Fractionating Column - Industrial Fractionating Columns

Industrial Fractionating Columns

Fractional distillation is one of the unit operations of chemical engineering. Fractionating columns are widely used in the chemical process industries where large quantities of liquids have to be distilled. Such industries are the petroleum processing, petrochemical production, natural gas processing, coal tar processing, brewing, liquified air separation, and hydrocarbon solvents production and similar industries but it finds its widest application in petroleum refineries. In such refineries, the crude oil feedstock is a complex, multicomponent mixture that must be separated, and yields of pure chemical compounds are not expected, only groups of compounds within a relatively small range of boiling points, also called fractions. That is the origin of the name fractional distillation or fractionation. It is often not worthwhile separating the components in these fractions any further based on product requirements and economics.

Distillation is one of the most common and energy-intensive separation processes. In a typical chemical plant, it accounts for about 40% of the total energy consumption. Industrial distillation is typically performed in large, vertical cylindrical columns (as shown in Figure 2) known as "distillation towers" or "distillation columns" with diameters ranging from about 65 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters or more.

Industrial distillation towers are usually operated at a continuous steady state. Unless disturbed by changes in feed, heat, ambient temperature, or condensing, the amount of feed being added normally equals the amount of product being removed.

It should also be noted that the amount of heat entering the column from the reboiler and with the feed must equal the amount heat removed by the overhead condenser and with the products. The heat entering a distillation column is a crucial operating parameter, addition of excess or insufficient heat to the column can lead to foaming, weeping, entrainment, or flooding.

Figure 3 depicts an industrial fractionating column separating a feed stream into one distillate fraction and one bottoms fraction. However, many industrial fractionating columns have outlets at intervals up the column so that multiple products having different boiling ranges may be withdrawn from a column distilling a multi-component feed stream. The "lightest" products with the lowest boiling points exit from the top of the columns and the "heaviest" products with the highest boiling points exit from the bottom.

Industrial fractionating columns use external reflux to achieve better separation of products. Reflux refers to the portion of the condensed overhead liquid product that returns to the upper part of the fractionating column as shown in Figure 3.

Inside the column, the downflowing reflux liquid provides cooling and condensation of upflowing vapors thereby increasing the efficacy of the distillation tower. The more reflux and/or more trays provided, the better is the tower's separation of lower boiling materials from higher boiling materials.

The design and operation of a fractionating column depends on the composition of the feed and as well as the composition of the desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele method or the Fenske equation can be used. For a multi-component feed, simulation models are used both for design, operation, and construction.

Bubble-cap "trays" or "plates" are one of the types of physical devices, which are used to provide good contact between the upflowing vapor and the downflowing liquid inside an industrial fractionating column. Such trays are shown in Figures 4 and 5.

The efficiency of a tray or plate is typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a fractionating column almost always needs more actual, physical plates than the required number of theoretical vapor-liquid equilibrium stages.

In industrial uses, sometimes a packing material is used in the column instead of trays, especially when low pressure drops across the column are required, as when operating under vacuum. This packing material can either be random dumped packing (1–3 in/2.5–7.6 cm wide) such as Raschig rings or structured sheet metal. Liquids tend to wet the surface of the packing, and the vapors pass across this wetted surface, where mass transfer takes place. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance.