Problem 2.1.6

 

Determine enthalpy of vaporization of benzene at 75 degrees centigrade.  Critical temperature is 561.65 Kelvin, Pitzer acentric factor is 0.205.

 

Solution: 

 

Enthalpy of Vaporization – There technique for estimating enthalpies of vaporization of pure liquids have been proposed.

 

  1. This is based on dPvp over dT is equal to delta Hv over (RT2/Pvp delta Zv).  It requires finding dPvp over dT from a vapor pressure-temperature correlation.  A separate estimation of delta Zv must be made before delta Hv can be obtained.  This procedure is inherently accurate, especially if delta Zv is obtained from reliable PVT correlation. 
  2. In the second category are the techniques from the law of corresponding states.  The Pitzer et al. form is one of the most accurate and convenient.  In an analytical form, this equation for delta Hv is approximated by delta Hv over RTc is equal to f(Tr, omega).  This equation is applicable when Tr is greater than 0.6 but less than 1.0.  A second corresponding states method is the two-reference fluid method.
  3. The third method is to estimate delta Hvb and then scale with temperature with appropriate functions as provided in the literature.

 

Reduced temperature is equal to 0.62.  Plugging in the values of Tr and omega in Perry’s Equation 3.73 group delta Hv/RTc is calculated to be 6.47.  Using the value of critical temperature, gas constant, enthalpy of vaporization at 75 degrees centigrade is found to be 3.024 time 104 joules per mole.