Advance Chemical Engineering Thermodynamics

1. Advance Chemical Engineering Thermodynamics By Dr.DangSaebea

2. The First Law and Other Basic Concepts Part I

3. A thermodynamic view of the world • System • The part of the world to which we are directing our attention • Surroundings • Everything that is not a part of the system constitutes • The system and surroundings are separated • Boundary

4. A thermodynamic view of the world opens system: allows energy as well as chemical transfer into and from the system closed system: allows only energy transfer isolated system: no transfer at all adiabatic system: only transfer of work

5. A thermodynamic view of the world Properties and the state of a system Properties of a system P, T, V, x • principle measurable values capable of assuming definite values. • The pressure, volume, and temperature determine the values of all the other properties. • They are therefore known as state properties

6. Internal energy and the First Law • Internal energy The sum of the kinetic energy of motion of the molecules, and the potential energy represented by the chemical bonds between the atoms and any other intermolecular forces that may be operative.

7. Example 1 Water flows over a waterfall 100 m in height. Take 1 kg of the water as the system, and assumes that it does not exchange energy with its surroundings (a) What is the potential energy of the water at the top of the falls with respect to the base of the falls? (b) What is the kinetic energy of the water just before it strikes bottom? (c) After the 1 kg of water enters the stream below the falls, what change has ocurred in its state?

8. “Energy cannot be created or destroyed“ • This fundamental law of nature, more properly known as conservation of energy • Under its more formal name of the First Law of Thermodynamics, it governs all aspects of energy in science and engineering applications. • It's special importance in Chemistry arises from the fact that virtually all chemical reactions are accompanied by the uptake or release of energy.

9. The First Law • Any change in the internal energy of a system is given by the sum of the heat q that flows across its boundaries and the work w done on the system by the surroundings. • If heat flows into a system or the surroundings to do work on it, the internal energy increases and the sign of q or w is positive. • heat flow out of the system or work done by the system will be at the expense of the internal energy, and will therefore be negative. ΔUt= Q + W 1

10. The First Law • Equation (1) applies to processes involving finite changes in the internal change in the internal energy. For differential changes 2 dUt = dQ + dW Ut= extensive properties U = intensive properties Ut =nU 3 Eq. 2 and 3 may now be written d(nU)=ndU=dQ+dW 4

11. Example 2 When a system is taken from state b in Figure along path acb, 100 J of heat flows into the system and the system does 40 J of work. (a) How much heat flows into the system along path aeb if the work done by the system is 20 J • (b) The system returns from b to a along path bda. If the work done on the system is 30 J, does the system absorb or ilberate heat? How much?

12. Pressure-volume work • The kind of work most frequently associated with chemical change occurs when the volume of the system changes owing to the disappearance or formation of gaseous substances. • This is sometimes called expansion work or PV-work. • It can most easily be understood by reference to the simplest form of matter we can deal with, the hypothetical ideal gas

13. Pressure-volume work • This will cause the gas to expand, so the piston will be forced upward by a distance Δx. Since this motion is opposed by the force x, so the work is given by w = – fΔx(1) w = – fΔx(2) w = – P ΔV (3)

14. Example 3 A gas is confined in a cylinder by a piston. The initial pressure of the gas is 7 bar, and the volume is 0.1m3. the piston is held in place by latches in the cylinder wall. (a) The whole apparatus is placed in a total vacuum. What is the energy change of the apparatus if the restraining latches are removed so that the gas suddenly expands to double its initial volume, the piston striking other latches at the end of the process? (b) If the process described in (a) is repeated, not in a vacuum but in air at atmospheric pressure of 101.3 kPa, what is the energy change of the apparatus? Assume the rate of heat exchange between the apparatus and the surrounding air is slow compared with the rate at which the process occurs.

15. THE REVERSIBLE PROCESS A process is reversible when its direction can be reversed at any point by an infinitesimal change in external conditions. • The work done on the system is given by this equation only when certain characteristics of the reversible process are realized. • Processes for which these requirements are met are said to be mechanically reversible The reversible process is ideal in that it produces the best possible result. It represents a limit to the performance of actual processes, but is never fully realized. The reversible work as the limiting value may be combined with an appropriate efficiency to yield a reasonable approximation to the work of an actual process.

16. Example 4 A horizontal piston/cylinder arrangement is placed in a constant-temperature bath. The piston slides in the cylinder with negligible friction, and an external force holds it in place against an initial gas pressure of 14 bar. The initial gas volume is 0.03 m3. The external force on the piston is reduced gradually, and the gas expands isothermally as its volume doubles. If the volume of the gas is related to its pressure so that the product PVt is constant, what is the work done by the gas in moving the external force? How much work would be done if the external force were suddenly reduced to half its initial value instead of being gradually reduced?

17. Heat changes at constant pressure and volume: the Enthalpy • For n moles of a homogeneous fluid contained in a closed system the energy balance is: The work for a mechanically reversible, here written: (1) (2) These two equations combine: (3)

18. Constant-Volume Process • If the process occurs at constant total volume, the work is zero. Moreover, for closed systems the last term of Eq. (3) is also zero, because n and V are both constant. Thus, (4) Integration yields: (5) Thus the heat transferred in a mechanically reversible, constant-volume, closed-system process equals the internal-energy change of the system.

19. Constant-Pressure Process (6) For a constant-pressure change of state: (7) (8) Thus, the mathematical (and only) definition of enthalpy is: (9) The differential form is: (10) It becomes an equation for a finite change in the system: (11)

20. Example 5 Calculate ΔU and ΔH for 1 kg of water when it is vaporized at the constant temperature of 100°C and the constant pressure of 101.33 kPa. The specific volumes of liquid and vapor water at these conditions are 0.00104 and 1.673 m3 kg-1. For this change, heat in the amount of 2,256.9 kJ is added to the water.

21. The End