The Enthalpy Chart Presented to CBE 317 Sept

The Enthalpy Chart Presented to CBE 317 Sept – 2003 Dick Hawrelak

A Typical Enthalpy Chart

The Liquid Line

The Vapor Line

The Latent Heat Line

Enthalpy Chart Zones

Equation of State • At low pressure (14. 7 psia, 101. 3 k. Pa) 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

A Centrifugal Compressor

A Refrigeration PFS

Refrigeration on Enthalpy Chart

Flashing

Flashing Around E-3

Solve the % Flash

De-Superheating a Vapor

Condensing the Vapor

Sub-Cooling The Liquid

Steam Expansion To Generate Power

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. ht ml