IntroductionDistillation is defined as a process in which liquid or vapor mixture of two or more

substances is separated into its component fractions of desired purity, by the

application and removal of heat.

Distillation is based on the fact that the vapor of a boiling mixture will be

richer in the components that have lower boiling points, therefore when this vapor is

cooled and condensed, the condensate will contain more volatile components. At the

same time, the original mixture will contain more of the less volatile material.

There are many types of distillation column, each designed to perform specific

types of separation, and each differs in terms of complexity;

I. Batch column: in batch operation, the feed to the column is introduced

batch-wise. That is, the column is charge with a `batch` and then the

distillation process is carried out. When the desired task is achieved, a

next batch of feed is introduced.

II. Continuous column: in contrast, continuous columns process a

continuous feed stream. No interruptions occur unless there is a

problem with the column or surrounding process unit. They are

capable of handling high throughputs and are the most common of the

two types, trays column and packed column;

a. Binary column – feed contains only two components

b. Multi-component column – feed contains more than two

components

c. Tray column(internal) – where trays of various design are used

to hold up the liquid to provide better contract between vapor

and liquid

d. Packed column – where instead of trays `packings` are used to

enhance contact between vapor and liquid.

1

Distillation columns are made up of several components, each of which is

used either to transfer heat energy or enhance material transfer. A typical distillation

contains several major components:

I. A vertical shell where the separation of fluid components is carried

out

II. Column internals such as trays/plates or packings which are used to

enhance components separations

III. A reboiler to provide the necessary vaporization for the distillation

process

IV. A condenser to cool and condensed the vapor leaving the top of the

column

V. A reflux drums to hold the condensed vapor from the top of the

column so that liquid (reflux) can be recycle back to the column.

Operation and terminology; the liquid mixture that is to be processed is known

as the feed and this is introduced usually somewhere near the middle of the column to

a tray known as the feed tray. The feed tray divides the column into a top (enriching)

section and bottom (stripping) section. The feed flows down the column where it is

collected at the bottom in the reboiler. Heat is supplied to the reboiler to generate

vapor. The source of heat input can be any suitable fluid, although in most chemical

plants this is normally stream. In vapor raised in the reboiler is re-introduced into the

unit at the bottom of the column. The liquid removed from the reboiler is known as

the bottom product.

The vapor moves up the column, and as it exits the top of the unit, it is cooled

by a condenser. The condensed liquid is stored in a holding vessel known as the reflux

drum. Some of this liquid is recycled back to the top of the column and this is called

the reflux. The condensed liquid that is removed from the system is known as the

distillate or top product.

Thus, there are internal flows of vapor and liquid within the column as well as

external flows of feeds and product streams, into and out of the column.

2

Main Components of Distillation Column

Figure 1,

• A vertical shell where separation of liquid components is done.

• Column internals e.g.trays/plates and/or packings which are used to enhance

component separations.

• A reboiler to provide the necessary vaporization for the distillation process.

• A condenser to cool and condense the vapour leaving the top of the column.

• A reflux drum to hold the condensed vapour from the top of the column so

that liquid (reflux) can be recycled back to the column.

Figure 1

3

Types of Distillation Column1. Staged columns in distillation

Distillation may be carried out in staged or plate columns in which each plate provides intimate contact between vapor and liquid in continuous countercurrent flow. Each plate constitutes a single stage where there is simultaneous partial condensation of vapor and partial vaporization of liquid. (Figure 2)

Figure 2

A plate column is a vertical column which consist a number of plates. In this column, liquid enters from the top, flows over every plate and from plate to plate via down comers.

4

Various plates used for phase contacting are:

1.1 BUBBLE CAP PLATE Bubble cap disperses the gas phase into liquid as fine bubbles; it prevents liquid

drainage through the gas passage at low gas rates. (Figure 3)

Figure 3

bubble caps columns can be operated with a wide range of vapour and flow rates.

Bubble cap plates are now rarely used for installation because of (1) their relatively high cost. (2) High pressure drop.

1.2 SIEVE PLATEVery common plates in use in recent year are the perforated plates because of their

simplicity and of low cost. (Figure 4)

Figure 4

A sieve plate is perforated plate that employs perforation for dispersing a gas into a liquid on the plate.

It is metal sheet having hundreds of round perforations init of the ranging between 3 to 12mm.

The total area of holes range from 5 to 15% of the plate area.

5

1.3 VALVE PLATE It is the recent development in perforated plates. A valve plate is nothing but a

perforated metal sheet wherein the perforations are covered with lift able caps.(Figure 5)

Figure 5

The valves are metal discs up to about 38 mm diameter and are held in the plate by means of legs which restrict the upward motion of the caps.

The valve provides variable orifices for gas flow.

2. Packed columns in distillation

Distilling columns dumped with packing material is an alternative arrangement to plate columns. The packing material provides high interfacial area for the exchange of the components between the vapor and liquid phases. A true countercurrent flow of vapor and returning liquid (reflux) occurs in packed columns, in contrast to the stage-to-stage flow in plate columns.

Packed columns are used for smaller diameter columns since it is expensive to build a staged column that will operate properly in small diameters. Packed towers have the advantage of a smaller pressure drop and are, therefore, useful in vacuum fractionation. Another advantage of packed columns is that they can be used to process corrosive materials. (Figure 6 – Random kinds of Packing)

Figure 6

6

A packed column is a pressure vessel that has a packed section. The column can be filled with random dumped packing or structured packing sections, which are arranged or stacked.

In the column, liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place.

Packing material can be used instead of trays to improve separation in distillation columns.

Differently shaped packing materials have different surface areas and void space between the packing. Both of these factors affect packing performance.

Packing Objective

Objectives for maximizing efficiency

To spread surface area uniformly.

To promote uniform distribution of vapour & liquid throughout the bed

To maximize the void space per unit column volume.

To minimize friction (good Aerodynamic characteristics).

To ensure uniform resistance to vapour & liquid flow throughout the bed.

Some kinds of Packing used for phase contacting:

2.1 Pall Rings

Primarily made of 304SS and 316L SS metal alloys for quick replacement in kind from stock materials. 

Carbon Steel and specialty alloys, such as Monel 400 and Hastelloy C276, made are used for specific applications. (Figure 7)

Pall Rings are available in various sizes such as (mm) 16, 25, 38, and 50.

Figure 7

7

2.2 Saddle Rings

In terms of performance, i.e., low-pressure drop and high efficiency.

Large effective interfacial area,

High mechanical strength.

And lower cost due to less metal than previous generations of random packing.

Available in various sizes, which give different combinations of efficiency and pressure drop. (Figure 8)

Figure 8

2.3 FLEXIRING(Random Packing)

Good capacity and low pressure drop.

Higher liquid hold-up and residence time.

Versatile standard packing. Figure 9

Figure 9

8

Criteria of Choosing Sieve or Packed Column

Characteristics of Tray Columns:

Predictable hydraulic and mass transfer behavior

Moderate to high pressure drop per tray

Can be scaled to large diameters

Low cost

Suitable for fouling service

Feed point flexibility is easy

Characteristics of Packed Columns:

Low pressure drop / smaller diameter

Random packing scale-up for HETP is difficult; structured packing scale-up is predictable

HETP prediction less well developed than for trays

Low to moderate cost for random packing; high cost for structured packing

Not suitable for fouling service

Feed point flexibility is difficult

9