Classification Equipment and Design of Distillation Columns

Distillation is the separation process most used in the chemical and petrochemical industry; as shown in Table 1, its operation is classified into several forms.

Most of the time, distillation is carried out in vertical columns or towers (like the packed column shown in Figure 2) where the liquid descends while the vapour ascends to the top of the column. The vapour left at the top of the tower is condensed and at least a fraction is returned back to the top of the tower as liquid reflux. Part of the liquid leaving the bottom of the column is vaporized in a reboiler and returned to the column as boil-up.

How the distillation equipment operates and how it is calculated has been modified over the years. Figure 3 shows some developments related to distillation.

There are three types of internal devices which provide the intimate contact between phases in a distillation column. These are tray, random packing and structured packing.

Table 1 Different classifications of distillation equipment or operation

Amount of compound to be separated Mode of operation

Mixing between phases Internal device used

System characteristics

Operating pressure

Solute recovery Fractionation Steady-state Unsteady-state Batch Semibatch Start and shut-down Stagewise contact Continuous contact None Plates Bubble cups Sieve Valve Packing Random Structured Flash

Fractionation Azeotropic Extractive Low High vacuum Medium vacuum Low vacuum Medium High

Bottom product Figure 2 Distillation column equipped with structured packing.

Each has advantages and disadvantages. Trays have been used for many years. Random packings have also been used over three generations of design. Structured packings have replaced trays, especially in applications at low and atmospheric pressures. At high pressure, trays perform better than packings.

For any applcation one must determine whether tray or packing is the most appropriate. The following factors are an indication of when trays or packings are favoured.

Factors favouring trays

• High liquid rate (this occurs when high column pressures are involved);

• large diameter (packing prone to maldistribution);

• complex columns with multiple feed/take-offs;

• feed composition variation;

• columns equipped with tray weigh less than those equipped with some packings.

Factors favouring packings

• vacuum conditions;

• low pressure drop required;

• in smaller diameter columns (where trays are more difficult to install, diameters 0.6-0.9 m or less);

• corrosive system (more construction materials available);

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