Important Unit Operations of Chemical Engineering

A. D. Little introduced the Unit Operations of Chemical Engineering concept in 1905.

The field of chemical engineering is the combination of two-part unit operations and unit processes.

The difference between unit operation and the unit process is unit operations involve only physical changes in materials handled in the system. In contrast, the unit process involves physical and chemical changes in materials handled by the system.

Unit Operations are the most important part of Chemical Engineering. Let’s understand this concept in detail.

What is Unit Operation?

The operations carried out in the chemical industries involving physical changes in the considered system are regarded as unit operations.

A physical change results in any property of matter and requires some driving force.

Features of Unit Operations

They are physical in nature.

They are common to all types of diverse industries.

Each operation has common techniques based on the same scientific principles regardless of the material being handled.

They are independent of the industries in which they are carried out.

Practical methods of carrying out unit operations may be different in different industries.

So, the concentration of solutions by evaporation is a unit operation that can be carried out in diverse industries. This operation is similar to the handling of sugar, salt, or fruit juices with some differences in the most suitable arrangement.

Unit operations are used to prepare the reactants, separate and purify the products, recycle the uncovered reactants, and control the energy transfer in and out of the chemical reactor.

A physical change in any property of matter occurs by the action of some driving force. The normal driving force in heat flow is the temperature difference.

Unit Operations of Chemical Engineering

Fluid Flow Operation

It is concerned with the principles that determine the flow or transportation of any fluid from one point or place to another.

A fluid is a substance or component capable of flowing if allowed to do so. Gas, liquid, and vapor have this capability, so they are called fluids.

The driving force for the fluid flow operation is Pressure Difference.

For More Details, Read Also: Fluid Flow Operation

Mechanical Operations

Mechanical Operation means mechanical power is used to perform these operations.

For example, Size reduction, Screening, and Mixing are mechanical operations.

Classification of Mechanical Operations

Size Reduction

Cutting

Crushing

Grinding

Classification

Screening

Froth Flotation

Magnetic Separation

Jigging

Tabling

Electrostatic Separation

Wet Classification

Solid-Fluid Separations

Filtration

Sedimentation

Centrifugal Separation

Gas-Solid Separations

Dust Collection

Bag Filtration

Electrostatic Precipitation

Solid Handling

Storage

Feeding

Conveying

Size Enlargement

Pelletization

Agglomeration

Granulation

Extrusion

Mechanical Separation

Many unit operations of chemical engineering are directed toward separating a substance into its parts.

These separation techniques apply to heterogeneous mixtures and are grouped into five headings as

Separation of Solids from Solids

Solids are separated in the dry state by Screening, Electrostatic Separation, and Magnetic Separation.

Separation of Solids from Solids in Liquids

Various methods used to separate solids from solids in liquids include Jigging, Flotation, Classification, and Tabling.

Separation of Solids from Liquids

Solids are separated from liquids by Filtration and Sedimentation.

Separation of Solids or Liquid drops from Gases

Solids are separated from gases by Gravity Settling, Centrifugal Separation/Settling Filtration and Electrostatic Precipitation.

Separation of Liquids from Liquids.

Immiscible liquids are separated by a technique called Decantation using either a Gravity Decanter or a Centrifugal Decanter.

Mixing and Agitation

Mixing is a process in which at least two separate materials (which may be present in the same or different phases) are taken and forced to be randomly distributed through one another by some mechanical means.

If materials present for mixing are in the same phase is called Homogeneous mixing.

If materials are in different phases is called Heterogeneous mixing.

Mixing and Agitation are not the same but different from each other.

Agitation means the induced motion of material in a circulatory pattern inside a tank or vessel. At the same time, mixing is the random distribution into and through one another of two or more initially separate phases.

Heat Transfer Operations

Heat transfer is the operation where heat/thermal energy is transferred from one place to another place of the body or one body to another body due to temperature differences.

The temperature difference is the driving force of the heat transfer.
Heat transfer can occur through three modes of heat transfer Conduction, Convection, and Radiation.

Classification

Phase Change Operations

Evaporation

Condensation

No-Phase Change Operations

Heating

Cooling

Operations in which Heat and Mass Transfer are both associated

Distillation

Crystallization

Sublimation

Evaporation

Drying

Condensation

For more details, Read Also: Fundamentals of Heat Transfer.

Mass Transfer Operations

Mass transfer is concerned with the transfer of mass from one phase to another different phase.

It depends on the molecules diffusing or vaporizing from one phase to another and is based on differences in vapor pressure, solubility, or diffusivity.

Concentration difference is the driving force for mass transfer.

Classification

Liquid – Gas (Vapour)

Distillation

Gas Absorption

Stripping

Humidification

Dehumidification

Liquid – Solid

Crystallization

Leaching (Solid Extraction)

Adsorption

Liquid – Liquid

Extraction

Solid – Gas (Vapour)

Adsorption

Drying

Sublimation

References

“Unit Operations of Chemical Engineering”, McCabe W L, Smith J C, Harriott P, Mc Graw Hill Publication, 7^th edition 2005.

“Process Heat Transfer”, D. Q. Kern, McGraw Hill.

“Chemical Engineers Handbook”, Perry R.H. & Chilton C.H., 7^th ed. McGraw hill.

“Mass Transfer Operations” by R. E. Treybal, Mc-Graw Hill International, 3^rd edition

FAQs

What are unit operations in chemical engineering?

Unit operations involve physical changes in a chemical system, such as separating and purifying products, preparing reactants, recycling uncovered reactants, and controlling energy transfer in and out of a chemical reactor.

What are the features of unit operations?

Unit operations are physical in nature, common to all types of industries, independent of the industries in which they are carried out, and have common techniques based on the same scientific principles. Practical methods of carrying out unit operations may be different in different industries.

What are the different types of mechanical operations in chemical engineering?

Mechanical operations in chemical engineering include size reduction, screening, mixing, solid-fluid separations, gas-solid separations, and solid handling.

What are the different types of mechanical separation in chemical engineering?

Mechanical separation techniques in chemical engineering include the separation of solids from solids, the separation of solids from solids in liquids, the separation of solids from liquids, the separation of solids or liquid drops from gases, and the separation of immiscible liquids.

What is the difference between mixing and agitation in chemical engineering?

Mixing refers to the random distribution into and through one another of two or more initially separate phases, while agitation means the induced motion of material in a circulatory pattern inside a tank or vessel.

What is heat transfer in chemical engineering?

Heat transfer is the transfer of thermal energy from one place to another due to a temperature difference. It can occur through three modes: conduction, convection, and radiation.

What are the different types of heat transfer operations in chemical engineering?

Heat transfer operations in chemical engineering can be classified into phase change operations (evaporation, condensation) and no-phase change operations (heating, cooling). Some operations involve both heat and mass transfer, such as distillation, crystallization, sublimation, evaporation, drying, and condensation.

What is mass transfer in chemical engineering?

Mass transfer is the movement of molecules or atoms from one place to another due to a concentration gradient. It is often associated with chemical reactions or changes in the phase of a substance.

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