2. Unit Process and Unit Operation
Topics Covered:
1. Unit Process
2. Unit Operation
3. Analysis of Manufacturing Processes:
3. Unit Process
Definition:
Unit processes involves principal chemical conversions leading to synthesis of various useful products and provide basic
information regarding reaction temperature and pressure, extent of chemical conversions, yield of products, nature of reaction
whether it is endothermic or exothermic and type of catalyst used.
It is a process in which chemical change occurs.
Examples: oxidation, reduction, calcination, etc.
Unit Processes in Chemical Industry:
❑ Combustion
❑ Oxidation
❑ Neutralization
❑ Silicate Formation
❑ Causticization
❑ Electrodialysis
❑ Double Decomposition
❑ Calcination and Dehydration
❑ Nitration
❑ Esterification
❑ Ammonolysis
❑ Reduction
❑ Halogenation
❑ Sulfonation
❑ Hydrolysis and Hydration
❑ Hydrogenation and Hydrogenolysis
❑ Alkylation
❑ Condensation
❑ Polymerization
❑ Diazotization and Coupling
❑ Fermentation
❑ Pyrolysis and Cracking
❑ Aromatization
❑ Isomerization
❑ Hydroformylation
❑ Ion Exchange
4. Unit Operation
Definition:
Unit operations involve the physical separation of products obtained during various unit processes. Here no chemical change is
there.
e.g. Screening, Distillation, Evaporation, Decantation, etc.
Types of Unit Operations in Chemical Engineering:
❖ Mass Transfer Operations:
It includes the operations in which mass is transferred by using chemical potential difference as driving force. E.g. gas absorption,
extraction etc.
❖ Fluid Flow Operations/ Momentum transfer Operations:
It includes the operations which involve fluid transportation, filtration and solid fluidization.
❖ Heat transfer Operations:
It includes the operations in which temperature difference is main driving force. E.g. Heat Exchange, Evaporation etc.
❖ Mechanical Operations:
It includes the operations in which external mechanical force is used as a driving force. E.g. Crushing, Screening etc.
❖ Thermodynamic Operations :
It includes the operations in which energy is transformed from one place to another so that we get desired output E.g.
Refrigeration, Liquefaction etc.
There are some operations in which two operations take place simultaneously. E.g. Distillation, Humidification etc.
5. Analysis of Manufacturing Processes:
1. Ammonia Manufacturing using Haber’s Process:
Steps Unit Process/Unit Operation Reason
Purification of input stream
(Feed Guard Reactor)
Unit Process In this step, CO and CO2 are converted
into CH4 and removal of traces of H2S,
H2O,P and Arsenic is done.
Main Reaction Unit Process Here catalytic hydrogenation of nitrogen
takes place
Cooling of product stream (condenser) Unit Operation Here, ammonia from vapor mixture is
converted into liquid ammonia i.e. only
physical state of ammonia is changed.
Separation of liquid ammonia
(Separator)
Unit Operation Here, liquid ammonia is separated from
vapor mixture. No chemical reaction is
there.
Refrigeration of remaining ammonia
(refrigerator)
Unit Operation Here, remaining ammonia is get
condensed. Again, only physical state of
ammonia is changed.
6. 2. Nitric Acid Manufacturing using Ostwald’s Process:
Steps Unit Process/Unit Operation Reason
Oxidation of Ammonia
(Platinum – Rhodium catalyst is used.)
Unit Process In this step ammonia reacts with oxygen
to produce nitric oxide (NO).
Cooling of mixture Unit Operation Here, The temperature of mixture is
reduced.
Absorption in water.
(Excess air is added to gaseous stream
before entering the absorption column)
Unit Process First of all, NO is get converted into NO2
due to the addition of excess air. Then NO2
gas is absorbed into water to get Nitric
acid.
❑ Many times, Unit operation and unit processes are integrated. Gas absorption is a unit operation because ,gas is
absorbed over the liquid and does not react with it. But if absorbed gas reacts with that liquid, then we cannot say that,
it is a unit operation. In this case a unit process is done by using the technique of unit operation. In Ostwald’s process
NO2 is absorbed in the water, but it react with water to form Nitric acid. Hence we have considered that as a unit
process.
❑ Sometimes side reactions take place while performing a unit operation. Those reactions do not alter the classification of
that particular step. Because the extent of such reactions is very low. In Ostwald’s process, output mixture of catalytic
reactor is cooled using direct contact with water, during this process small amount of nitrous acid is formed which is
removed in that step only.
