Introduction to Chemical Engineering Thermodynamics

1. CHEMICAL ENGINEERING THERMODYNAMICS
Sasidhar Gumma
Department of Chemical Engineering
Indian Institute of Technology Guwahati
s.gumma@iitg.ac.in
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2. Function of Thermodynamics
“It is the function of thermodynamics to relate those properties of a
system required for practical or theoretical purposes to the
parameters that are most readily measured, and thus to provide the
maximum return of information for any investment in experiment.”
- H.C. VAN NESS
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3. Classical and Chemical Engineering Thermodynamics
 Classical Thermodynamics
 First/second laws, Essentially energy balance over a process , heat-work, work-
heat conversion device
 PVT behavior, thermodynamic variables
 Phase and reaction equilibria
 Application to chemical engineering problems: Work requirement
and heat effects in mixing and separation processes
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4. Thermodynamics of Pure Species
 Generic thermodynamics deals with property estimates of pure species
– For example, steam tables for energy balance can be readily used
Example
Answer ?
Simple! Use steam tables (turns out T = 518.47 K)
 Will anything condense?
 Look in steam tables
Steam, 533.15 K, 10 bar, 1 kg/s 533 .07 K, 9.95 bar, 0.995 kg/s
0.005 kg/s, 1 bar, T = ?
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5. Thermodynamics of Mixtures
Gas mix: 85% CH4, 7% C2, 4 % C3, 2% C4, 1% C5, 0.6% C6, 0.4% C7
What is the temperature, T? Will anything condense?
Not so straightforward anymore!
Gas mix, 298.15, 10 bar, 1 kg/s 296 K, 9.95 bar, 0.995 kg/s
0.005 kg/s, 1 bar, T = ?
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6. Thermodynamics of Mixtures
 50 cc H2O + 50 cc methanol mixture, final volume < 100 cc
 Isoproponol + Methyl ketone, ΔV > 0
 Freezing point of water + ethylene glycol < freezing point of either
 Freezing point of C6H6 + C6F6 > freezing point of either
 C6H6 : 278 .7 K C6F6 : 278.1 K Equi-molar: 297 K
 Chemical industry deals with virtually infinite number of species and their
mixtures (22 million compounds in CAS registry)
 How do they behave?
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7. Thermodynamics and Reactions
 Blast furnace
Fe2O3 + 3CO = 2Fe+ 3CO2
 Gas leaving chimney contains considerable amount of CO, carrying away
unutilized heat.
 Can we increase CO conversion?
Longer contact time: Furnaces as long as 30 m were made in England
 Reaction is equilibrium limited (No improvement achieved!!)
 Enormous sum of money was wasted which could have been prevented if laws of
thermodynamics were applied
Story as described by Le Chatelier (1888)
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8. Role of Thermodynamics in Chemical Engineering
 The two laws provide
 Constraints and interconversions between heat and work
 Basis for establishing states of pure substances and their mixtures
 Direction in which they move when stimulated by external force!
 Typical Applications
 Phase equilibrium (separation of components from mixtures)
 reaction equilibrium (ultimate extent of a reaction)
 Plays supervisory role
 Necessary in design of equilibrium controlled operations
 In design of rate controlled separations
 It sets the boundaries (driving force)
 Processes are often treated as departure from equilibrium
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9. Feasibility of a Process
 For a process to be feasible it must
 Obey the thermodynamic constraints
 Operate at a reasonable rate so that size of the equipment will be economical
 Any process design usually involves a feasibility study before detailed design
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10. Engineering Aspects
 Measured variables: T,P, V, x (or y) and change in energy
 Other variables like G,S, fugacity, activity coefficient etc. are all “created” to find
easy solutions to thermodynamic problems in abstract domain
 Thermodynamics in abstract domain is more or less solved (science)
 Challenge lies in the third step (engineering)
 Nature unfolds itself without any reference to energy, entropy , fugacity,
compressibility factor (Z) etc.
 These variables transform the problem into a convenient domain for us to solve
 Recall quote from Prof Van Ness!!
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11. Course Objectives
 Provide undergraduate students with the fundamentals of Chemical Engineering
Thermodynamics.
 Demonstrate the application of the fundamental concepts to a wide variety of
processes occurring in Chemical Engineering.
 Develop skills necessary to make appropriate assumptions in specific Chemical
Engineering problems.
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12. Expected Outcomes
 Formulate and manipulate the thermodynamic treatment of processes.
