Example 3.18: Enthalpy concentration diagram: One pound of pure sulfuric acid at 150 °F is mixed with 1 pound of 20% sulfuric acid solution initially at 200 °F. Calculate the temperature of adiabatic mixing and the weight of water evaporated.

Solution: Enthalpy concentration chart

From a material balance the resultant mixture contains (1(1) + 0.2(1))/2 = 60% H₂SO₄ based on combined liquid and vapor phase.

C Draw a tie line de connecting the enthalpies of the two initial solutions

C Locate the intersection (g) of tie line with the 60% abscissa line.

Enthalpy of the mixture = 58 Btu/lb

Temperature of the mixture = 300 °F

Liquid phase (h) has a composition of 63% H₂SO₄.

Amount evaporated, y (neglecting the small amount of H₂SO₄ in the vapor phase), can be calculated from either a material balance or energy balance.

C Material balance:

0(1) + (1 - 0.2)(1) = y + (1 - 0.63)(2 - y)

y = 0.095 lb water vapor

C Energy balance:

Enthalpy of H₂SO₄ at 150 °F = 40 Btu/lb

Enthalpy of 20% H₂SO₄ at 200 °F = 76 Btu/lb

Enthalpy of water vapor at 300 °F and 1 atm = 1194 Btu/lb

Enthalpy of 63% H₂SO₄ at 300 °F = 1.0 Btu/lb

40´1 + 76´1 = 1194y + 1.0(2 - y)

y = 0.095 lb