# Physical meaning of some earned values using DePriester Chart

Using DePriester Chart and Given one of mole fractions ($z$), pressure and temperature we can acquire K-values for that properties and bubble and dew properties.

Now there are sometimes that we get some K's and using the summation of $y/k$ we get some quantities that are not 1, like 0.9 or 1.2. My question is about those values, are they showing some physical property?

If I wanted to calculate the dew point pressure at a given temperature and I take arbitrary pressures (including the dew point pressure) I get the following results.

[p] = psi    sum of y/k
- - - - - - - - - - - -
100         0.828
126         1.000
150         1.174


Do the values for the pressures above and below the dew point pressure have a physical meaning?

For example, showing us that we are in two-phase, SH vapor or SC liquid region? Or are they just showing there isn't VLE with that given property?

• Could you give an example of where this was done? I have a hunch where you are going with this. Mar 19 '16 at 7:54
• @idkfa Ok an example,we have mole fractions and temperature, and we want dew pressure, so we take two arbitrary pressures and calculate summation of y/k , which y=z because we are in dew point,and k earned by DePriester,so if we try this summation in 100psi we get 0.828 and if we try it in 150psi we get 1.174 , and finally if we try 126psi we get 1 and thats the dew pressure,i wanna know about those 1.174 and 0.828,do they give us some physical signs about which regions are we in,by that pressure? Mar 19 '16 at 9:07
• These numbers came from somewhere, it would have been nice if you'd provided the calculations. And please make sure to include important information in the question body. Mar 25 '16 at 12:38

You can represent K as $K_i = \frac{p_i^{sat}}{p}$ for an ideal mixture using Raoult's law.

This equation directly yields the correspondance of $K$ and $p$. If the pressure is below the dew point pressure $K_i$ will be larger. Hence $\frac{z_i}{K_i}$ will be smaller and subsequently the sum over it.

What this tells you is

$p$ yields $\sum \frac{z_i}{K_i} < 1$ pressure is below dew point pressure

$p$ yields $\sum \frac{z_i}{K_i} > 1$ pressure is above dew point pressure

Note: This is for a fixed temperature.

Alternatively you can make the same deductions with $K_i = \frac{y_i}{x_i}$ and the knowledge that at dew point the first drop of liquid forms.