CHE 312Chemical

Engineering Thermodynamics II

Credit Hours 3 – 0

By Dr. Qazi Nasir

Lecture # 1

CHE 312 Chemical Engineering Thermodynamics II• In nineteenth century, the science of thermodynamic was

born which describe the operation of steam engines

• The name was given as power developed from heat is the main application of steam engine

• The principle which valid for engines are generalized as first and second laws of thermodynamics

• These laws have no mathematical sense

• Chemical engineers among other problems , calculation of heat and work requirement for physical and chemical processes, determination of equilibrium conditions for chemical reactions and transfer of chemical species between phases

CHE 312 Chemical Engineering Thermodynamics II• The application of thermodynamics to any real problems

starts with the identification of body

• Body = system

• Thermodynamic state = Measurable macroscopic properties such as length , time, mass, temperature and amount of substance (Mass, no of moles, total volume, specific volume)

• Temperature: commonly measured in liquid-in-glass thermometer, wherein the liquid expands when heated

• Pressure: The normal force exerted by the fluid per square meter or Nm-2. (dead-weight gauge)

CHE 312 Chemical Engineering Thermodynamics IIWork: when a force acts through a distance then work is performed

• A work which accompanies a change in volume of fluid is often encountered in thermodynamics, e.g. compression and expansion of fluid in a cylinder

• The force exerted by the piston on the fluid is equal to the product of the piston area and pressure of the fluid.

• The displacement of the piston is equal to the total volume change of the fluid divided by area

CHE 312 Chemical Engineering Thermodynamics II

CHE 312 Chemical Engineering Thermodynamics IIArea is constant

CHE 312 Chemical Engineering Thermodynamics IIHeat: Hot object brought in contact with cold object becomes cooler whereas cooled object become warmer.

• Something is transferred from hot object to cold object is Heat Q

• Temperature is a driving force for the transfer of energy as heat

• Rate of heat transfer from one body to another is proportional to the temperature difference. Between two bodies

• 1/100th quantity of heat which when transferred to one kilogram mass of water raised its temperature from 0 to 100 0C

CHE 312 Chemical Engineering Thermodynamics IIThermodynamic properties are classified into two types

Direct measureable properties

• Pressure-volume-temperature (PVT) relationships

Indirect measureable properties

• Determined indirectly using relationships that express in terms of physical properties

• Internal energy, entropy, enthalpy, Helmholtz free energy, Gibbs free energy and association properties such as chemical potential and fugacity)

CHE 312 Chemical Engineering Thermodynamics II

Four quantities called ‘thermodynamic potential’ are useful

in the chemical thermodynamics of reactions and non-

cyclic processes.

• Internal energy

• Enthalpy

• Helmholtz free energy

• Gibbs free energy

CHE 312 Chemical Engineering Thermodynamics IIHelmholtz Free Energy

• Internal energy U might be thought of as the energy required to create the system in the absence of changes in temperature or volume

• But in environment of temperature T, some the energy can be obtained by spontaneous heat transfer in TS

CHE 312 Chemical Engineering Thermodynamics IIGibbs Free Energy

• Internal energy U might be thought of as the energy required to create the system in the absence of changes in temperature or volume

• Amount of work PV must be done if the system is created from a very small volume.

CHE 312 Chemical Engineering Thermodynamics IIFugacity

• In chemical thermodynamics, the fugacity (f) of a real gas is effective pressure which replaces the true mechanical pressure in accurate chemical equilibrium calculations.

• For example, nitrogen gas (N2) at 00C and a pressure of P = 100 atm has a fugacity of f = 97.03 atm

• This also means the chemical potential of real nitrogen at a pressure of 100 atm is less than if nitrogen were an ideal gas.

• Fugacities are determined experimentally or estimated from various model

• The ideal gas pressure and fugacity are related through fugacity coefficient

CHE 312 Chemical Engineering Thermodynamics IIFundamental equations

J. W. Gibbs give the names because they contain all information needed for the complete description of the thermodynamic state of a system.

Fundamental equations for closed system

System that do no exchange mass with the surroundings is considered as closed system

CHE 312 Chemical Engineering Thermodynamics IIFundamental equations for the Internal EnergyA closed homogenous system which experience the transfer of a small of heat dQ and an amount of volumetric work dW. The change in internal energy according to first law is given as

dU = dQ – dW (1)

If changes are reversible, then:

dW = PdV ; dQ = TdS

dU = TdS – PdV

U = f(S, V)

CHE 312 Chemical Engineering Thermodynamics IIFundamental equations for the Internal Energy

Consider a differential change in S and V. The corresponding change in U is given as.

Comparison of equation (1) indicates that

CHE 312 Chemical Engineering Thermodynamics IIThe system enthalpy is also specified from the definition

H = U + PV

Similarly, the two energy properties

A = U – ST

Gibbs free energy

G = U + PV - ST

Therefore, the availability of the functional relationship U = f(S,V) gives complete description on the thermodynamic state of the system i.e. its fundamental equation

CHE 312 Chemical Engineering Thermodynamics IIThe fundamental Equation for H, A, and G

H = U + PV (2)

The enthalpy differential

dH = dU + PdV + VdP

As, TdS = dU + PdV

dH = TdS + VdP

which indicates that:

H = f(S,P) (3)

CHE 312 Chemical Engineering Thermodynamics IIThe differential change in S and P leads to following form

Comparison with Eq (2) indicates that

(4)

Eq (3) and Eq (4) completely describe the system once S and P are specified. It is therefore, also a fundamental equation

CHE 312 Chemical Engineering Thermodynamics IISimilarly, Helmholtz & Gibbs free energy can be describe as

CHE 312 Chemical Engineering Thermodynamics II• Recommended Books

• Smith J.M., Van Ness H.C., Abbott M.M. “Chemical Engineering Thermodynamics” 6th Ed. 2001. McGraw Hill International Edition.

• Daubert Thomas E. “Chemical Engineering Thermodynamics”, 1st Ed. 1985, McGraw Hill Book Company.

• Sandler Stanley I. “Chemical and Engineering Thermodynamics” 3rd Ed. John Wiley and sons, Inc.