Distillation is a method of separating chemical compounds based on differences in their volatilities

1
Control of Distillation Column

• Distillation is a method of separating chemical
  compounds based on differences in their
  volatilities (boiling points).
• Distillation is used in various process
  industries
• Crude oil separation into various fractions
• Distillation of water to remove impurities, such
  as sea salt
• Distillation of air, obtaining nitrogen, oxygen,
  hellium
• Distillation of fermented solutions to provide
  alcoholic beverages with a higher alcohol
  content.

2
Control of Distillation Column
Feed contains mixture of 2 or more compounds,
which have different boiling points. This mixture
enters a distillation column. Mixture is heated
by means of the re-boiler, which is highlighted
in red in the figure below. This process section
operates by taking a fraction of the liquid
mixture at the bottom of the distillation column
and heating it up by using steam (through the
heat exchanger). As a result of heat input,
temperature of the mixture rises and those
compounds in the mixture that are more volatile
(i.e. have lower boiling point) than others start
to evaporate. Vapour travels upwards through the
distillation column. It eventually exits
distillation column and enters heat exchanger
that condenses vapour, with the resulting
condensate entering the so-called reflux drum.
Heat exchanger that is used to condensate vapour
is highlighted in blue in the figure below.
Reflux drum is highlighted in green. One
fraction of liquid inside the reflux drum is
recycled back into distillation column. Other
fraction is extracted from a process. Liquid that
is extracted at this end of the process (product
1) contains mainly those compounds that have
lower boiling points (higher volatility).
This reflux is used to achieve a more complete
separation of products. Inside the distillation
column reflux liquid provides cooling and
condensation of the upflowing vapours, thereby
increasing the efficacy of the distillation
column. In general, this reflux results in better
separation of high-volatility and low-volatility
components of the feed mixture. Liquid at the
bottom of the distillation column is
predominantly composed of those components that
have higher boiling point. One portion of this
liquid in recycled back into the distillation
column through the re-boiler (as explained
above). Other portion of this liquid is extracted
from a process (product 2).
CW IN
CW OUT
Product 1
DISTILLATION COLUMN
Feed
St IN
St OUT
Product 2
3
Control of Distillation Column
CW IN
CW OUT
LI1
V1
XI3
FI3
FI5
PI1
V3
V5
FI1
TI1
XI1
Product 1
TI3
Feed
TI2
FI4
V4
LI2
St IN
St OUT
Manipulated Variables V1, V2, V3, V4, V5
XI2
Controlled Variables PI1, FI2, FI3, FI4, FI5,
LI1, LI2, TI2, TI3, XI2, XI3
V2
Feedforward Variables FI1, TI1, XI1
FI2
Product 2
4
Control of Distillation Column
First step is to close all of the flow loops
(FI2, FI3, FI4, FI5). Also, we can control
pressure inside the distillation column by
manipulating flowrate of cooling water using
valve V1
CW IN
CW OUT
LI1
V1
XI3
FC3
FI3
PC1
FI5
FC5
PI1
V3
V5
FI1
TI1
XI1
Product 1
TI3
Feed
FC4
TI2
FI4
V4
LI2
St IN
St OUT
Manipulated Variables PC1, FC2, FC3, FC4, FC5
XI2
V2
Controlled Variables LI1, LI2, TI2, TI3,
XI2, XI3
FC2
Feedforward Variables FI1, TI1, XI1
FI2
Product 2
5
Control of Distillation Column
Next step could be to close primary loops of the
cascade controls.
CW IN
CW OUT
LI1
V1
XI3
FC3
FI3
PC1
FI5
FC5
PI1
V3
V5
FI1
TI1
XI1
Product 1
TI3
Manipulated Variables PC1, LC2, FC3, FC4, FC5
Controlled Variables LI1, TI2, TI3, XI2, XI3
Feed
FC4
TI2
FI4
Feedforward Variables FI1, TI1, XI1
V4
One approach could be to close most of the
potential primary loops such as temperature
control loops for TI2 and TI3 using FC4 and FC5
as manipulated variables, respectively. Also, LI1
could be controlled by manipulating FC3.
However, there is a strong interaction between
FC5 and TI2 as well as between FC4 and TI3.
Furthermore, both FC3 and FC5 have a strong
impact on XI3 and LI1. Hence, it is recommended
that multivariable control (e.g. MPC) be used to
control remaining controlled variables.
LI2
St IN
St OUT
XI2
V2
LC2
FC2
FI2
Product 2
6
MPC Control of Distillation Column (Block
Diagram Form)
XI1
TI1
FI1
LI1_SP
Distillation Column
Model Predictive Controller
TI2_SP
PC1_SP
LI1
TI3_SP
LC2_SP
TI2
XI2_SP
XI3_SP
FC3_SP
TI3
FC4_SP
XI2
FC5_SP
XI3