Example 3.18: 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.
Draw a tie line de connecting the enthalpies of the two initial solutions
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.
Material balance:
0(1) + (1 - 0.2)(1) = y + (1 - 0.63)(2 - y)
y = 0.095 lb water vapor
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