# Boiling Point Elevation Discussion

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If we plot the curves for both pure water (dashed) and a liquid containing impurities (such as salt water or Bayer Liquor), we
find the latter has moved up at any point. The difference between these two curves is the *Boiling Point Elevation* (BPE) at that
particular pressure.

Note that according to this definition, [math]\displaystyle{ \beta }[/math] is intrinsically a function of pressure. Formally

- [math]\displaystyle{ \beta = T_L^{s}(p) - T_w^{s}(p) }[/math]

where:

- [math]\displaystyle{ T_w^{s}(p) }[/math] is the boiling point of water expressed as a function of pressure,
- [math]\displaystyle{ T_{L}^{s}(p) }[/math] is the boiling point for any liquor.

We have a number of possible ways of interpreting BPE:

## BPE at Stream Pressure

The BPE *at stream pressure* is difference between the temperature at which the liquor would boil and that which water would boil at, at the stream pressure.

## BPE at Stream Temperature

Care must be taken if the boiling point elevation is specified as a function of *temperature* - Does the correlation refer to the liquor boiling temperature or water boiling temperature?

See the graph below:

The BPE *at stream temperature* can be interpreted in two ways:

- We can choose the liquor vapour pressure at that temperature as the basis for saturation (point 1 in the above diagram); or
- We can choose the water vapour pressure at that temperature as the basis for saturation (point 2 in the above diagram).

Please see Boiling Point Elevation for a description of the Boiling Point Elevation equation used in the Standard Species model in SysCAD.