ChemE 260 Reversibility and Irreversibility

April 27, 2005

Dr. William BaratuciSenior Lecturer

Chemical Engineering Department

University of Washington

TCD 6: DCB 5: 7

Reversible and Irreversible Processes

• Reversible Processes– Processes in which the system and the surroundings can both

simultaneously return to their initial states after the process is completed.

• Irreversible Processes– Processes in which the system and the surroundings cannot

both simultaneously return to their initial states after the process is completed.

BaratuciChemE 260April 27, 2005

Sources of Irreversibility• Heat Transfer through a finite temperature difference• Friction• Unrestrained, or fast, expansion or compression

• Mixing of two different substances• Spontaneous chemical reactions• Electric current flowing through a finite resistance• Inelastic deformation of solids

BaratuciChemE 260April 27, 2005

Heat Transfer• Heat transfer from the system to the surroundings occurs

spontaneously because T1 > Tsurr.• But the system cannot spontaneously return to the initial state

because heat cannot spontaneously flow from Tsurr = 25oC to Tsys = 30oC.

• We could use a heat pump to transfer heat from the surroundings into the system to return the system to state 1.– But the HP would require work from the surroundings to run and this

would leave the surroundings in a different state than they started in !

P1, V1

T1=100oC

PA = P1

VA < V1

1 A

cool

Tsurr = 25oC TA = 30oC

• Any heat transfer through a finite T is therefore irreversible.

BaratuciChemE 260April 27, 2005

Friction• Friction converts kinetic energy into internal energy when two

bodies in contact with each other are in motion relative to each other.

• Consider what happens when the block spontaneously slides down the inclined plane.

BaratuciChemE 260April 27, 2005

Friction• As the block slides down the inclined plane, the potential energy

of the block decreases.• Friction acts in the direction that opposes motion.

• The block does work on the surroundings in order to overcome the frictional force that resists motion.

• Friction converts the work into an increase in T and U of the system and surroundings

BaratuciChemE 260April 27, 2005

Friction• The block never spontaneously slides back up the inclined plane !• Work must be input to increase Epot and to overcome friction.• Friction again converts the work input into another increase in T and U of the

system and surroundings.

• To return both the block and the surroundings to their original states, we must take heat out of the block and the surroundings and completely convert it to work and return this work to the surroundings.

• A device that completely converts heat into work violates the K-P Statement of the 2nd Law and is impossible !

• We conclude that any process that includes friction is irreversible !

BaratuciChemE 260April 27, 2005

Expansion & Compression• Even without friction, fast expansion and compression are

irreversible.

P1, V1, T1

P2 > P1

V2 < V1

T2 > T1

1 2

CompressRapidly

ExpandVery

Slowly

1

P1, V1, T1

• Force that must be overcome during the fast compression is greater than P1 Apiston.

• Force that is overcome during the slow expansion is equal toP1 Apiston.

• We put more work in during the fast compression than we got back out during the slow expansion !

Assume:Q = 0Epot = 0Ekin = 0

BaratuciChemE 260April 27, 2005

Expansion & Compression• If we put in more work than we got out, then Usys and Tsys must

have increased !

P1, V1, T1

P2 > P1

V2 < V1

T2 > T1

1 2

CompressRapidly

ExpandVery

Slowly

1

P1, V1, T1

Assume:Q = 0Epot = 0Ekin = 0

• To return both the system and the surroundings to their original states, we must take heat out of the system and completely convert it to work and return this work to the surroundings.

• A device that completely converts heat into work violates the K-P Statement of the 2nd Law and is impossible !

• We conclude that compression or expansion at a finite rate are irreversible !

BaratuciChemE 260April 27, 2005

Internally Reversible Processes• No irreversibilities within the boundary of the system

– No friction inside the system– No rapid expansion or compression inside the system– No mixing of two different substances inside the system– No spontaneous chemical reactions inside the system– No electric current flowing through a finite resistance inside

the system– No inelastic deformation of solids inside the system

• Irreversibilities can exist outside the system– This usually means that heat can be exchanged between the

system and surroundings through a finite temperature difference.

BaratuciChemE 260April 27, 2005

Externally Reversible Processes

• Irreversibilites do not exist in the surroundings

– This usually means that heat can only be exchanged between

the system and surroundings through an infinitely small or

infinitessimal temperature difference.

• Irreversibilites can exist inside the system

BaratuciChemE 260April 27, 2005

Reversible Processes• No irreversibilities exist within the system or in the

surroundings• No real process is reversible• Some real processes are very nearly reversible

– They have very little friction– They involve only very slow compression or expansion– Heat transfer only takes place between bodies at very similar

temperatures.

• Why bother studying reversible processes if they do not exist ?– Reversible processes are easier to analyze– Reversible processes provide a best-case to which we can

compare the performance of any real process.BaratuciChemE 260April 27, 2005

Next Class …• The Carnot Cycle

– The gold-standard !

– Maximum efficiency !

– A reversible cycle to which we will compare the performance of all of the real cycles we will analyze in te rest of this course.

– We will return to example problems and numbers !

BaratuciChemE 260April 27, 2005