July 29, 2009

Latent Heat Calculation

The most used & ever needed property is the latent heat of vaporization which is critical also for various calculations. A process engineer must understand it properly.

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This is one of the property of a pure fluid or a mixture which is never available to you when you need it most specially when you need it urgently. That time you are never able to recall the source where did you see it last time.

So don't worry now. There are few easy methods available which can give you quick & accurate estimate of this be it for pure fluid or for a liquid mixture.

The first method I am discussing is Riedel's Correlation.
This method is limited to calculating latent heat at normal boiling point only. However, this is a property which can be used for the derivation of other properties also. We will see it in future & coming posts.

Hvb = 1.093 R Tc [ Trb x {Ln(Pc)-1}/{0.930 - Trb}]

Hvb = Latent heat at normal boiling point. (Remember this is an important definition)
Trb = Tb / Tc Reduced Boiling Point Ratio in Kelvin
Pc = Critical Pressure atm

Hvb Unit is Lit-Atm/Gm-mole
R = Gas Constant in Lit-Atm/Gm-mole/K

So change R value in different unit & you will get Hvb in desired unit accordingly, because the value in parantheses is unitless.
Also note that 1 Lit-Atm/Gm-mole is equal to 24.12 Cal/gm-mole

The second method I am discussing is Pitzer's Correlation.
This method is applicable for a wide range from normal boiling point to critical point.

The equation given is as below.

(Hv / R Tc) = 7.08 ( 1 - Tr)^0.354 + 10.95 * omega ( 1 - Tr)^0.456

Hv = Latent heat at t °C.
Tr = (t+273.15)/Tc
omega is accentricity factor - a std property

The third method I am discussing is Watson's Correlation.
This method is most useful for the fluid where you know the latent heat at a given temperature & want to calculate it at another temperature, or using some simulation where accurate estimate is required so instead of using basic equations you can use this escalation equation.

Hv2 = Hv1 [ (1 - Tr2) / (1 - Tr1)]^0.378

Hv1 = latent heat at T1
Hv2 = latent heat at T2

Units are same as described in first method.

List of other property estimation methods on this Blog.

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