Page 192 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

190 Chapter 6 Evaporators

6.6.3 Design data
Elevation of boiling point (BPE)
Usually evaporators deal with concentrated solutions whose specific heat, latent heat of vaporization
and boiling point deviate from water. There may be other thermal phenomena (e.g., heat of crystal-
lization) that need to be considered. The elevation in boiling point for different solute percentages at
different pressures is estimated from Duhring plot and the change in other thermodynamic properties
are estimated from enthalpy-concentration plots.
Duhring plots: These are plots of boiling point of pure solvent (usually water) in the abscissa and
the corresponding boiling point of the solution in the ordinate at the same pressure. Different lines are
for solutions at different concentrations. The points on a particular line are for a solution of same
concentration but different pressures. These are straight lines for practical purposes that make inter-
polation and extrapolation easier. Boiling point elevation (BPE) of a solution at a particular concen-
tration is thus obtained from the difference between the ordinate and abscissa at that concentration
where the abscissa corresponds to the pure solvent boiling point at the effect pressure. The linearity of
the lines suggests that the boiling point elevation is primarily a function of solute concentration and
does not change much with pressure and temperature. This is a fact that is often used for simplifying
calculations. Although linear, the lines are not parallel and in general have a steeper slope for more
concentrated solution, i.e., boiling point elevation (BPE) increases faster with increasing pressure for
more concentrated solution than for dilute solutions. Fig. 6.20 presents a typical Duhring plot for
sodium chloride solution.

FIGURE 6.20
Duhring Plot for sodium chloride solution.