Chemical Engineering Thermodynamics is the study of energy, heat, and work interactions in chemical processes. It primarily focuses on understanding energy transfer within a system and between the system and its surroundings.
Thermodynamics plays a critical role in designing, analyzing, and optimizing chemical processes. It offers insights into energy changes during reactions, phase transitions, and mixing processes. Understanding thermodynamics is fundamental. It is essential for predicting the behavior of fluids, solids, and gases. Therefore, it is an essential subject in chemical engineering.
This discipline combines classical thermodynamic principles with the specific requirements of chemical processes. This integration allows engineers to efficiently design and operate reactors, distillation columns, heat exchangers, and other critical equipment.
The term “Thermodynamics” originates from the Greek words therme (heat) and dynamics (power). Thermodynamics deals with energy transformations and describes the direction of natural changes.
Thermodynamics also establishes conditions under which systems attain equilibrium, a state where no further changes occur. The laws of thermodynamics explain processes where material properties change due to energy transfer, helping engineers design efficient systems.
Basic Definitions in Chemical Engineering Thermodynamics
System
A system is a substance or group of substances selected for analysis.
Examples include reaction vessels, distillation columns, and heat engines.
Process
A process refers to changes occurring in a system.
For instance, the combustion of hydrocarbon and oxygen forms water and carbon dioxide, constituting a process.
Surroundings
The surroundings comprise everything outside the system, separated by boundaries. These boundaries can be physical or imaginary, rigid or movable.
For example, in a shell and tube heat exchanger, steam is the system. The cooling water, which absorbs latent heat, represents the surroundings.
Types of Thermodynamic Systems
Open System
An open system exchanges both energy and mass with its surroundings. Example: a tubular flow reactor.
Cyclical processes like power and refrigeration cycles are closed when considered as a whole. Each component of the cycle, such as the compressor, pump, and heat exchanger, is open.
Closed System
A closed system exchanges energy but not mass with its surroundings.
Examples: a closed steel container of hot water or a batch reactor.
Isolated System
A system that neither exchange energy nor mass with surroundings is called isolated system.
The system is unaffected by the changes in the surroundings.
An example of an isolated system is thermoflask.
Homogeneous System
Properties of the homogeneous system are the same throughout the system or vary smooth without showing any surface of discontinuity. A homogeneous system is also called a phase.
Examples of the homogeneous system are liquid water in a beaker and a column of dust-free air.
Heterogeneous System
A system consists of two or more distinct homogeneous phases separated by phase boundaries called a heterogeneous system.
Sudden changes in properties occur at phase boundaries.
An example of a heterogeneous system is a mixture of water and toluene.
Define State and Properties of System
State
Specifications like pressure, volume, and temperature define the state of a system.
Properties
System properties are classified as:
- Extensive Properties: Depend on the system’s mass (e.g., volume).
- Intensive Properties: Independent of the system’s mass (e.g., temperature, pressure).
Some intensive properties are derived from the extensive properties by specifying the unit amount of substance concerned.
Degrees of Freedom
The minimum number of variables required to define a system’s state.
Some intensive properties are derived from the extensive properties by specifying the unit amount of substance concerned.
State Functions
A state function describes a system’s current state, not its history. It depends only on the system’s state, not the path taken to reach it. The difference in a state function between two states depends only on these states, not the process connecting them.
Change in property value from initial state to final state
They are exact differentials.
Change in state function/ property is zero for a cyclic process.
Path Functions
A path function depends on the route taken between the initial and final states of a system.
Examples of path functions includes heat and work.
The path is the series of equilibrium states the system passes through during the change. Path functions are expressed as inexact differentials.
Energy Concepts
Work
Energy is used as work when a force acts over a distance (dW):
Where 𝑊 is the work done, 𝐹 is the force acting, and 𝑍 is the displacement.
The unit of work in the SI system is N.m (Newton meter) or J (joule).
For expansion or compression work in a cylinder, the gas is confined in a cylinder. It has pressure (P) and volume (V). If the piston has an area (A), the force (F) acting on the piston is given by:
The displacement of the piston in the direction of the force 𝑑Z is related to the change in volume 𝑑𝑉 of the gas as
Thus we get,
If the volume of the gas changes from the initial value 𝑉1 to the final value 𝑉2. You can integrate the above equation. This will calculate the work done on the face of the piston.
Heat
Heat is the quantity transferred between one body to another. This transfer occurs due to a difference in temperatures between them.
Heat can not be stored within the system. Heat can be absorbed only during the change of state of the system. It can evolve during this change as well. Heat is energy in transit, like work. Heat is a path function depends on how the process is carried out.
Heat is expressed in joules, calorie, BTU.
Energy Forms
Energy is the quantity that can be stored in the system, can be exchanged between the system and surroundings.
The exchange of energy occurs either as heat or work (heat and work can’t be stored in the system).
Potential Energy (PE)
Energy the system possesses due to its position above some arbitrary reference plane.
