Introduction to Chemical Engineering

Thermodynamics

Chapter 7

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Throttling Process

Fluid flows through a restriction results in a pressure drop No change in kinetic or potential energy No shaft work produced and no heat transfer

∆H=0 For an ideal gas: no change in T occurs For real gases: T reduction results from a throttling process

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Turbines (Expanders)

Expansion of a gas in a nozzle to produce a high-velocity stream This kinetic energy is converted into shaft work when the stream impinges on blades attached to a rotating shaft Overall result is conversion of internal energy of a high-pressure stream into shaft work Steam : device is called a turbine High-pressure gas (e.g., ethylene) device called expander

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Neglect potential energy, heat transfer is negligible, inlet and

outlet are sized so that fluid velocities are roughly equal

T1, P1 and P2 are known but ignore T2, Ws and H2

Reversible isentropic turbine (constant entropy) S2=S1.

Thus, one can determine H2 and then Ws.

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Compressors Rotating blades (not too high discharge pressure) or

cylinders with reciprocating pistons (for high pressures)

Kinetic and potential energy changes negligible.

Adiabatic compression

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Pumps

Need to determine enthalpy of compressed subcooled liquids

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Other useful relations:

Both equations are usually integrated assuming that:

CP, β and V are constant