5.3. Boiling point elevation (BPE)

Most evaporators produce concentrated liquor having a boiling point considerably higher than that of pure solvent (or water). This phenomenon is called boiling point elevation (BPE). BPE occurs as the vapor pressure of a solution (usually aqueous solution) is less than that of pure solvent at the same temperature. Boiling point of a solution is a colligative property . It depends on the concentration of solute in the solution for a pair of solute and solvent.

BPE of the concentrated liquor reduces the effective temperature driving force compared to the boiling of pure solvent. Equilibrium vapor generated from a solution exhibiting boiling point elevation is superheated with respect to vapor generated during boiling of pure solvent. The vapor is generated at the solution boiling point, which is higher than the pure component boiling point. The vapor, however, is solute free, so it won't condense until the extra heat corresponding to the elevation is removed, thus it is superheated. Therefore the BPE of the concentrated solution must be known for evaporator design.

Determination of BPE : For strong solutions, the BPE data is estimated from an empirical rule known as Dühring rule . This states that the boiling point of a given solution is a linear function of the boiling point of pure water at the same pressure. Thus if the boiling point of the solution is plotted against the corresponding boiling point of pure water at the same pressure, a straight line is generated. Different lines are obtained if such plots made for solution of different concentrations. The main advantage is that a Dühring lines can be drawn if boiling points of a solution and water (read from steam table) at two different pressures are known. This line can be used to predict boiling point of a solution at any pressure. A Dühring plot for the NaOH-water system can be found in heat transfer text books ( [1] (page 472) and [2] (page 386).

5.4. Selection of suitable evaporator

The selection of the most suitable evaporator type depends on a number of factors. Mainly these are: (i) throughput, (ii) viscosity of the solution (and its increase during evaporation), (iii) nature of the product and solvent (such as heat sensitivity and corrosiveness), (iv) fouling characteristics and, (v) foaming characteristics. A selection guidelines based on these factors is given in Figure 3.8 .

Figure 3.8. Selection guide of evaporators [3] .