If mass (m) is at an elevation (z) above the ground,
Where, g is the acceleration due to gravity.
Kinetic Energy (KE)
Energy possessed by the system by virtue of its motion.
If mass (m) moving at a velocity (u),
Kinetic Energy (KE) and Potential Energy (PE) are not thermodynamic properties of the system. They don’t change with a change in temperature or pressure of the system.
Internal Energy (IE)
Internal Energy is a definite property of the system, denoted by U. It is the state function. Kinetic Energy and Potential Energy are not included in Internal Energy U.
A system under a given set of conditions has a definite internal energy.
In a cyclic process, the internal energy of the system does not change. It remains the same as it was before all the changes.
For a non-cyclic process, some energy gets stored in the system or some energy gets removed from the system. These changes in stored energy are measured as the change in the internal energy of the system.
The absolute values of internal energy are not known. Change in internal energy is needed in the thermodynamic analysis.
Enthalpy
For a system kept at constant volume, no work of expansion or any other kind is done. In such a case, the change in internal energy equals the heat supplied to it. This is because the work of expansion is zero for a constant volume process.
When the system expands against a constant external pressure, the change in internal energy is different. It is no longer equal to the energy supplied as heat.
The system is free to expand against constant external pressure. In this situation, the change in internal energy does not equal the energy supplied as heat. This difference occurs because some energy is used to do work on the surroundings.
A part of the energy supplied is utilized by the system to occupy a new volume. The utilized energy equals the work needed to push the surroundings against constant pressure. The heat supplied at constant pressure can be measured. This measurement is the change in another thermodynamic property of the system known as enthalpy.
Enthalpy is denoted by H.
Where, 𝑈 is the internal energy of the system. 𝑃 is the absolute pressure. 𝑉 is the volume of the system. Since 𝑈,𝑃, and 𝑉 are all state functions, a combination of them is 𝐻. Which is also a state function.
Enthalpy is treated as ‘total energy.’ It includes both the intrinsic energy it possesses (U). It also includes the energy due to the expansion possibilities of the system.
In differential form,
Noting that 𝑑𝑊 = 𝑃𝑑𝑉 for a reversible non-flow process and 𝑉𝑑𝑃 = 0 for a constant pressure process,
Power
The time rate of doing work is called Power. Unit is (J/s) or watt ((W).
Equilibrium State
Properties (on a macroscopic scale) are uniform throughout the system and do not vary with time.
Thermal equilibrium – No heat exchange occurs between various points of the system. The temperature is uniform throughout the system.
Mechanical equilibrium – Pressure is uniform throughout the system.
Thermodynamic equilibrium – There is no heat and work exchange. There is no mass transfer between phases. There is no diffusion of mass within the phase. No chemical reaction occurs between constituents.
A state of equilibrium implies a state of rest. All forces are in exact balance.
State of equilibrium will be retained by the system after any small, but short mechanical disturbance in external conditions.
Steady State
A system interacting with the surroundings has attained a steady state condition. This occurs when the properties at a specified location in the system do not vary with time.
Consider the walls of a furnace. The inside surface is exposed to hot combustion gases. The outside surface is exposed to atmospheric air. Heat transfer occurs from inside to outside.
At a steady-state, the temperature at a specified point in the wall will be constant. However, the temperature at different points would be different.
The system at steady state exchanges mass, heat, or work with surroundings, even when exhibiting time invariable for the properties.
Process Classifications
Reversible and Irreversible Processes
Processes occur when there is a driving force, which changes state between the parts of the system. If the driving force is finite, the process is Irreversible, and if it is infinitesimal in magnitude, the process is Reversible
Irreversible Processes
Irreversible Processes can’t be reversed without the use of external energy.
To bring back the system to its initial state, surroundings have to undergo some change through heat or work interactions.
In irreversible processes, changes remain in the surroundings or in the system after the process.
Example of Irreversible Processes: all spontaneous processes. Free expansion of gas. Diffusion of a solute from concentrated region to dilute region. Transfer of heat from a hot body to a cold body. Rusting of iron. Mixing of gases.
Reversible Processes
In Reversible Processes, the direction of the process can change. A tiny alteration in the forces acting on the system can cause this change.
Reversible Processes can be brought back to their original state leaving no change in surroundings.
Driving and opposing forces are in exact balance. By increasing or decreasing the forces by a tiny amount, the process can reverse direction. Reversible Processes take infinite time to finish, complete absence of dissipative forces like friction. Never attained in practice. Regarded as the limit of realizable processes.
The reversible process occurring in a work-producing machine will deliver the maximum amount of work. The process occurring in a work requiring machine requires a minimum amount of work.
Example: A process very close to a reversible process consists of water and water vapor in equilibrium. This equilibrium occurs in a cylinder provided with the frictionless piston. The external force on the piston matches the force due to the vapor pressure of water. This occurs at a given temperature. By increasing the force acting on the piston by an infinitesimally small amount, the vapor will condense. Decreasing it slightly will make the water vaporize. To condense a certain amount of water vapor requires the same amount of work as vaporizing that quantity of water.