 Formulate and analyze specific Chemical Engineering problems using fundamental
concepts.
 Select appropriate approximations for practical problem solving.
 Understand the implications of approximations on the efficiency and accuracy of
the solution.
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13. Necessary Prior Knowledge
 Mass and energy balances
 Differential and integral calculus
 Spreadsheet software for simple calculations / graphing such as (Openoffice Calc /
Microsoft Excel® or similar)
 Software for simple algebraic problems such as finding roots of a polynomial,
solving simultaneous non-linear algebraic equations etc. (Octave / Matlab® or
similar )
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14. Grading Scheme
 Assignments: 30 points (10 assignments with equal weight)
 Mid-term examination: 30 points
 Final examination: 40 points
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15. Learning Strategy
 Course is challenging
 Prepare to solve many problems (8-10 per week)
 Practice as many problems on the topic as you can from the textbook
 DO NOT randomly pick an equation from the book to solve the problem.
 Understand where a particular equation comes from and the underlying
assumptions
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16. Course Outline
 First law, reversibility of a process
 enthalpy, heat capacity, mass and energy balances for open systems
 PVT behavior of pure substances
 Processes involving ideal gas, sensible heat calculations
 2nd law, heat engines, T-scale
 Entropy, entropy generation, entropy “balance”
 Thermodynamic properties, diagrams/tables
 Properties of ideal gases, residuals
 Equations of state (virial, cubic etc.)
 Corresponding states, generalized correlations
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17. Course Outline
 Two-phase systems, liquid mixtures, solution thermodynamics
 Ideal gas mixture and concept of fugacity
 Ideal solution and excess properties
 Vapor-liquid equilibrium (VLE), Raoult’s law, K-factors, Henry’s law
 Modified Raoult’s law and activity coefficients
 Gibbs-Duhem Equation and Thermodynamic Consistency
 Equilibrium and Stability of Liquid Mixtures , VLLE
 Chemical Reaction Equilibria
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18. Book(s)
 Introduction to Chemical Engineering Thermodynamics, J.M.Smith, H C Van Ness,
M M Abbott, Adapted by B I Bhatt (7th Ed.), Tata McGraw Hill Pvt. Ltd. (2004)
 Chemical Engineering Thermodynamics, Y V C Rao (2nd Ed.) Universities Press
(1997)
 Introductory Chemical Engineering Thermodynamics, J.R.Elliott and C.T.Lira (2nd
Ed.), Pearson Eductaion India Pvt. Ltd. (2013).
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19. Development of Thermodynamics
 Based on general understanding of interconversion between heat and work, first and
second laws were developed
 They have no proof, but there is no contrary experience either
 These laws were later developed into a network of equations
 Have wide ranging application in all branches of engineering
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20. Size of a system
 System: Body of matter that is of interest
 Size of the system: Expressed as mass (m), moles (n) or total volume (Vt)
 𝑛 =
𝑚
𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡
 Molecular weight of water: 18 gm/mol, 18 kg/kmol or 18 lb/lb-mol
 𝑀𝑜𝑙𝑎𝑟 𝑣𝑜𝑙𝑢𝑚𝑒 𝑉 =
𝑉𝑡
𝑛
(SI Units: 𝑚3
𝑘𝑚𝑜𝑙−1
)
 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑣𝑜𝑙𝑢𝑚𝑒 𝑉 =
𝑉𝑡
𝑚
(SI Units: 𝑚3𝑘𝑔−1)
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21. Density
 𝑀𝑜𝑙𝑎𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝜌 =
1
𝑚𝑜𝑙𝑎𝑟 𝑣𝑜𝑙𝑢𝑚𝑒
(SI Units: 𝑘𝑚𝑜𝑙 𝑚−3 )
 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝜌 =
1
𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑣𝑜𝑙𝑢𝑚𝑒
(SI Units: 𝑘𝑚𝑜𝑙 𝑚−3 )
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22. Temperature
 Galileo (Early 1600’s): Thermoscope
 Romer (early 1700’s) : Developed numerical scale
 Farenheit (used mercury, increased resolution of the scale)
 Celsius (1744) : Celsius scale
 Amontons (early 1700’s) provided approximate estimate of absolute zero
 Absolute temperature scales (Kelvin and Rankine)
 Exercise: Learn interconversion between various temperature scales i.e. ℃, ℉, 𝐊 and OR
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