What is meant by the thermal process?

Thermal processes are that set of processes that uses the thermal energy of a steady or a flowing fluid, to perform required work. The concepts of the thermal process are generally studied under thermodynamics and heat transfer in mechanical engineering. The subject of thermodynamics focuses on the methods to convert heat into work, or their interconversion. It also focuses on the methodology to reverse various spontaneous processes, like the flow of heat energy from a cold reservoir to a high-temperature reservoir. Thermodynamics deals with heat energy and its related energy conversion, whereas heat transfer in the context of mechanical engineering, deals with the rate of flow of heat and its various consequences.

Thermodynamics, and heat and mass transfer are the two major areas of a system executing a thermal process. Apart from mechanical, the concept of thermal process covers other broader areas in science and technology like thermal process engineering, chemical, biochemical, physics, and environmental engineering. The major issues taken care of by the thermal process are rectification, absorption, adsorption, chemical kinetics, membrane processes.

First law of thermodynamics and internal energy

Different types of thermal processes are mostly studied in the branch of thermal engineering. The area of thermal engineering mostly deals with the expansion of gas due to the transfer of heat energy to the gas, and effectively utilizing it in conversion to useful work. This is usually given by the first law of thermodynamics. The first law deals with the interconversion of heat and work. It signifies the energy conversion principle.

Concept of internal energy, U

Every system is composed of molecules and there exists associated energy with them. The internal energy of a thermodynamic system can be divided into two types, macroscopic and microscopic. The microscopic approach deals with energy possessed by a system at the molecular level, whereas the macroscopic approach deals with the bulk matter of the system. In thermodynamics, emphasis is majorly provided at the molecular level. Hence, the internal energy of a system due to energy possessed by molecules is given by:

$U=\mathrm{vibrational} \mathrm{energy}+\mathrm{translational} \mathrm{energy}+\mathrm{spin} \mathrm{energy}$

All these energies associated with the molecules account for the rise in the temperature of the system. Hence, according to thermodynamics, the internal energy is a function of temperature.

$U=f\left(T\right)$, where $T$ represents temperature.

The first law of thermodynamics introduces the concept of internal energy. The general equation of the first law of thermodynamics is,

$∆Q=∆W+∆U$,

where

 $∆Q$ represents the net heat interaction between a system and a surrounding

$∆W$ represents the net work interaction between the system and the surrounding, and

$∆U$ represents the change in internal energy of the system

or, $∆Q-∆W=∆U$

The equation signifies, the net change in heat and work interaction of a system is stored as the change in internal energy of a system. 

Different thermodynamic processes

Quasi-static process

When gas is confined inside a piston and cylinder arrangement, the expansion and contraction of the gas lead to the movement of the piston from its initial state to the final state. The initial and final state of the piston involves infinite piston positions. The state of the system and its associated properties at any intermediate position is not possible to analyze under such conditions. Hence, the process and position of the piston should be idealized to consist of finite steps with each step under equilibrium, it then permits the entire process to analyze and represent the thermodynamic process in a Pressure- volume diagram (PV diagram). Hence, the quasi-static process is a thermodynamic process that occurs infinitesimally slowly and represents each step in the thermodynamic process in equilibrium.

Constant pressure process

A thermodynamic system consisting of a gas confined in a cylinder is said to undergo a constant pressure process where, during the entire piston movement the pressure of the gas remains constant.

Isenthalpic process

A thermodynamic system is said to undergo an isenthalpic system if, during the process execution or movement of the piston, the enthalpy of the gas remains constant. 

Cyclic process

A thermodynamic process is said to be under a cyclic process when after the process termination, the system comes back to its initial position following a different path. 

Isothermal process

When during the piston movement, or process execution, the temperature of the system remains constant throughout, the system is said to have executed the process isothermally. 

Thermal process in chemical engineering

The basic fundamental methodology in chemical engineering is the thermal separation process of a mixture consisting of two or more components. The components may be in the form of a fluid or other materials. The heat and materials transfer process is extensively used to alter the chemical concentrations. 

The concept of the thermal process in chemical engineering leads to the development of chemical and oil processing plants. Some of the important processes where thermal process finds its application are:

Distillation and rectification

Distillation is a unit operation process, which is used for fractionating fluid mixtures. Different volatility of the materials in the mixture forms the basis of separation. Rectification is specially used to refine crude oil. It uses the methodology of distillation and is a multi-stage distillation process. 

Absorption and adsorption

Absorption is a technique of separating different gas components from a gas flow, A solvent is usually used to perform the separation. In the case of adsorption, individual components from a fluidic mixture are removed, these removed components generally occur bonded to a solid surface. 

Crystallization and membrane separation process

Crystallization is a cleaning process and is also used for the separation and shaping of different substances and materials. Membrane separation is used to remove smaller substances including dissolved ions and viruses that occur in the liquid. 

Context and Applications

Some of the key areas where thermal processes find their application are:

• Separation and purification of gaseous and liquid mixtures
• Cryogenic systems
• Water treatment
• Bio-refining
• Industrial production process
• Thermal processing and thermal imaging
• Multiphase heat transfer
• Applied thermodynamics and thermal engineering, etc.

This topic is widely used in various undergraduate and postgraduate degree courses of:

• Bachelors in Technology (Mechanical Engineering)
• Bachelors in Technology (Chemical Engineering)
• Masters in Technology (Thermal and Fluid engineering)
• Masters in Technology (Chemical and Thermal process)

Practice Problems

1. In which of the following processes does the pressure of the system remains constant?

1. Reversible process
2. Boltzmann proses
3. Convection process
4. Constant pressure process

Answer: Option d

Explanation: In the constant pressure process, the pressure of the system remains constant throughout.

2. Which of the following processes uses a solvent to initiate the separation process?

1. Absorption process
2. Transient process
3. Denaturation process
4. Computational and unit operation processes

Answer: Option a

Explanation: The absorption process uses a solvent to initiate the separation process.

3. Which law introduces the concept of internal energy?

1. Carnot law
2. First law
3. Second law
4. Zeroth law

Answer: Option b

Explanation: The first law introduces the concept of internal energy.

4. Which of the following is the correct equation of the first law?

1. $∆Q=∆W-∆U$
2. Q=nRT
3. $∆Q=∆W+∆U$
4. None of these

Answer: Option c

Explanation: The correct equation for the first law is $∆Q=∆W+∆U$

5. Which of the following process is used to remove dissolved ions and other materials that naturally occur in fluid substances?

1. Membrane separation
2. Distillation
3. Rectification
4. Heat transfer process by using heat exchangers

Answer: Option a

Explanation: The membrane separation process is used to remove dissolved ions and other materials that naturally occur in fluid substances.

Related Concepts

• Heat transfer and heat exchanger efficiency
• Carnot cycle
• Carnot engine
• Carnot cycle efficiency