Applications
Understanding chemical engineering thermodynamics is crucial for designing efficient and sustainable industrial processes. This knowledge forms the backbone of innovations in energy systems, material development, and environmental conservation.
Conclusion
Chemical Engineering Thermodynamics is an essential field of study. It is crucial for understanding and optimizing the energy interactions involved in chemical processes. It combines the principles of classical thermodynamics with the specific needs of chemical engineering. This combination equips engineers with the tools to design efficient systems. These systems include reactors, distillation columns, and heat exchangers. The study of thermodynamic properties includes energy, heat, work, and various state functions. This study enables the prediction of system behaviors. It also aids in designing processes that minimize energy consumption and maximize performance. Mastery of thermodynamics is thus crucial for developing sustainable, cost-effective solutions in chemical engineering.
FAQs
What is chemical engineering thermodynamics?
Chemical Engineering Thermodynamics is the study of energy, heat, and work interactions within chemical processes. It helps in understanding how energy is transferred between systems and their surroundings, which is crucial for the design, optimization, and analysis of chemical engineering processes.
Why do we study chemical engineering thermodynamics?
We study Chemical Engineering Thermodynamics to understand and control energy and matter behavior in chemical processes. It helps optimize energy use, improve process efficiency, ensure safety, design systems like reactors and distillation columns, predict chemical reactions, and minimize environmental impact. It’s essential for process design, stability, and sustainability in chemical engineering.
Why is Thermodynamics Important in Chemical Engineering?
Thermodynamics plays a vital role in chemical engineering by enabling engineers to design efficient systems like reactors, distillation columns, and heat exchangers. It aids in predicting fluid behaviors, phase transitions, and energy changes, ensuring optimal performance and energy conservation in chemical processes.
What are the Types of Thermodynamic Systems in Chemical Engineering?
There are three primary types of thermodynamic systems:
Open System: Can exchange both mass and energy with surroundings (e.g., a tubular flow reactor).
Closed System: Can exchange energy but not mass (e.g., a batch reactor).
Isolated System: Does not exchange energy or mass with surroundings (e.g., a thermos). Understanding these systems helps engineers design processes with precise energy and mass balances.
What is the Difference Between Intensive and Extensive Properties in Thermodynamics?
Extensive Properties: Depend on the system’s size or mass (e.g., mass, volume).
Intensive Properties: Independent of the system’s size (e.g., temperature, pressure). Intensive properties help determine the condition of a system without depending on the quantity of material.
What Are State Functions in Thermodynamics?
State functions are properties that depend only on the current state of the system, not on the path taken to reach that state. Examples include temperature, pressure, and internal energy. These are essential for analyzing and predicting system behaviors during chemical processes.
What is the Role of Work and Heat in Chemical Engineering Thermodynamics?
In chemical engineering thermodynamics, work is energy used when a force acts over a distance (e.g., expansion work in a piston). Heat is energy transferred due to temperature differences. Both work and heat are crucial for understanding energy exchanges in chemical processes and ensuring efficient energy use.
What is the Significance of Enthalpy in Chemical Engineering?
Enthalpy is a thermodynamic property used to measure heat transfer in processes at constant pressure. It combines internal energy and the energy associated with pressure and volume, making it essential for analyzing heat exchange in chemical reactions and industrial operations.
What Are Reversible and Irreversible Processes in Thermodynamics?
Reversible Processes: Can return to their original state without affecting the surroundings. They represent ideal conditions and are used to calculate maximum efficiency.
Irreversible Processes: Cannot be reversed without external energy. These processes occur in real-world applications and are characterized by energy losses such as friction or heat dissipation.
How Does Chemical Engineering Thermodynamics Contribute to Process Optimization?
Thermodynamics enables engineers to predict energy changes and optimize chemical reactions, phase separations, and heat exchanges. By understanding the laws and principles governing energy flow, engineers can design more efficient and sustainable chemical processes, minimizing energy consumption and maximizing output.
What Are the Key Applications of Thermodynamics in Chemical Engineering?
hermodynamics is used in various chemical engineering applications, including:
Reactor Design: Optimizing energy transfer during chemical reactions.
Distillation Columns: Designing efficient separation processes based on temperature and pressure changes.
Heat Exchangers: Managing heat exchange processes for energy recovery.
Phase Transition Analysis: Understanding how materials change from one phase to another under different conditions.
What Is the Concept of Equilibrium in Thermodynamics?
Equilibrium refers to the state where all macroscopic properties (e.g., temperature, pressure) are uniform throughout the system and do not change with time. In chemical engineering, equilibrium is crucial for understanding and controlling reactions and separations that are in balance, ensuring process stability.
Further Reading and Resources
To delve deeper into thermodynamics in chemical engineering, check out the following resources:
