The Enthalpy Chart

Presented to CBE 317

Sept – 2003

Dick Hawrelak

A Typical Enthalpy Chart

The Liquid Line

Tc,Pc

PSIA

BTU/LB

*

Liquid at its Bubble Point

The Vapor Line

Tc,Pc

PSIA

BTU/LB

* Vapor at its Dew Point

The Latent Heat Line

Tc,Pc

PSIA

BTU/LB

HvHLLatent Heat = Hv - HL

*

Enthalpy Chart Zones

PSIA

BTU/LB

Pc, Tc

*

The Vapor Zone

14.7 psia

The LiquidZone

Dense Fluid Zone (Dense Vapor or a Light Liquid)

The TwoPhaseZone

Ideal

Non - Ideal

VacuumZone

Equation of State

• At low pressure (14.7 psia, 101.3 kPa) the Ideal Gas Law applies where PV = (z)(n)(R)(T) and Z = 1.0

• At high pressure, the Gas Law is non-ideal and Z is less than 1.0

• Vapor Density = (MW)(P) / [(Z)(R)(T)]

• More mistakes are made with vapor density than any other physical property.

Super-Critical Region

• Above the Pc and the Tc which design equations apply?

• Given temperature and pressure above the criticals, a compressor vendor will treat the data as a dense vapor and use vapor correlations.

• Given the same data, a pump vendor may treat the data as a light liquid and use liquid correlations (Dow Cochin Pipeline system). Pump power required will be much lower than compressor power required.

Just below the Pc and the Tc

• This is a critical zone for distillation because the latent heat approaches zero as the Pc and Tc are approached.

• Hence, Vapor flow = (BTU / hr) / (LH) becomes very high.

• Design equations at conditions near the criticals are very complicated and many errors are made in this region.

Low Pressures

• Vacuum condition below 14.7 psia.• Low vapor densities mean high vapor

flows by (cf / hr) = (lb / hr) / (lb / cf).• Compared with high pressure, pressure

drop calculations in the vacuum zone have very little margin for error.

• Hence, equipment such as exchangers and distillation towers and lines can be severely under-sized.

Constant Entropy Line

PSIA

BTU/LB

P1, T1, S1, H1

P2, T2, S1, H2

A Reciprocating Compressor

Pc, Tc

*

HP = (lb/hr)(BTU/lb) / 2547

A Centrifugal Compressor

PSIA

BTU/LB

P1, T1, S1, H1

P2, T2, S2, H2

A Centrifugal Compressor

Pc, Tc

*

HP = (lb/hr)(BTU/lb) / 2547

A Refrigeration PFS

Refrigeration on Enthalpy Chart

PSIA

BTU/LB

P1, T1, S1

P2, T2, S2

A RefridgerationCycle

Pc, Tc

*

Flashing

PSIA

BTU/LB

P2, V

P1, F,

Pc, Tc

*

P2, L

Flashing Around E-3

to KO Pot

Level Control Valve

PIC

LC

Solve the % FlashDetermine the E-3 flash from 104 deg F to 130 psia

Assume F = 100 lbs F = High Pressure Liquid, V = Flash Vapour, L = Flash Liquid

Mass balance = 100 = V + L or V = 100 - L substitute into Heat balance eqn

Heat balance = (100)Hf = V(HV) + L(HL) V @ 117 btu.lb F at 60F & 130 psia 130 psia 117 btu/lb

HF = -6 btu/lb Feed at -6 BTU/lbHV = 117 btu/lb shell side feed, F E-3HL = -34 btu/lb L

-34 btu/lb100(Hf) = -600 130 psiaV(HV) = (100-L)(117)L(HL) = L(-34)

Solve for L -600 = (100 - L)(117) + L(-34)

L = 81.46 % liquid HL = -34 btu/blV = 18.54 % vapour HV = 117 btu/lb

flash

De-Superheating a Vapor

PSIA

BTU/LB

P1, T1 P1, T2

Pc, Tc

*

Condensing the Vapor

PSIA

BTU/LB

P1, T1, V P1, T1, L

Pc, Tc

*

Sub-Cooling The Liquid

PSIA

BTU/LB

P1, T1, LP1, T2, L

Pc, Tc

*

Steam Expansion To Generate Power

12,435 lb/hr 95 deg F 120 deg F Air To Atm650 psia 1.6927 psia750 deg F Vacuum Pump

75 deg F Horrizontal KO Pot

120 deg FTo BFW system

HP = 1716.9 Eff'y = 77%

C-1 ST-1

Steam Expansion

Which Route To Get From A to B?

Summary• The Enthalpy Chart is one of the most useful

tools for solving chemical engineering problems.

• Learn to communicate with it as shown in this demonstration. If done properly, you’ll make fewer mistakes in your design work.

• An internet web site that allows you to draw enthalpy charts from a large chemical database can be found at:

• http://www.questconsult.com/~jrm/enthpres.html