The concept of unit operations was introduced by A. D. Little in 1905. The operations carried out in the process industries involving a physical change in the system under consideration are regarded as unit operations.
Chemical engineering is the branch of engineering that is concerned with the design and operation of industrial chemical plants. A chemical or process plant is required to carry out the transformation of raw materials into design products efficiently, economically, and safely.
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Broadly, unit operations are (i) Mechanical operations e.g. size reduction, conveying, filtration, etc. (ii) Fluid flow operations in which the pressure acts as a driving force, (iii) Heat transfer in which the temperature difference acts as a driving force, e.g. evaporation and (iv) mass transfer operations in which the concentration difference acts as a driving force, e.g. distillation.
Usually, a large number of unit operations of chemical engineering are directed toward separating a substance into component parts. For heterogeneous mixtures, such separations may be entirely mechanical, e.g. the separation of solid particles according to their size or the filtration of a solid from a suspension in a liquid.
Features of Unit Operations
1. These are physical in nature, i.e. changes involved in them are primarily physical.
2. These are common to all types of diver industries.
3. Individual operations have common techniques and are based on the same scientific principles regardless of the material being processed.
4. They are independent of the industries in which they are carried out.
5. Practical methods of carrying them out may be more or less different in different industries.
Hence, the concentration of solutions by evaporation is a unit operation that can be carried out in all types of diverse industries. This operation is basically similar to the handling of sugar or salt, or fruit juices with some differences in the most suitable arrangements.
They are basically used to conduct the physical steps of preparing the reactants, separating and purifying the products, recycling the unconverted reactants, and controlling the energy transfer in and out of the chemical reactor.
Theory of Unit Operations
The theory of unit operations is based on the fundamental laws of physical science such as the law of conservation of mass, the law of conservation of energy, Newton's laws of motion, the ideal gas law, Dalton's law of partial pressure, Newton's law of cooling, Raoult's law, etc.
Classification of Unit Operations
1. Fluid flow: It is concerned with the principles that determine the flow or transportation of any fluid from one point to another.
2. Mechanical operations: These involve size reduction of solids by crushing, grinding, pulverizing, etc., mixing, conveying, and mechanical separation such as decantation, filtration, settling and sedimentation, screening, flotation, etc.
3. Heat transfer: It deals with the principles that determine the accumulation and transfer of heat energy (thermal energy) from one place to another owing to the existence of a temperature difference. Heat transfer occurs in heating, cooling, phase change, evaporation, drying, distillation, etc. The modes/mechanisms by which heat transfer may occur are conduction, convection, and radiation.
4. Mass transfer: It is concerned with the transfer of mass from one phase to another distinct phase. Mass transfer operations depend on molecules diffusing or vaporizing from one district phase to another and are based on differences in vapor pressure, solubility, or diffusivity. Molecular diffusion and turbulent/eddy diffusion are the mechanisms of mass transfer. Mass transfer operations include separation technology like distillation, gas absorption, drying, extraction, crystallization, etc.
This text covers heat and mass transfer operations - a part portion of the unit operations of chemical engineering.
Unit Systems
The various systems of units and the basic/fundamental quantities associated with them are given below.
Symbolic abbreviations of the units are given in brackets.
The international system of units abbreviated as SI units has now been adopted throughout the world, in particular, in the field of science and technology.
Basic SI Units
Mass: Kilogram (kg)
Length: Meter (m)
Time: Second (s)
Temperature: Kelvin (K)
Mole: Kilogram mole (kmol)
Force: Newton (N)
Pressure: Newton/meter2
Energy: Newton・ meter (N.m)
Power: Newton ・meter/second (N.m/s)