Continuous distillation, a form of distillation, is an ongoing separation in which a mixture is continuously fed into the process and separated fractions are removed continuously as output streams. Distillation is the separation or partial separation of a liquid feed mixture into components or fractions by selective boiling, the process produces at least two output fractions. Batch distillation is used when smaller quantities are distilled. In a continuous distillation, each of the streams is taken simultaneously throughout operation, therefore. In practice when there are multiple distillate fractions, the exit points are located at different heights on a fractionating column. The bottoms fraction can be taken from the bottom of the column or unit. Each fraction may contain one or more components, when distilling crude oil or a similar feedstock, each fraction contains many components of similar volatility and other properties. Although it is possible to run a small-scale or laboratory continuous distillation, distillation is one of the unit operations of chemical engineering. Continuous distillation is used widely in the process industries where large quantities of liquids have to be distilled. These fractions are the origin of the fractional distillation or fractionation. It is often not worthwhile separating the components in these fractions any further based on product requirements, if this vapor is then separated and condensed into a liquid, it becomes richer in the lower boiling point component of the original mixture. This is what happens in a distillation column. A mixture is heated up, and routed into the distillation column, on entering the column, the feed starts flowing down but part of it, the component with lower boiling point, vaporizes and rises. However, as it rises, it cools and while part of it continues up as vapor, image 3 depicts a simple continuous fractional distillation tower for separating a feed stream into two fractions, an overhead distillate product and a bottoms product. The lightest products exit from the top of the columns and the heaviest products exit from the bottom of the column, the overhead stream may be cooled and condensed using a water-cooled or air-cooled condenser. The bottoms reboiler may be a steam-heated or hot oil-heated heat exchanger, in a continuous distillation, the system is kept in a steady state or approximate steady state. Steady state means that quantities related to the process do not change as time passes during operation, such constant quantities include feed input rate, output stream rates, heating and cooling rates, reflux ratio, and temperatures, pressures, and compositions at every point. Unless the process is disturbed due to changes in feed, heating, ambient temperature, or condensing, even when a variation in conditions occurs, modern process control methods are commonly able to gradually return the continuous process to another steady state again
Image 1: Typical industrial distillation towers
Image 2: A crude oil vacuum distillation column as used in oil refineries
Image 3: Chemical engineering schematic of Continuous Binary Fractional Distillation tower. A binary distillation separates a feed mixture stream into two fractions: one distillate and one bottoms fractions.
Image 4: Simplified chemical engineering schematic of Continuous Fractional Distillation tower separating one feed mixture stream into four distillate and one bottoms fractions