upwards, thereby increasing the effectiveness of the distillation tower. The more reflux is provided for a given number of theoretical plates, the better the tower’s separation of lower boiling materials from higher boiling materials. Alternatively, the more reflux provided for a given desired separation, the fewer theoretical plates are required.
In solvent recovery plants fractional distillation is used to separate solvents from each other and from non-volatile material. Distillation towers designed for recovery systems are mostly in batch unsteady state and have more stages per height to prevent reflux changes. Normally a distillation tower is designed under typical conditions and then expanded to other processes. Fractional distillation can be used to separate water form many solvents yet some solvents form low boiling azeotropes and cannot be completely dehydrated. Azeotropes are not limited to solvent-water mixtures which makes it the main limitation of distillation as many waste materials contain solvent-solvent and solvent-water azeotropes that may need an alternative process to reach desired results.