Fractional Distillation – Working of Fractional Distillation unit.

Fractional distillation differs from simple distillation because in simple distillation vapor passes through a condenser and condensate collected into the receiver while in fractional distillation vapor passes through fractionating columns where partial condensation of vapors occurs and a part of condensing vapor goes to the still.

Rectification is the process of separation between components of the mixture containing miscible volatile liquids having different but close boiling points.

Construction of Fractional Distillation:

• On Small Scale, the apparatus consists of

1. Still in which liquid is boiled.
2. Fractionating column: It is inserted between the still and condenser.
3. Condenser: It is used to condense vapors. Some part of condensate goes back to the column which is called reflux and the rest is obtained as the product. The ratio of reflux to product is called the reflux ratio.

• On large scale, provision is made to supply heat at the bottom of the column. At the top of the column, a condenser is attached. The fractionating column has a large area for providing sufficient flow conditions. The top of the column is cooler than the bottom so that the liquid stream becomes progressively hotter as it descends and the vapor stream becomes progressively cooler as it rises.

• The efficiency of a fractionating column is measured by several theoretical plates. Smaller the boiling point difference, the greater the number of theoretical plates for efficient separation. The heat input still should also be controlled otherwise packing will remain wet if it is low.

A. Plate column:

1. Bubble cap column:

• The bubble cap column is widely used in large distillation plants. The column is divided into sections using a series of horizontal plates. Each plate carries several caps that are secured by a bolt with the plate. The edge of the cap is serrated or the sides may be slotted. Vapor rises from plate A. This vapor passes through the liquid on plate B and is partially condensed. The process of condensation and vaporization continues at plate C.

2. Sieve plate columns

• All the constructions are the same as bubble cap columns. Instead of bubble cap plates, flat plates with a large number of relatively small perforations, drilled in them are used. These perforations are usually 3/16 to 4 inches in diameter. The pressure and velocity of the vapor rising in the column through these holes and prevent downflow. Nowadays Turbo-grid plates are used which consist of several horizontal bars with narrow slots between them.

B. Packed column

• The column is filled with some sorts of material that offers a large surface area supposedly wetted by the liquid. Raschig rings are popular. A Raschig ring is a hollow cylinder whose length is equal to its diameter. This may be made of metal, stoneware, ceramics, carbon, plastics, or other materials. Packed columns can be used for corrosive materials. Berl Saddles are also used widely. Lessing Ring is modified Raschig rings

Advantages of Fractional Distillation:

1. Highly effective and efficient
2. Fractional distillation is easy to use. Modern fractional distillation columns have been equipped with microprocessors that fully control the distillation process from beginning to end. Automation minimizes the time needed to operate the equipment.

Disadvantages of Fractional Distillation:

1. High capital cost
2. Not flexible
3. It is not economical for small-volume productions
4. Not easy to sterilize
5. Not preferable for processing of temperature-sensitive products

Uses:

• Fractional distillation is a method used in separating volatile liquids. Fractional distillation has been used traditionally in the separation of mixtures and purification of solvents, chemicals, and other naturally occurring material. Fractional distillation has found widespread use in oil refineries.

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